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Overlap and competition for nest holes among Eclectus Parrots, Palm Cockatoos and Sulphur-Crested Cockatoos


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We examined the extent of overlap in the characteristics of nest holes used by eclectus parrots (Eclectus roratus), palm cockatoos (Probosciger aterrimus) and sulphur-crested cockatoos (Cacatua galerita) in patches of rainforest and woodland in and around Iron Range National Park, Cape York Peninsula, Australia. Eclectus parrots nested only in rainforest and palm cockatoos nested mostly in eucalypt woodland adjacent to rainforest. Sulphur-crested cockatoos nested in both habitats. Nest holes of eclectus parrots and rainforest sulphur-crested cockatoos were in trees of larger DBH (diameter at breast height) and higher off the ground than those of palm cockatoos and sulphur-crested cockatoos in woodland. Palm cockatoos differed from the other parrots in their use of deeper holes with entrances that faced skywards rather than sideways. Both palm cockatoos and woodland sulphur-crested cockatoos used nests with smaller entrances than eclectus parrots and rainforest sulphur-crested cockatoos. All species showed intraspecific competition for nest holes. Behavioural conflict was also common between sulphur-crested cockatoos and the other two species. Each year 9.7–25.8% of eclectus parrot nests were taken over either permanently or temporarily by sulphur-crested cockatoos. Only one palm cockatoo nest was taken over by sulphur-crested cockatoos. Nest-holes were destroyed by natural causes at similar rates in rainforest (3.8% per annum over 174 nest-years) and woodland (5.4% per annum over 93 nest-years). Four nest trees fell over, and the floor of the nest collapsed at a further four holes. Three woodland nest trees burnt down during dry-season fires (August–October). New eclectus parrot and rainforest sulphur-crested cockatoo holes originated from incipient hollows on the tree that were modified by the parrots. We discuss the intense competition between these large parrots in light of the apparent shortage of appropriate nest holes in Cape York rainforest and eucalypt woodland.
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CSIRO PUBLISHING Australian Journal of Zoology, 2003, 51, 81–94
© CSIRO 2003 10.1071/ZO02003 0004-959X/03/010081
Overlap and competition for nest holes
among eclectus parrots, palm cockatoos
and sulphur-crested cockatoos
Robert Heinsohn
, Stephen Murphy
and Sarah Legge
Centre for Resource and Environmental Studies, Australian National University,
Canberra, ACT 0200, Australia.
School of Botany and Zoology, Australian National University, Canberra,
ACT 0200, Australia.
We examined the extent of overlap in the characteristics of nest holes used by eclectus parrots (Eclectus
roratus), palm cockatoos (Probosciger aterrimus) and sulphur-crested cockatoos (Cacatua galerita) in
patches of rainforest and woodland in and around Iron Range National Park, Cape York Peninsula,
Australia. Eclectus parrots nested only in rainforest and palm cockatoos nested mostly in eucalypt
woodland adjacent to rainforest. Sulphur-crested cockatoos nested in both habitats. Nest holes of eclectus
parrots and rainforest sulphur-crested cockatoos were in trees of larger
DBH (diameter at breast height) and
higher off the ground than those of palm cockatoos and sulphur-crested cockatoos in woodland. Palm
cockatoos differed from the other parrots in their use of deeper holes with entrances that faced skywards
rather than sideways. Both palm cockatoos and woodland sulphur-crested cockatoos used nests with smaller
entrances than eclectus parrots and rainforest sulphur-crested cockatoos. All species showed intraspecific
competition for nest holes. Behavioural conflict was also common between sulphur-crested cockatoos and
the other two species. Each year 9.7–25.8% of eclectus parrot nests were taken over either permanently or
temporarily by sulphur-crested cockatoos. Only one palm cockatoo nest was taken over by sulphur-crested
cockatoos. Nest-holes were destroyed by natural causes at similar rates in rainforest (3.8% per annum over
174 nest-years) and woodland (5.4% per annum over 93 nest-years). Four nest trees fell over, and the floor
of the nest collapsed at a further four holes. Three woodland nest trees burnt down during dry-season fires
(August–October). New eclectus parrot and rainforest sulphur-crested cockatoo holes originated from
incipient hollows on the tree that were modified by the parrots. We discuss the intense competition between
these large parrots in light of the apparent shortage of appropriate nest holes in Cape York rainforest and
eucalypt woodland.
Ne st hole competition in t ropi cal Australian parrotsR. He insohn
al .
Many bird species rely on holes in trees (also called cavities or hollows) for breeding.
Although some (e.g. woodpeckers) dig their own holes, most are ‘secondary hole-nesters’
that rely on pre-existing holes formed through the action of insects or fungi, or those that
are dug by other species (Newton 1994). Nests in tree holes provide security from predation
compared with open nests in other locations such as branches, the canopy or ground, and
may support microclimatic conditions that aid thermoregulation and help control water loss
(reviewed by Gibbons and Lindenmayer 2002). Limited availability of holes creates intra-
and interspecific competition for nest sites and limits the population size of many
hole-nesting birds (Newton 1994). Entrance size and the internal dimensions of the hole,
especially depth, are the primary determinants of occupancy by birds of different sizes.
Deep holes with small entrances are probably preferable because they hinder access by both
predators and larger competitors (Newton 1994; Gibbons and Lindenmayer 2002).
Worldwide, obligate hole-nesting is concentrated in seven orders of birds, the
Passeriformes (songbirds), Piciformes (woodpeckers), Apodiformes (swifts), Coracii-
82 Aust. J. Zoology R. Heinsohn et al.
formes (rollers and kingfishers), Strigiformes (owls), Psittaciformes (parrots), and Anseri-
formes (waterfowl) (Newton 1994). Saunders et al. (1982) and Newton (1994) noted that
Australia has a higher proportion of hole-nesting birds than other continents, possibly due
to the larger number of holes found in Australian forests and woodlands. In total, 11% of
Australia’s birds are obligate hole-nesters compared with 5% in Europe, 4% in North
America, and 6% in Africa (Gibbons and Lindenmayer 2002). Australian hole-nesters are
not evenly distributed across the seven orders mentioned above, as there are no
woodpeckers, only one species of roller, and none of Australia’s swifts nest in tree holes
(Pizzey 1997). However, Australia does have approximately one-fifth of the world’s parrot
species and the greatest taxonomic diversity of that group (Forshaw and Cooper 1989).
Parrots account for a high proportion (almost 40%) of the 119 Australian hole-nesting bird
species compiled by Gibbons and Lindenmayer (2002).
Our research is aimed at determining the ecology and population viability of two large
parrot species, eclectus parrots (Eclectus roratus) and palm cockatoos (Probosciger
aterrimus), found in patches of rainforest and woodland on eastern Cape York Peninsula.
Rainforest within the study area comprises a semi-deciduous mesophyll vine forest, up to
35 m high and dominated by the canopy tree species Castenospermum australe,
Beilschmedia obtusifolia, Nauclea orientalis, Syzgium bamagense and Terminalia
sericocarpa. Woodland, on the other hand, is a more open and lower (up to 20 m) forest type
dominated by the sclerophyllous species Eucalyptus tetradonta, Corymbia nesophila and
C. clarksoniana. Woodland in the region is also characterised by a grassy understory
dominated by Imperata cylindrica.
In contrast to the growing number of studies from temperate Australia (e.g. Saunders
1982; Rowley and Chapman 1991; Krebs 1998), information on the ecology of parrots (and
indeed most hole-nesting birds) in tropical Australasia is lacking (for rare examples see
Marsden 1992; Marsden and Jones 1997). On Cape York, such knowledge is essential
because many species have limited ranges and are threatened by changing land tenure and
management practices. For example, changed fire regimes may affect the availability of
nest holes in eucalypt woodland (Gibbons and Lindenmayer 2002), and logging of
rainforest could remove key hole-bearing tree species (Heinsohn and Legge 2003).
The specific aim of this paper is to outline the nesting requirements and degree of
competition for nest holes between eclectus parrots, palm cockatoos, and sulphur-crested
cockatoos (Cacatua galerita). These parrots all actively defend their nest holes, and engage
in aggressive intra- and inter-specific encounters that suggest competition for limited
nesting resources. Female eclectus parrots have high between-year nest-site fidelity and
reoccupy their holes up to eight weeks before laying their clutches. They rarely leave the
nest tree for the entire 8–9-month breeding period (Heinsohn and Legge 2003).
