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Diversity and abundance of landbirds in spring reorientation flights in the Pelee region, Canada

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Introduction
The reorientation flight of landbirds
during migration (often termed “reverse
migration”) is a phenomenon that
involves birds flying, diurnally, in the
opposite direction of normal migration
in North America (Lewis 1939, Gunn
1951). Noted primarily in spring, reori-
entation flights also occur in varying
intensity in the fall in the Atlantic mar-
itime provinces of Canada (Richardson
1982, McLaren et al. 2000), at Cape
May, New Jersey (Weidner et al. 1992,
Van Doren et al. 2015), and in
Fennoscandia (Alerstam 1978, Åkesson
1999). Reorientation flights have seldom
been studied in the Great Lakes region
and have only been documented there in
the spring (Lewis 1939, Gunn 1951).
Lewis (1939) made perhaps the earliest
observations about reorientation flights.
He made the observation that species
which were common during spring
reorientation flights in the Pelee region
seem to become increasingly uncommon
or absent in days following intense reori-
entation flights. Gunn (1951) conduct-
ed an observational study and reported
that reorientation flights occurred
between one and four hours after sun-
rise, were most intense in May and were
mainly comprised of blackbirds (Icteri-
dae), wood warblers (Parulidae) and pip-
its (Motacillidae).
This paper describes the species com-
position and abundances associated with
reorientation flights in the Pelee region
of southwestern Ontario. We conducted
daily visual observations to identify and
count landbird species engaged in spring
reorientation flights and estimate their
abundance at Point Pelee National Park
and Fish Point Provincial Nature Reserve
on Pelee Island (Figure 1). Our study
had three main objectives. First, we
wanted to document the composition
and abundance of species that partici-
pated in spring reorientation flights and
determine their relative abundances. Sec-
ond, because of population declines
70 Ontario Birds Decembe r 2015
Diversity and abundance of
landbirds in spring reorientation
flights in the Pelee region, Canada
Kenneth G.D. Burrell, Stephen D. Murphy and Bradley C. Fedy
noted among Neotropical species (Sauer
et al. 2014), we wanted to know if there
were significant differences in composi-
tion and abundance of Neo tropical-
wintering versus temperate-wintering
migrant species. Finally, we wanted to
compare differences in composition and
abundance of species between the main-
land and island study sites.
Methods
Data Collection
We developed a standardized fixed point
survey, similar to that employed by the
Cape May Bird Observatory’s ‘Morning
Flight’ program (New Jersey Audubon
2014), and to the Thunder Cape Bird
Observatory’s migration monitoring
protocol (Wojnowski et al. 2010). Daily
Volu me 33 Number 3 71
Figure 1. The Pelee region, showing the locations of both study sites (Fish Point Provincial Nature Reserve
and Point Pelee National Park).
obser vations were conducted by two
trained observers between 26 April and
20 May, 2010-2012 at the southern tip
of Fish Point Provincial Nature Reserve
(41.4° N, 82.4° W) on Pelee Island and
in 2012 at the southern tip of Point Pelee
National Park (41.5° N, 82.3° W). The
timing of observations (late April to
May) corresponded with peak spring
abundances of migrating landbirds.
Surveys were conducted during the
first three hours following local sunrise
at both locations. Birds flying in a per-
sistent southerly direction out of sight
over Lake Erie were recorded as partici-
pating in reorientation flights. Identifi-
cation and counting occurred while birds
were in flight. Using binoculars, we iden-
tified birds to species whenever possible;
otherwise birds were assigned an identi-
fication as close to species level as possi-
ble (e.g., black bird species). Where nec-
essary and possible, some birds were
photo graphed to aid in identification;
however, identification was greatly aided
by call notes, as well as by birds landing
before continuing south. Only landbirds
were counted, as these species have been
shown to commonly participate in reori-
entation flights (Lewis 1939, Gunn
1951). One family (swallows) was ex clu -
ded, as foraging extends over large areas
(Kerlinger 1995, Faaborg 2002), making
it difficult to differentiate between for-
aging birds and those engaging in reori-
entation flights and to accurately record
numbers.