Unpublished aerial survey data confirms the scarcity of trees of the required height and
species: two different extrapolation methods from the surveys show that there are only
approximately 400 eclectus parrot nest trees in the 500 km
of rainforest found in and
around Iron Range National Park (Legge and Heinsohn, unpublished data). Nest holes of
sulphur-crested cockatoos are also constantly guarded by at least one member of each pair
during breeding, but palm cockatoos have a different form of nest-site defense. Males
defend holes and advertise their presence most mornings and afternoons with displays that
include loud calling and whistling, bowing, crest erection, and drumming on the nest tree
with a stick held in the foot. Aggressive interactions between male palm cockatoos, with
growling, full body contact during flight and major loss of feathers are common (S.
Murphy, personal observation).
Nest hole competition in tropical Australian parrots Aust. J. Zoology 83
The similar size of these species, overlapping breeding seasons, and observations of
behavioural conflict all suggest that they compete for large tree holes. Here we outline the
physical characteristics of the nest holes of each species, and compile behavioural
observations of nest usurpation and delays in breeding due to such competition. We also
compare the rate at which nest holes were both created and lost in rainforest and woodland
due to natural causes.
Study species
In Australia, eclectus parrots are restricted to a narrow strip of coastal rainforest from the McIlwraith Range
to the Iron Range (Forshaw and Cooper 1989; Juniper and Parr 1998). They weigh 550–650 g, lay a clutch
of two eggs, and breed from July to February. They exhibit a unique form of reversed sexual dichromatism,
in which the females are spectacular red and blue whereas the males are shiny green. Their social system is
also unusual amongst parrots. In Australia, they breed cooperatively, with multiple males providing all food
to the female and her chicks (Heinsohn and Legge 2003). The female incubates and broods alone but, unlike
other parrots, remains guarding the hole throughout the entire nesting period. She also has strong control
over the sex of her offspring (Heinsohn et al. 1997). Incubation takes 30 days, and the nestling phase takes
72–97 days (Heinsohn and Legge 2003).
Palm cockatoos in Australia are found in woodlands and rainforest on northern Cape York Peninsula
(Forshaw and Cooper 1989; Juniper and Parr 1998). They weigh 650–1000 g and lay a single egg in a hole
in a tree or in the upward-facing end of a tree-stump. In New Guinea they are primarily a rainforest species,
but most nests in Australia have been found in eucalypt woodlands close to rainforest patches. Like eclectus
parrots, they breed from August to February but some nest holes are not used every year (S. Murphy,
unpublished data). A stick platform up to 2 m deep is constructed above the bottom of the hole; its function
is believed to be to keep eggs and chicks dry during heavy rain and to allow excreta to wash away.
Incubation takes about 30 days, and chicks take 60–70 days to fledge (S. Murphy, unpublished data).
Sulphur-crested cockatoos occur in a variety of habitats in lowland New Guinea and Australia roughly
along the eastern half of Australia to Tasmania (Forshaw and Cooper 1989; Juniper and Parr 1998). On
Cape York, they nest in both rainforest and woodland, and most breeding occurs between August and
December. They weigh 600–700 g and lay a clutch of two eggs. Although larger than eclectus parrots, their
chicks fledge more quickly (62–72 days) (Heinsohn and Legge, unpublished data).
Study site
Our ongoing studies of parrots are being conducted at Iron Range National Park on Cape York Peninsula,
in far north Queensland, Australia (12°45S, 143°17E) (Fig. 1). The national park includes the coastal plain
and low ranges of hills east of the Tozer and Janet Ranges and comprises patches of rainforest, eucalypt
woodland and heath in a complex mosaic. The mean annual rainfall for the national park is 2780 mm (M.
Blackman, personal communication), with most rain falling during a distinct ‘wet’ season from December
to April.
Two of the authors (RH and SL) began studying eclectus parrots in July 1997 and the third (SM) began
work on palm cockatoos in June 1999. Both projects have included opportunistic observations of tree hole
use and nesting habits of sulphur-crested cockatoos. To find eclectus parrot nest trees, RH and SL walked
along tracks and creek beds listening for eclectus parrot vocalisations. Females call very loudly at the
beginning of the breeding season, apparently to advertise their presence at the nest hole. Colour banding
shows that nest trees are re-used by the same individuals each year (Heinsohn and Legge 2003). Breeding
efforts at 34 trees with 46 nest holes have been monitored throughout the study. Sixteen of the eclectus
parrot nests had at some stage also been used by sulphur-crested cockatoos for breeding (see Results), and
a further eight nest holes used only by sulphur-crested cockatoos were also located in rainforest.
Single-rope techniques were used to gain access to each nest once every two weeks to monitor breeding
activity, and breeding behaviour and competition were observed from permanent hides built in
neighbouring trees (Heinsohn and Legge 2003).
SM located 24 active palm cockatoo nests using similar search methods in the eucalypt woodlands.
Search effort was initially concentrated in areas where the birds were seen displaying in the late afternoon,
and in patches of woodland close to rainforest. Later surveys, which were aimed at determining habitat
requirements, extended searches into areas of low palm cockatoo density (S. Murphy, unpublished data).
84 Aust. J. Zoology R. Heinsohn et al.
SM mounted a small video camera on an extendable pole and attached it with light cables to a monitor to
look into all potential nest holes up to 12 m high. This system allowed nests to be checked for eggs, chicks,
and the broken sticks that palm cockatoos place in holes to build nesting platforms. Single-rope techniques
and extendable ladders were used to climb to higher holes. Palm cockatoos build platforms in many more
holes than they use for nesting; the data used in this paper refer only to those holes known to be used for
breeding. SM also located and monitored breeding activity at seven sulphur-crested cockatoo nest holes.
We recorded/measured the following characteristics of each nest hole for all three parrot species: (1)
tree species, (2) whether the tree was dead or alive, (3) diameter at breast height (DBH, m) (4) height of
hole entrance above the ground (m), (5) depth of hole (entrance to floor, cm), (6) entrance angle (from
facing horizontal = 0° to facing skywards = 90°), (7) angle
of hole (from vertical chamber = 0° to horizontal
chamber = 90°), (8) shortest entrance dimension (cm), (9) longest entrance dimension (cm), and (10) the
number of entrances to the hole (the main entrance was used for the measures outlined above). We recorded
the annual rate of hole loss and its causes (e.g. fire damage, tree fall), and in the case of eclectus parrots
and rainforest sulphur-crested cockatoos we also recorded the rate and cause of new nest hole formation.
During our regular nest checks we also noted opportunistically the incidences of behavioural competition
(e.g. fighting) and actual displacement (e.g. hole occupation) between species at nest holes.
Tree species
Eclectus parrots nested in 11 tree species, with 36 out of 46 holes occurring in only five
species (Table 1). Rainforest sulphur-crested cockatoos used five of the same species with
one additional species, Palaquium galactoxylum (Table 1). Many of the palm cockatoo nest
trees were dead and could only be identified to genus level. Palm cockatoos nested in at
least eight tree species, with 17 out of 24 nests occurring in either Eucalyptus tetradonta or
Corymbia sp. Sulphur-crested cockatoos in woodland nested in species that were a subset
of those used by palm cockatoos. Overlap in tree species between eclectus parrots and palm
cockatoos was small and included only one species commonly used by eclectus parrots
Iron Range
National Park
Fig. 1. Location of the study area at Iron
Range National Park (12°45S, 143°17E).
Nest hole competition in tropical Australian parrots Aust. J. Zoology 85
(Melaleuca dealbata) and one species rarely used by both (Eucalyptus tessellaris). There
was one nest hole in an Alstonia scholaris that was used by all three parrot species in
succession, first by eclectus parrots, followed by sulphur-crested cockatoos, and then by
palm cockatoos.
Physical characteristics of nest holes
Statistical tests of single characteristics of nest trees and holes indicated some differences
between the physical characteristics used by the three parrot species. Most eclectus parrots
and sulphur-crested cockatoos nested in live trees whereas 7 of 24 palm cockatoo nests were
in dead trees. Eclectus parrots and rainforest sulphur-crested cockatoos nested in trees with
significantly greater DBH, and in holes that were significantly higher from the ground than
palm cockatoos (Table 2). Palm cockatoos used holes that were, on average, more
skyward-facing than those used by sulphur-crested cockatoos, and both palm cockatoos and
woodland sulphur-crested cockatoos used nests with smaller shortest-entrance dimensions
than those of eclectus parrots. The longest entrance dimension, hole angle, and number of
entrances did not differ between the species (Table 2).
Overlap in nest-hole dimensions
We used a principal components analysis (JMP Statistical Package) to determine the overall
similarity of the physical characteristics of nest holes used by the three parrot species. We
included six measures of nest holes that could vary similarly in rainforest and woodland
trees, but excluded the height of the hole and DBH as these strongly reflect the different
tree types available in the two habitats. The first three principal components accounted for
29.1%, 21.9%, and 19.9% of the variation. The eigenvalues and eigenvectors for these are
shown in Table 3.
Plots of PC1 versus PC3 and PC2 versus PC3 suggested that palm cockatoos differed in
their values of PC3 (Fig. 2). Kruskal–Wallis tests (single-factor analysis of variance by
ranks) confirmed significant differences between palm cockatoos, eclectus parrots and
rainforest and woodland sulphur-crested cockatoos in values of PC3 (χ
= 20.1, P = 0.002)
but not PC1 (χ
= 2.6, P = 0.455) or PC2 (χ
= 4.2, P = 0.239). The eigenvectors indicated
Table 1. Tree species used by eclectus parrots, sulphur-crested cockatoos and palm cockatoos for
nest holes
Numbers in parentheses indicate the number of times that nest-tree species was used
Parrot species Tree species
Eclectus roratus Alstonia scholaris (10), Castenospermum australe (7), Ficus albipila
(9), Alstonia actinophylla (4), Melaleuca dealbata (6), Ficus sp.
(dead) (1), Tetrameles nudiflora (2), Endospermum
myrmecophilum (1), Lophostemon sp. (3), Syzigium sp. (2),
Eucalyptus tessellaris (1)
Cacatua galerita (rainforest) Alstonia scholaris (2), Melaleuca dealbata (2), Tetrameles nudiflora
(1), Castenospermum australe (1), Palaquium galactoxylum (1),
Lophostemon sp. (1)
Cacatua galerita (woodland) Melaleuca leucadendra (4), Melaleuca dealbata (3), Eucalyptus
tetradonta (1)
Probosciger aterrimus Eucalyptus tetradonta (10), Corymbia sp. (7), Blepharocarya
involucrigera (1), Eucalyptus brassiana (1), Eucalyptus
tessellaris (1), Lophostemon sauveolens (1), Melaleuca dealbata
(1), Melaleuca sp. (1), Alstonia scholaris (1)
86 Aust. J. Zoology R. Heinsohn et al.
Table 2. The physical characteristics of nest holes, with comparison of these characteristics for the three parrot species
Numbers denote means ± standard errors
Character Eclectus roratus Cacatua galerita
Cacatua galerita
No. of nests 46 24 7 24
No. of nests in live trees 44 23 7 17
DBH (m) 01.5 ± 0.1 01.3 ± 0.1 00.9 ± 0.2 00.6 ± 0.1 F
= 29.1, P < 0.001
Hole height (m) 22.2 ± 0.7 21.7 ± 0.9 13.5 ± 1.7 09.1 ± 0.9 F
= 50.6, P < 0.001
Hole depth (cm) 84.8 ± 4.9 66.5 ± 6.8 58.7 ± 12.7 99.2 ± 6.7 F
= 5.1, P = 0.003
Entrance angle 15.4 ± 4.4 18.9 ± 6.1 18.6 ± 11.3 51.9 ± 6.1 F
= 8.4, P < 0.001
Shortest entrance dimension (cm) 30.4 ± 1.2 31.7 ± 1.6 22.3 ± 3.0 25.2 ± 1.6 F
= 4.9, P = 0.004
Longest entrance dimension (cm) 34.3 ± 1.4 35.8 ± 2.0 37.3 ± 3.7 33.3 ± 2.0 F
= 0.5, P = 0.718
Hole angle 23.6 ± 4.9 24.6 ± 6.7 10.0 ± 12.5 16.3 ± 6.7 F
= 0.6, P = 0.608
No. of entrances 01.2 ± 0.1 01.1 ± 0.1 01.3 ± 0.1 01.1 ± 0.1 F
= 0.5, P = 0.674
Nest hole competition in tropical Australian parrots Aust. J. Zoology 87
that hole-entrance dimensions had the greatest influence in PC1, hollow angle had the
greatest influence in PC2, and that entrance angle and hole depth were the greatest
contributors to PC3. Thus, entrance angle and hole depth were the chief factors separating
palm cockatoos from the other parrots. Specifically, palm cockatoos nested in deeper holes
than the other two parrots, and in holes that faced skywards rather than side-ways.
Behavioural conflict at nest sites
Behavioural conflict between eclectus parrots and sulphur-crested cockatoos at nest holes
was common in each year of the study. Female eclectus parrots and sulphur-crested
cockatoos (unsexed) were observed to attack and chase each other, and in some instances
hole ‘ownership’ changed between and even within breeding seasons. Table 4 shows the
number of eclectus parrot nests where some form of competition between the two species
was observed, and the number of nests where take-overs by sulphur-crested cockatoos
prevented the eclectus parrots from breeding there that year or potentially caused them to
delay breeding. This was based on instances where sulphur-crested cockatoos physically
occupied holes that had been previously used by eclectus parrots, while the latter species
milled around waiting for the cockatoos to complete breeding. In all, 9.7–25.8% of eclectus
parrot nests were affected in this manner each year (Table 4). A further three of the 16 holes
used by both sulphur-crested cockatoos and eclectus parrots were used by the former at the
start of our study but were later taken over by the latter. One nest hole that was used by
eclectus parrots for three years was taken over in 2000 by rufous owls (Ninox rufa) and later
by European bees (Apis mellifera).
Direct behavioural conflict between palm cockatoos and sulphur-crested cockatoos was
observed at six nests. In three cases, palm cockatoos were known to drop sticks and
splinters inside nests used previously by sulphur-crested cockatoos, which promptly
removed them. In one case, palm cockatoos and sulphur-crested cockatoos both regularly
visited the same nest hole. The palm cockatoos dropped sticks inside the hole and the
sulphur-crested cockatoos removed them, but neither species used the hole for breeding.
Sulphur-crested cockatoos were only once known to take over a hole previously used for
breeding by palm cockatoos. Sulphur-crested cockatoos appeared regularly at two other
palm cockatoo nests but were chased away each time. One of these nest holes had
previously been used by eclectus parrots before being used by sulphur-crested cockatoos
and ultimately by palm cockatoos (Table 5).
Table 3. Results of a principal components analysis describing the physical
characteristics of nest holes of eclectus parrots, sulphur-crested cockatoos,
and palm cockatoos to the first three principal components
Eigenvalue 1.75 1.32 1.19
Percentage variation 29.20 21.94 19.91
Hole depth (cm) 0.21 0.36 0.58
Entrance angle 0.03 –0.24 0.76
Shortest entrance diameter (cm) 0.65 –0.21 –0.19
Longest entrance diameter (cm) 0.65 –0.24 –0.06
Hole angle 0.09 0.69 –0.18
No. of entrances 0.32 0.49 0.09
88 Aust. J. Zoology R. Heinsohn et al.
Fig. 2. Pair-wise plots of first three principal components of physical characteristics of nest holes. (a)
First principal component (PC1) versus PC2, (b) PC1 versus PC3, and (c) PC2 versus PC3. Symbols refer
to nest holes used by: palm cockatoos (crosses), eclectus parrots (filled diamonds), eclectus parrots and
sulphur-crested cockatoos (open diamonds), sulphur-crested cockatoos in rainforest (open circles),
sulphur-crested cockatoos in woodland (closed circles), all three species (closed squares).
Nest hole competition in tropical Australian parrots Aust. J. Zoology 89
None of us have ever observed any form of aggression or competition at nest holes
between eclectus parrots and palm cockatoos.
Creation, destruction, and turnover of nest holes
We documented the destruction of existing holes and the creation of new holes for both
eclectus parrots and rainforest sulphur-crested cockatoos, but only the destruction of nest
holes for palm cockatoos and woodland sulphur-crested cockatoos. Three eclectus parrot
nest trees fell over, and the floor of the nest collapsed at a further three eclectus parrot nests
and one sulphur-crested cockatoo hole. One sulphur-crested cockatoo nest tree fell over. In
the eucalypt woodland, two palm cockatoo nest trees and one sulphur-crested cockatoo nest
tree burnt down during dry-season fires (August–October). All three of these trees were
already dead (Table 6). One further palm cockatoo nest hole was lost when it was taken over
and altered dramatically by extensive gouging and chewing by sulphur-crested cockatoos.
All new eclectus parrot and rainforest sulphur-crested cockatoo holes originated from
existing holes in the tree, mostly where branches had broken off. We observed the parrots
actively enlarging these holes by gouging out and removing the inner wood in six out of
seven cases (Table 6). Palm cockatoos were never observed enlarging or manipulating the
hole by chewing or gouging. Holes were lost at approximately the same rate they were
gained for the rainforest parrots (i.e. eight lost, seven gained over five years) (Table 6).
In 174 nest-years for rainforest and 93 nest-years for woodland, the mean annual rate of
hole loss for each species was 4.0% for eclectus parrots, 2.5% and 9.8% for sulphur-crested
cockatoos in rainforest and woodland, respectively, and 5.3% for palm cockatoos. Despite
the different causes, the overall rates of loss for rainforest (3.8%) and woodland (5.4%)
were similar (Table 7).
Table 4. Instances of behavioural competition (fighting and opposing bird in hole), and the number
of breeding attempts by eclectus parrots that were potentially delayed because of nest take-overs by
sulphur-crested cockatoos over five years of the study
Year Total no. of nests No. of nests at
which competition
No. of nests at
which breeding
Percentage of nests
at which breeding
1997 16 08318.8
1998 31 09309.7
1999 31 12 8 25.8
2000 31 07309.7
2001 25 04312.0
Table 5. Behavioural interactions between parrot species at six tree holes
PCs, palm cockatoos; SCCs, sulphur-crested cockatoos; EPs, eclectus parrots
Nest Nest used by: Form of competition
1 SCCs 1999–2001 PCs irregularly add sticks, SCCs remove them
2 SCCs 1999 PCs add sticks in 2000–01, SCCs remove them
3 PCs and SCCs visit hole 1999–2001 PCs add sticks, SCCs remove them
4 PCs 1999–2000 SCCs take over nest in 2001
5 PCs 1999–2001 SCCs regularly at hole
6 EPs 1997–98 SCCs chased away
SCCs 1999, PCs 2001 SCCs chased away
90 Aust. J. Zoology R. Heinsohn et al.
Table 6. Tree holes lost and gained, including causes, by eclectus parrots, sulphur-crested cockatoos, and palm cockatoos
Parrot species Cause of hole loss and tree species involved Cause of hole gain and tree species involved
Eclectus roratus Tree fell over (1 × Castanospermum australe, 1 × Melaleuca
dealbata, 1 × Ficus sp. – dead)
Rot at small hole (1 × M. dealbata, 1 × Tetrameles nudiflora)
Nest floor collapsed (1 × C. australe, 1 × Syzigium sp., 1 × Ficus
Rot at small hole, enlarged by parrot (1 × Alstonia scholaris, 1 ×
F. albipila, 1 × M. dealbata, 1 × C. australis)
Cacatua galerita (rainforest) Tree fell over (1 × Melaleuca dealbata) Rot at small hole, enlarged by parrot (1 × Endospermum
myrmecophilum, 1 × Lophostemon sp.)
Nest floor collapsed (1 × C. australe)
Cacatua galerita (woodland) Tree burnt (1 × Melaleuca sp. – dead) None recorded
Probosciger aterimmus Tree burnt (1 × Corymbia clarksoniana, 1 × Eucalyptus
tetradonta), both trees already dead
None recorded
Nest hole lost to take-over/modification by sulphur-crested
cockatoos (1 × Melaleuca dealbata)
Nest hole competition in tropical Australian parrots Aust. J. Zoology 91
On northern Cape York, the major ecological separation in nest-hole use by the three largest
parrot species was whether they bred in rainforest or woodland, rainforest being a tall (25+
m), closed forest community with a high diversity of mesophyllic species, and woodland
having low diversity of mainly sclerophylous species that are more widely spaced and have
a low canopy height (10–20 m). Although palm cockatoos nest primarily in rainforest in
New Guinea (Paul Igag, personal communication), only two active palm cockatoo nests
have been recorded inside rainforest in our study. Unpublished survey results of habitat use
confirms that the highest breeding densities of palm cockatoos are in woodland (S. Murphy,
unpublished data). Thus, most potential for overlap in nest sites between the three parrots
in this study is between eclectus parrots and sulphur-crested cockatoos in rainforest, and
between palm cockatoos and sulphur-crested cockatoos in woodland. Consequently,
eclectus parrots and palm cockatoos used few of the same nest-tree species. Eclectus
parrots used tall rainforest trees, especially of the genera Alstonia, Castenospermum and
Ficus whereas palm cockatoos mainly used Eucalyptus and Corymbia. Melaleuca was used
frequently by eclectus parrots but only occasionally by palm cockatoos; for both species
these were usually found in creek-beds or swamps.
In contrast, sulphur-crested cockatoos in each habitat used a subset (with one exception)
of the tree species used by the other two parrots. In woodland, a higher proportion of
sulphur-crested cockatoo nests were in Melaleuca than for palm cockatoos. Sulphur-crested
cockatoos and eclectus parrots also used more live trees (only 4% were in dead trees)
whereas 29% of palm cockatoo nests were in dead trees.
Comparisons of the physical attributes of nest holes indicated some differences between
parrot species. For example, palm cockatoos used deeper holes with entrances that faced
skywards. Both eclectus parrots and sulphur-crested cockatoos used entrances that opened
sideways. This accords with the observation that only palm cockatoos actively build
platforms inside the holes to overcome flooding. Losses of young due to flooding are a
major determinant of reproductive success in eclectus parrots (Heinsohn and Legge 2003).
Sulphur-crested cockatoos were less choosy in this regard and had shallower holes, on
average, in both environments. Flooding may be less of a problem for them because their
breeding season is shorter and overlaps less with the wet season.
Rainforest nest holes were higher from the ground and in trees with larger DBH than
those in woodland, which reflects the size of available nest trees in each habitat. High holes
are safer from some predators than low holes (e.g. Nilsson 1984; Marsden and Jones 1997)
and it is noteworthy that sulphur-crested cockatoos nested much higher in rainforest (i.e.
Table 7. Annual loss of nest holes for eclectus parrots and sulphur-crested cockatoos in rainforest,
and for palm cockatoos and sulphur-crested cockatoos in woodland
Ye a r E c l e c t u s
ed cockatoos
ed cockatoos
1997 0/16 (0%) 0/8 (0%) 0/24 (0%)
1998 2/31 (6.5%) 0/8 (0%) 2/39 (5.1%)
1999 1/31 (3.2%) 0/8 (0%) 1/39 (2.6%) 0/7 (0%) 0/24 (0%) 0/31 (0%)
2000 2/31 (6.5%) 0/8 (0%) 2/39 (5.1%) 1/7 (14.3%) 1/24 (3.2%) 2/31 (6.5%)
2001 1/25 (4.0%) 1/8 (12.5%) 2/33 (6.1%) 1/7 (14.3%) 2/24 (12.8%) 3/31 (9.7%)
Mean 4.0% 2.5% 3.8% 9.5% 5.3% 5.4%
92 Aust. J. Zoology R. Heinsohn et al.
when taller trees were available). Palm cockatoos had the lowest nest heights, mostly due to
their habit of nesting in holes in broken stumps. The shortest entrance dimension was also
smaller for the woodland parrots such that entrances were usually ‘slits’ rather than round
openings. Entrance size is important for exclusion of both competitors and predators, and
most species choose the smallest entrance they can fit through (Newton 1994; Gibbons and
Lindenmayer 2002). The narrower entrances in woodland probably reflect the larger
number of available holes in that habitat (Gibbons and Lindenmayer 2002), and therefore
the greater choice available for hole nesters.
Our principal components analysis of the physical attributes of the nest holes (i.e. PC1
v. PC3 and PC2 v. PC3) indicated a high degree of overlap between the three parrots. This
analysis excluded DBH and hole height as these features relate almost exclusively to the
type of trees available in rainforest and woodland, whereas the other physical attributes (e.g.
entrance size) are more independent of habitat. Palm cockatoos showed the most
separation. They differed significantly from the other parrots in values of the third principal
component, which reflected their preference for deeper holes and holes that faced
skywards. Hole depth is clearly important for palm cockatoos and was probably
underestimated owing to the birds’ habit of building stick platforms inside the hole, which
makes accurate estimates of depth difficult. Eclectus parrots and sulphur-crested cockatoos
in both habitats overlapped considerably in the physical characteristics of their nest holes
(Fig. 2).
Behavioural conflict was observed most frequently between eclectus parrots and
sulphur-crested cockatoos. These species showed a high convergence in nest-site
characteristics in rainforest, and indeed actively competed for the same hole in many
instances. Temporary or permanent take-overs by sulphur-crested cockatoos led to delays
in breeding at 9.7–25.8% of eclectus nests each year. Such delays often led to reproductive
failure for the eclectus parrots, especially when the hole was prone to flooding, because the
breeding effort was extended further into the wet season (Heinsohn and Legge 2003).
Behavioural conflict was also regularly observed between woodland sulphur-crested
cockatoos and palm cockatoos, even though only one palm cockatoo nest was ever taken
over by sulphur-crested cockatoos. Most interactions took the form of sticks being added to
the hole by the palm cockatoos and removed by the sulphur-crested cockatoos, or with the
former chasing the latter.
No behavioural conflict was recorded between eclectus parrots and palm cockatoos.
This is unsurprising since they use different habitats and the physical dimensions of their
nest holes are relatively divergent. It is nonetheless interesting that palm cockatoos in New
Guinea nest primarily in rainforest, whereas those on Cape York seem to prefer to breed in
the woodlands adjacent to rainforest. Two possible and non-mutually exclusive reasons for
this are (1) that palm cockatoos face stiff competition for rare nest hollows inside the
rainforest, and (2) that they have gained access to the more abundant source of hollows in
woodlands that are near enough to rainforest for them to retain access to the food and shelter
it provides.
The rate of loss of nest holes was similar in rainforest and woodland, but losses occurred
for different reasons. In rainforest, all losses were either from the entire tree falling over or
from the floor of the hole collapsing; in most cases the wood was damp, decayed and
attacked by insects and fungi. In contrast, the greatest cause of hole loss in the woodlands
was from nest trees burning down during dry-season fires. In rainforest, eight new nests
were created over five years compared with the seven that were lost, supporting the
expectation of an equilibrium in undisturbed forest (e.g. Sedgwick and Knopf 1992;
Nest hole competition in tropical Australian parrots Aust. J. Zoology 93
Newton 1994). The same agents responsible for hole demise (insect and fungal pests) were
also responsible for hole creation, although in most cases existing holes in the tree were
further enlarged by the parrots. It is important to note that while eclectus parrots and
sulphur-crested cockatoos do enlarge existing holes, thereby making otherwise unsuitable
holes usable, this behaviour does not abate the problem of hollow limitation as it does for
true ‘primary’ hole-nesting species. Thus, eclectus parrots and sulphur-crested cockatoos
are midway between ‘primary’ (e.g. woodpeckers) and ‘secondary’ hole-nesters (most
hole-nesting species), whereas palm cockatoos are clearly in the latter category.
In conclusion, our data demonstrate a high degree of overlap and competition between
eclectus parrots and sulphur-crested cockatoos in rainforest, and a smaller degree of overlap
between palm cockatoos and sulphur-crested cockatoos in woodland. Together with our
observations of conflict, this supports the hypothesis that nest holes are a limiting factor for
these large birds (e.g. Newton 1994; Gibbons and Lindenmayer 2002). This is despite the
fact that both eclectus parrots and sulphur-crested cockatoos can modify existing holes to
create cavities suitable for breeding.
There are many accounts of competition and usurpation of holes between parrots and
other species (e.g. galahs, Eolophus roseicapillus: Rowley 1990) but our results provide a
rare demonstration that competition can also be intense between large hole-nesting species
in mature tropical rainforest. Studies from temperate Australia and elsewhere in the world
(e.g. Mannan et al. 1980) show that the abundance and diversity of hole-nesting birds
increases as the forest ages. Indeed, eucalypts with holes appropriate for large parrots in
Western Australia and southern Australia have been shown to be over 100 and 200 years
old respectively (Saunders 1979; Nelson and Morris 1994). The rarity of hole-bearing trees
of the required DBH and height in the Iron Range rainforest (approximately 0.8 km
Legge and Heinsohn, unpublished data) is suggestive of a small and geographically limited
breeding population of eclectus parrots, and our unpublished surveys for palm cockatoos
suggest that they are also highly concentrated in just a few areas. Our future research is
aimed at determining what affects the spatial distribution of palm cockatoo density, as well
as conducting population viability analyses for both palm cockatoos and eclectus parrots.
At this stage we caution against any management practice that might cause the destruction
of old hole-bearing trees in this region. In rainforest, this includes logging for important
nest-tree species such as Castenospermum, and in woodland any fire practice that increases
the likelihood of loss of hole-bearing Eucalyptus, Corymbia, or Melaleuca species.
We thank Peter and Emma Huybers, Karl Goetz, Andrea Cook, Greg and Alice Daniels, and
Mick and Clare Blackman for their generous logistical support, and Michelle Hall, Anjeli
Nathan, Trish Pontynen, and Paul Igag for their help in the field. Our research is supported
by grants from the Australian Research Council, World Parrot Trust, National Geographic
Society, Winifred Violet Scott Foundation, Birds Australia, and The Gould League of New
South Wales.
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Manuscript received 10 January 2002; accepted 18 December 2002
... Logging and poaching are commonly associated with the edges of deforested areas and usually coexist (Brodie et al., 2014). While poaching for the illegal pet trade directly impacts parrot populations by the extraction of individuals (Pires, 2012), logging has an indirect but long-lasting negative effect on these populations by reducing their nesting sites (Heinsohn, Murphy, & Legge, 2003;Olah, Vigo, Heinsohn, & Brightsmith, 2014). ...
... Traditionally, this threat was mainly evaluated locally on a caseby-case basis (e.g., Heinsohn et al., 2003;Olah et al., 2014) but here we analyzed it on a global scale. The current trend is worrying, as a very high rate of timber extraction occurs over a large area of the Amazon Basin, the most important hotspot for parrot conservation. ...
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Human advance on natural habitats is a major cause of biodiversity loss. This transformation process represents a profound change in wooded environments, disrupting original communities of flora and fauna. Many species are highly dependent on forests, especially parrots (Psittaciformes) with almost a third of their species threatened by extinction. Most parrot species occur in tropical and subtropical forests, and given the forest dependence of most species, this is the main reason why habitat loss has been highlighted as the main threat for the group. Such habitat loss acts in synergy with other important threats (e.g. logging and poaching), which become especially problematic in certain developing countries along tropical latitudes. In this study, we used available information on parrot distributions, species traits, IUCN assessment, habitat loss and timber extraction for different periods, and distribution of protected areas, in order to determine conservation hotspots for the group, and analyze potential changes in the conservation status of these species. We detected four conservation hotspots for parrots: two in the Neotropics and two in Oceania, all of them facing different degrees of threat in regard of current habitat loss and agricultural trends. Our results suggest that the future of the group is subject to policymaking in specific regions, especially in the northeastern Andes and the Atlantic Forest. In addition, we predicted that agricultural expansion will have a further negative effect on the conservation status of parrots, pushing many parrot species to the edge of extinction in the near future. Our results have conservation implications by recommending protected areas in specific parrot conservation hotspots. Our recommendations to mitigate conservation risks to this group of umbrella species would also benefit many other coexisting species as well.
... The decline of large trees in human-dominated environments is of major conservation concern, as it reduces the number of natural tree hollows suitable for nesting (Gibbons and Lindenmayer 2002;Lindenmayer et al. 2012;Le Roux et al. 2016a, 2016bStojanovic et al. 2016). For some species, nest boxes can supplement the lack of natural tree hollows (Tidemann et al. 1992;Heinsohn et al. 2003;Brazill-Boast et al. 2010;Webb et al. 2012;Goldingay et al. 2015). However, boxes that are accessible to invasive species or native predators may not be suitable for many native species (Lindenmayer et al. 2017), creating challenges for native cavity-nesting species conservation in urban areas. ...
... The resource competition hypothesis postulates that when resources limited, infanticide is always the result of resource competition and the perpetrator will kill unrelated offspring in the vicinity in order to increase the available resources and the survival chances of his own offspring (Ebensperger 1998;Inoue et al. 2010). Competition for nesting sites is considerable as birds rely on them in order to breed (Newton 1994;Heinsohn et al. 2003;Ye et al. 2019). The Oriental Magpie Robin is a cavity-nesting species reliant on cavities for breeding and mainly feeds on insects and small vertebrates including young frogs. ...
Although a number of infanticide cases have been recorded among birds, since the incidence of this behavior is, overall, low and difficult to confirm, the importance of infanticide in the cause analysis of breeding failure is likely to be underestimated. During the breeding season of March to August 2019, we observed two cases of non-kin infanticide in Oriental Magpie Robins Copsychus saularis in nest boxes. The first incident occurred during the absence of the nesting male during the brooding period and in the sole presence of the female; another male entered the nest box and continuously attacked and pecked all four eight-day-old nestlings in the nest until they died. A second incident occurred in the presence of both parents, but during the feeding interval; a male perpetrator entered the nest box and pecked and pushed the single three-day-old nestling out of the box. The parents continued to carry food to the nest and tried to feed their nestling even after the latter had been killed. The parents were observed behaving anxiously once they detected no sign of activity from their nestling. We speculated that these examples of non-kin infanticide by conspecific Oriental Magpie Robin males could be attributed to competition for food resources or mates.
... Assemblages of arboreal vertebrates in savannas have also been shown to have a positive association with higher tree density (Tassicker et al. 2006;Veiga et al. 2013). Some studies have indicated that suitable den sites for arboreal vertebrates are limiting in savannas or there is strong competition for suitable hollows within taxa (Kurucz 2000;Aguirre et al. 2003;Heinsohn et al. 2003;Brazill-Boast et al. 2011). High-intensity fires can cause mortality of large trees and readily consume dead trees and hollow logs (Williams et al. 1999;Cook et al.2020). ...
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Tree hollows are a critical resource for many arboreal vertebrates, including many threatened species. In northern Australia’s vast tropical savannas, arboreal mammals are of particular conservation concern, as many have exhibited rapid population declines in recent decades. To understand the extent to which arboreal vertebrates compete for tree hollows in these savannas, we used motion-activated cameras to estimate rates of hollow visitation (i.e. individuals seen at the hollow entrance) and hollow use (i.e. individuals seen entering or exiting a hollow) by all vertebrate species at 29 hollows over a nine-month wet season period in the tropical savanna of Melville Island, Australia. We assessed the frequency of interactions between vertebrate species at the tree hollows. We recorded 21 vertebrate species visiting hollows, with larger hollows most frequently visited: 84% with entrance diameter > 10 cm. Larger-bodied mammal species were more likely to use larger hollows, in larger trees, at greater heights. However, smaller-bodies species also visited and used large hollows. The brush-tailed rabbit-rat (Conilurus penicillatus) used relatively shallow hollows close to the ground, and these hollows were most frequently visited by monitors (Varanus spp.) and snakes. This arboreal rodent has declined severely in recent decades, most likely due to predation by feral cats, and it is noteworthy that their hollow selection behaviour may be associated with increased predation risk. Our study highlights the importance of large tree hollows for a range of taxa in the tropical savannas of northern Australia and the potential impact of reductions in the density of large hollows on hollow-dependent species. Frequent, intense fires are a threat to large hollow-bearing trees, with appropriate fire management needed to conserve this resource and help to stem mammal declines.
... Ongoing anthropogenic disturbance has led to declining numbers of hollow-bearing trees in landscapes used for agriculture, timber harvesting, and urban development (Manning, Gibbons, Fischer, Oliver, & Lindenmayer, 2013). This represents a significant threat to hollow-dependent fauna, and can lead to localized population declines and ultimately the loss of biodiversity from impacted areas (Heinsohn, Murphy, & Legge, 2003;Lindenmayer et al., 2014;Strubbe & Matthysen, 2009). Management strategies are therefore required to increase localized availability of hollows, and to ensure that, where possible, mature hollow-bearing trees are retained (Le Roux et al., 2014). ...
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Artificial tree hollows (e.g., nest‐boxes) are commonly deployed to mitigate the loss of mature trees within human‐disturbed landscapes. Their effectiveness as a habitat resource, and thus conservation management tool, is strongly influenced by the suitability of internal microclimate conditions. In south‐eastern Australia, spout hollows are a nesting resource used by a diverse community of vertebrate species. We tested the suitability of a novel nest box design (spout boxes) that mimicked the physical characteristics of spout hollows. We monitored the occupancy (n = 193) and internal microclimate (n = 131) of natural hollows and spout boxes within a woodland where natural tree hollows were once abundant. Both natural hollows and spout boxes were occupied and used for breeding by birds and mammals. Natural hollows had consistently higher humidity, and thermal maxima and minima were buffered, when compared with spout boxes. These differences were largely explained by wall thickness. Spout boxes displayed even more extreme temperature variation and lower humidity when not shaded. While more extreme microclimate conditions did not prevent usage, tolerable thresholds for hollow‐dependent species may soon be exceeded under current climate change projections. Managers need to carefully consider nest box design and positioning to ensure the suitability of these supplementary resources for conservation purposes.
... It seems likely that this is due to two reasons. First, and similar to other large parrot species, nest-hollows are a highly limited resource (Heinsohn, Murphy, & Legge, 2003;Legge, Heinsohn, & Garnett, 2004;Olah, Vigo, Heinsohn, & Brightsmith, 2014) and the large majority of sulphur-crested cockatoos are non-breeders without an clearly observable bonded mate. Indeed in our urban population, nest-hollows are particularly limited (Davis, Major, & Taylor, 2013, 2014. ...
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Parrots are often referenced in discussions of social and cognitive complexity, yet relatively little is known of their social organisation in the wild. In particular, the presence of long-lasting social ties has been highlighted as a hallmark of social complexity, however the presence of such ties can be masked in fission-fusion systems like that exhibited by most parrot species. Social network analysis has the potential to elucidate such multi-level dynamics. While most parrot species are tropical canopy dwellers, a subset have successfully colonised urban habitats, where they are often the focus of much public interest. Our study takes advantage of this to use citizen-science to collect observations of wing-tagged sulphur-crested cockatoos in central Sydney and record their social associations over multiple years. Using a specifically designed mobile phone application "Wingtags", we collected over >27,000 citizen-science reports of wing-tagged cockatoos, and built social networks from spatial-temporal co-occurrences in observations for 130 tagged birds. To validate this novel methodology, we GPS-tagged a subset of wing-tagged birds and compared networks built from both data collection methods. We then examined correlates of social network structure before exploring the temporal dynamics of network structure and social associations. Social networks constructed from GPS data and citizen science data were highly correlated, suggesting that this novel methodology is robust. Network structure exhibited little seasonal variability and was largely driven by roost site choice; however, individuals also showed a surprising degree of mixing between roosts in their foraging associations. Finally, within this larger fission-fusion system, individuals tended to maintain specific social ties for long periods of time. There was an effect of age on these temporal dynamics, with aging individuals increasing both social stability and longevity of associations. Our findings highlight the utility of citizen-science to measure social networks in urban species, and add to the evidence that long-lasting social associations can persist in fission-fusion social systems such as those observed in wild sulphur-crested cockatoos.
... It resembles the situation with other cavity adopters, which are often limited by various physical characteristics of the cavities and their surroundings. This dependence is particularly known for cavity adopters among birds, which are known to reflect not only the number of available cavities (Newton, 1994;Gibbons and Lindenmayer, 2002;Heinsohn et al., 2003;Cockle et al., 2010) but also their quality (Cockle et al., 2008). The limited availability of cavities is characteristic particularly for human-altered habitats (Wiebe, 2011) whereas the cavities are not considered limiting nesting resources when being superabundant as are the cavities in mature forests (Wesołowski, 2007). ...
Observational studies have suggested that the presence of snail shells correlates with the presence of specialized adopters, particularly bees and wasps (Hymenoptera: Aculeata). However, should the empty shells be considered limiting resources once they are present in the respective habitat even if observational studies suggested low ratio of occupied relative to total shells? We performed a manipulative experiment, which consisted of the addition of marked snail shells of six terrestrial species that dominate the open and semi-open central European habitats to 21 sites with naturally present shells. We deployed the shells for the spring and summer 2017, allowed their inhabitants to undergo a diapause during the follow-up winter period and complete metamorphosis. The specialized bee and wasp species abundantly occupied the provided shells and the occupancy rates were several times higher in experimentally provided shells compared to the naturally present shells. These differences in occupancy rates were higher at anthropogenic compared to natural sites and at sites with more limited availability of naturally present shells. In contrast, the few sites at which the examined resource was superabundant, both the naturally present and the experimentally provided shells were occupied only to a limited extent. The species composition of assemblages that occupied the experimentally provided and naturally present shells were similar, and the differences in species composition between the natural and anthropogenic sites resembled those exhibited in the naturally present shells. In conclusion, we experimentally confirmed that empty snail shells serve as limiting resources for specialized bees and wasps even at sites where the naturally present shells are perceived as abundant.
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Knowledge of breeding success and its limiting factors is crucial in assessing species’ conservation needs. As cavity-nesters, parrots are particularly influenced by the availability of suitable cavities and low breeding output, whether due to natural processes or trapping. On the island of Sumba, Indonesia, the Critically Endangered Citron-crested Cockatoo (Cacatua citrinocristata) has the added problem of co-existing with an unusually rich hole-nesting bird community in a forested environment much constrained by habitat loss. We monitored 95 nesting cavities of cockatoos and their competitors and potential nest-predators, over one to four breeding seasons, using a combination of camera-traps, direct checks on nest contents, and observations from the ground. Competition for suitable cavities was intense among three large parrot species, two owls and a hornbill. Visitation rates by potential competitors were higher at unoccupied cavities than at those containing active nests, reflecting the guarding behaviour of the occupants. The Endangered Sumba Hornbill (Rhyticeros everetti) dominated observed direct confrontations and was the most frequent visitor to active parrot nests, suggesting a further role as a potential nest-predator. Cockatoos prospected many cavities but rarely then attempted to nest: instead the sites were usually occupied by other cavity-nesters, or by bees. At the few cavities where cockatoos did breed, predation pressure was likely low, and observed success rate high (10 successful of 15 nests), although the low number of nests found early in the breeding cycle suggests that some may have failed before detection. Intense competition for cavities suggests a shortage of suitable nest-sites, the need for preservation of old hole bearing trees and a role for nestboxes. Accessible, known, safe artificial nest-sites would also provide opportunities to assess the scale of nest-site shortage, allow camera placements to study productivity, exclude some competitors and predators and prevent illegal trapping. Especially given continued trapping pressure, the species would benefit from targeted local awareness-raising and law enforcement, with the whole endeavour backed up by longer-term forest restoration.
Conserving wide-ranging species is challenging and requires an understanding of resource selection at multiple spatial scales, including how functional resources interact. Understanding this interaction is likely to be particularly important if resources are rare, such as tree hollows, or likely to change into the future under climate change, such as water availability and distribution. We examined nest selection by Karak (Calyptorhynchus banksii naso), a threatened obligate tree hollow nester endemic to south-western Australia, globally one of the regions most affected by climate change. We aimed to identify factors influencing nest hollow selection at multiple spatial scales including interactions between functional resources. We found that nest selection occurred primarily at the hollow scale, with deeper, higher hollows selected, and at the landscape scale, with hollows closer to ephemeral and permanent drink sites selected. The preference for specific types of hollows indicated that suitable hollows are likely to be scarce in the landscape and that management prescriptions need to be developed to maintain the supply of suitable hollows. Maintenance of drink sites in an area experiencing significant declines in rainfall will require more novel management prescriptions, which could potentially include the provision of artificial drink sites. Overall, our study demonstrated the importance of understanding interactions between functional resources at large spatial scales for the effective conservation of wide-ranging species.
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Indirect transmission of pathogens can pose major risks to wildlife, yet the presence and persistence of wildlife pathogens in the environment has been little studied. Beak and feather disease virus (BFDV) is of global conservation concern: it can infect all members of the Psittaciformes, one of the most threatened bird orders, with infection often being lethal. Indirect transmission of BFDV through contaminated nest hollows has been proposed as a major infection source. However, data on whether and for how long nest sites in the wild remain contaminated have been absent. We determined the BFDV status of birds (parents and nestlings) for 82 nests of Crimson Rosellas, Platycercus elegans and Eastern Rosellas, Platycercus eximius . In 11 of these nests (13.4%, 95% confidence interval 6.9–22.7), we found an infected parent or nestling. Using nest swabs, we then compared BFDV presence at three points in time (before, during and after breeding) in three groups of nest boxes. These were nest boxes occupied by infected birds, and two control groups (nest boxes occupied by uninfected birds, and unoccupied nest boxes). Detection of BFDV on nest swabs was strongly associated with the infection status of parents in each nest box and with the timing of breeding. During breeding, boxes occupied by BFDV-positive birds were significantly more likely to have BFDV-positive nest swabs than boxes occupied by BFDV-negative birds; nest swabs tested BFDV-positive in 80% (28.4–99.5) of nests with parental antigen excretion, 66.7% (9.4–99.2) of nests occupied by parents with BFDV-positive cloacal swabs and 66.7% (22.3–95.7) of nests occupied by parents with BFDV–positive blood. 0% (0–52.2) of nests with BFDV–positive nestlings had BFDV–positive nest swabs. Across all boxes occupied by BFDV-positive birds (parents or nestlings), no nest swabs were BFDV–positive before breeding, 36.4% (95% CI 10.9–69.2) were positive during breeding and 9.1% (0.2–41.3) remained positive after breeding. BFDV was present on nest swabs for up to 3.7 months. Our study provides novel insights into the potential role of nest cavities and other fomites in indirect transmission of BFDV, and possibly other pathogens, and offers a non-invasive method for surveillance of pathogens in wild bird populations.
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More than 300 species of Australian native animals — mammals, birds, reptiles and amphibians — use tree hollows, but there has never been a complete inventory of them. Many of these species are threatened, or are in decline, because of land-use practices such as grazing, timber production and firewood collection. All forest management agencies in Australia attempt to reduce the impact of logging on hollow-dependent fauna, but the nature of our eucalypt forests presents a considerable challenge. In some cases, tree hollows suitable for vertebrate fauna may take up to 250 years to develop, which makes recruiting and perpetuating this resource very difficult within the typical cycle of human-induced disturbance regimes. Tree Hollows and Wildlife Conservation in Australia is the first comprehensive account of the hollow-dependent fauna of Australia and introduces a considerable amount of new data on this subject. It not only presents a review and analysis of the literature, but also provides practical approaches for land management.
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Data on nest hollows were collected from four study areas throughout the range of the short-billed form of the white-tailed black cockatoo, Calyptorhynchus baudinii latirostris, in south Western Australia. Hollows in trees are formed as a result of some destructive agent such as termites or fungi attacking the heartwood of the tree and breaking down the structure of the wood cells. The breaking off of part of the tree provides access to the hollow from the outside, and allows it to be used as a nest site. Throughout their breeding range, white-tailed black cockatoos will nest in any species of eucalypt which has a hollow of suitable size. The aspects of the entrances of hollows are not randomly distributed among compass groups, but the birds' selection of hollows was random. The aspect, depth to the floor and height of the entrance from the ground do not affect the success or failure of the nesting attempt. Female white-tailed black cockatoos searching for and preparing nest hollows chase female conspecifics from an area around their prospective nest tree. They continue this activity until they are incubating; this may result in suitable hollows not being accessible to other females. The rate of loss of hollows was 4.8 and 2.2% at two of the study areas. Hollows are being destroyed by all causes, particularly clearing for agriculture, faster than they are being created. Guidelines for the management of woodland must be drawn up so as to maintain a continuing supply of mature trees and protect hole-nesting species.
Galahs are the commonest of the cockatoos (Psittacidae: Cacatuinae), and one of the few native species that have thrived throughout European settlement in Australia. Previously confined to within flying distance of tree-lined water courses, galahs have now expanded into rangelands and have invaded the wheatlands where their consumption of grain and fouling activities made them pests. Following comments on the study area and methods of research, this monograph describes: the environment, food resources and pest status of this species; elements of behaviour; vocalizations; social behaviour; behaviour of the breeding pair; social organization and daily activity pattern; breeding biology (nest hollows, from eggs to hatching, and nestlings); and productivity and survival. Previous conflict between galah and man has virtually disappeared in Western Australia, and the main problem will be the need to plant or encourage in reserves and shelter belts trees capable of developing hollows when mature (an increasingly rare resource) in order to conserve this species. -P.J.Jarvis
This volume consist of eight main sections. Initially origins and evolutionary relationships are examined, followed by a brief section on the classification of the parrots. Next a section reviews the natural history of the parrots, briefly covering: general behaviour; distribution; habitat; movements; social behaviour; diet; breeding; and nocturnal species. Conservation status ics covered next. The main threats to parrots are then outlined and discussed: habitat loss; live bird trade; introduced species; persecution and hunting; and storms'climatic change. A brief section then looks at captive breeding. The mian body of the book is taken up with colour plates and a systematic section. The systematic section contains the following information, for each species: identification notes; voice; distribution and status (including distribution maps); ecology; description; sex/age; measurements; geographical variation; and references.
Predation was the major cause of total nest failure. Predation rates on nests in natural holes were higher than in nest-boxes for great tit Parus major and pied flycatcher Ficedula hypoleuca, while no significant difference was found for blue tit P. caeruleus and marsh tit P. palustris. In tits, woodpeckers were responsible for 17% of predation on nests in natural cavities but for 48% on nests in boxes. Of nests that were preyed upon, woodpeckers destroyed a lower proportion of those of great than of those of blue tit and marsh tit. Minimum nest entrance widths were correlated with the size of the species. Depths of nesting holes were generally similar for different species, but blue tit occupied shallower holes than did great tit. Starling Sturnus vulgaris, nuthatch Sitta europaea, and blue tit occupied holes higher up in the trees than did great tit and pied flycatcher. Marsh tits nested very low. Total rates of nest failure and predation were greater in low nests than in higher ones for starling, blue tit, and marsh tit. All 4 species that vary their nest heights in relation to density prefer to nest high. This indicates that there is competition for safe nest sites. Starling reduced the breeding success of nuthatch by taking over holes occupied by the latter. -from Author
Use of standing dead trees, or "snags," by birds was investigated in the Douglas-fir (Pseudotsuga menziesii) region of western Oregon in spring and summer, 1975 and 1976. Snags were examined in Douglas-fir forests approximately 10, 35, 75, 110, and 200+ years of age. Use of snags was quantified on the basis of evidence of past and present use by hole-nesting birds. On the average, hole-nesting birds used Douglas-fir snags over 60 cm in dbh and over 15 m tall for foraging and nesting; these snags usually had broken tops, few or no branches, decayed sapwood and heartwood, and less than 100% bark cover. Snags of this size and type occurred primarily in forests over 110 years of age; consequently, use of snags by hole-nesting birds was concentrated in older forests (>110 years old). Bird censuses were conducted in 1 representative area from each forest age-class. Density and species diversity of hole-nesting birds increased with forest age. Density of hole-nesting birds was positively correlated (r = 0.98, P < 0.05) with mean dbh of snags. Intensive management of Douglas-fir forests does not allow for the production or retention of large snags. A reduction in the number of large snags could reduce populations of hole-nesting birds. Possible means for retaining and producing large snags are discussed.
A fundamental factor regulating the numbers of secondary cavity nesting (SCN) birds is the number of extant cavities available for nesting. The number of available cavities may be thought of as being in an approximate equilibrium maintained by a very rough balance between recruitment and loss of cavities. Based on estimates of cavity recruitment and loss, we ascertained equilibrium cavity densities in a mature plains cottonwood (Populus sargentii) bottomland along the South Platte River in northeastern Colorado. Annual cavity recruitment, derived from density estimates of primary cavity nesting (PCN) birds and cavity excavation rates, was estimated to be 71-86 new cavities excavated/100 ha. Of 180 active cavities of 11 species of cavity-nesting birds found in 1985 and 1986, 83 were no longer usable by 1990, giving an average instantaneous rate of cavity loss of r = -0.230. From these values of cavity recruitment and cavity loss, equilibrium cavity density along the South Platte is 238-289 cavities/100 ha. This range of equilibrium cavity density is only slightly above the minimum of 205 cavities/100 ha required by SCN's and suggests that cavity availability may be limiting SCN densities along the South Platte River. We submit that snag management alone does not adequately address SCN habitat needs, and that cavity management, expressed in terms of cavity turnover and cavity densities, may be more useful.
The breeding behaviour of crimson rosellas (Platycercus elegans) in Canberra, Australia, was studied between 1993 and 1996. Female rosellas initiated breeding in late September or early October, laying a mean of 5.3 ± 0.1 (s.e.) eggs at 1–4-day intervals. Of all eggs laid, 50% fledged successfully. Rosellas had the highest breeding success in the wettest year (1995), when they bred earlier, laid larger clutches and fledged more young. Unexpectedly, breeding success was not lowest in the driest year (1994), although fewer birds attempted breeding and hatching success was low. In this study, poor environmental conditions for breeding were counterbalanced by decreased levels of conspecific interference through egg destruction. Overall, 55.8% of all clutches initiated were destroyed during laying, and more than half of this was attributed to rosellas. The reasons for egg destruction by rosellas were not clear. Boxes where clutches were destroyed were not quickly reoccupied and egg destruction was not highest when competition for nesting hollows was most intense. Clutch size and egg-laying intervals decreased over the breeding season, but the length of incubation did not. Large clutches did not produce more fledglings, because more eggs failed to hatch, especially early in the season. Eggs in a clutch hatched over a period of 0.5–7 days. Total hatching asynchrony increased over the breeding season and was not strongly correlated with clutch or brood size. This suggests that female rosellas initiated incubation at different times during laying. Clutches with longer hatching intervals took longer to incubate. If females in poor condition are inefficient incubators, female condition may affect the degree of hatching asynchrony.
The breeding biology of the Major Mitchell or pink cockatoo, Cacatua leadbeateri, was studied over six years at Yandegin on the margin of the Western Australian wheatbelt. Neither winter nor annual rainfall influenced the timing of egg-laying, which was spread over August and September. Most clutches (n = 63) were of three or four eggs, 84% of which hatched; 64% of nestlings fledged successfully and, on average, 1.27 independent young per pair were added to the population each autumn. Breeding pairs remained together throughout the year, spending the non-breeding season in a Local Nomadic Flock of 30-50 individuals. Pairs usually renested in the same area each season, never closer than 1 km to another pair (mean nearest neighbour 2732 m). Hollows used by C. leadbeateri were characteristically shallow (< 1 m) and high up, whereas other cockatoos tend to be less selective. Such specific nest hollow requirements led to regular reuse of scarce traditional sites and consequent easy 'farming' of nestlings for the aviary trade; specificity also rendered the species more susceptible to loss of rare nest sites by clearing than were other cockatoos. Although C. leadbeateri are long-lived (adult c. 88% survival) and productivity at Yandegin appeared adequate to maintain a stable population, the shrinking distribution in the face of expanding wheat farming is irreversible. Conservation of the species depends on the protection of large areas of the remaining suitable semiarid woodland and the continuing strict enforcement of the complete ban on harvesting wild birds for the aviary trade.