Data Analysis
Species were identified as Neotropical-
wintering or temperate-wintering mig-
rants based on Sibley (2000) and Dunn
and Alderfer(2011).We compared abun -
dance and species composition between
the two study sites and among years at
Fish Point. Differences in daily counts
were tested for significance using a
Wilcoxon rank sum test (Crawley 2013).
72 Ontario Birds Decembe r 2015
Figure 2. A male Baltimore Oriole engaged
in a reorientation flight; this species is one
of the most conspicuous participants to
spring reorientation flights (n = 2783).
Photo: Brandon R. Holden. May 2011,
Fish Point Provincial Nature Reserve.
Results
The number of days of observation in
2010, 2011 and 2012 at Fish Point was
24, 24, 25 respectively, and 24 in 2012 at
Point Pelee. Eighty species, totalling
61,677 individuals, were recorded partic-
ipating in spring reorientation flights. Of
these individuals, 38,337 were identified
to species and 23,340 were identified to
family level only. During our three hour
early morning observation periods, very
few birds were observed flying to the
north, presumably because most north-
bound spring migrants engage in noctur-
nal migration, whereas reorienting birds
fly south diurnally.
Blackbirds (9 species) and wood war-
blers (27 species) were the most common
participants (
n
= 42,686 and 10,842,
respectively) (Table 1; Figure 2), account-
ing for 87% of all reorienting migrants.
Woodpeckers (Figure 3) and pipits were
comparatively scarce, with just 58 and
136 individuals noted (0.09 and 0.22%
of all observed migrants, respectively).
The remaining species and numbers are
listed in Table 1. Thrushes (
Catharus
spp.)
were absent in all surveys, while tyrant fly-
catchers (Figure 4), vireos and sparrows
were observed in relatively low numbers.
These results were surprising based on the
number of observations of the species at
these locations (K. Burrell pers. obs.).
Volu me 33 Number 3 73
Figure 3. Red-headed Woodpeckers
were observed infrequently during
spring reorientation flights (n = 44).
Photo: Brandon R. Holden. May
2011, Fish Point Provincial
Nature Reserve.
Neotropical wintering migrants
species (
n
= 42) represented just over half
of all species (
n
= 80) participating. How-
ever, individuals of temperate-wintering
migrant species outnumbered individuals
of Neotropical species almost 4:1, largely
as a result of the high number of black-
birds. There was a difference in individu-
als of the two groups between study sites;
at Fish Point, Neotropical wintering
migrant species comprised approximately
12.6% (2011 and 2012) and 9% (2010)
of the tally of birds observed reorienting
per year, compared to only 7.2% of the
total at Point Pelee in 2012. Certain
Neotropical wintering species also
engaged in high abundance during reori-
entation flights, including Nashville (
n
=
831) and Yellow warblers (
n
= 581), as
well as Indigo Bunting (
n
= 788), all of
which are common breeding species in
Ontario (Table 1; Cadman
et al
. 2007).
The number of reorienting birds var-
ied across study sites and years (Table 1).
The highest annual total was recorded at
Fish Point in 2011 (
n
= 20,828) and the
lowest annual total count was in 2012 at
Fish Point (
n
= 10,768). The mean daily
count did not vary significantly between
the two study sites in 2012; at Fish Point
it was 675 and at Point Pelee it was 517
(Wilcoxon rank sum test, P= 0.790).
While there was not a substantial amount
of variation between study sites, there was
considerable variation among the mean
daily count among the three study years
at Fish Point, where the mean daily count
was 736 in 2010, 906 in 2011, and 431 in
2012. There was a significant difference
in pairs of study years at Fish Point, with
2010 and 2012, and 2011 and 2012
being significantly different (Wilcoxon
rank sum test, P=0.001); 2010 and 2011
were not significantly different, P=0.776).
74 Ontario Birds Decembe r 2015
Figure 4. Eastern Kingbirds were noted
to participate in spring reorientation
flights (n = 282).
Photo: Brandon R. Holden, May 2011,
Fish Point Provincial Nature Reserve.
Volu me 33 Number 3 75
Common name Latin name 2010 Fish 2011 Fish 2012 Fish 2012 Point Total
Point Total Point Total Point Total Pelee Total Individuals
Rock Pigeon Columba livia 00 01 1
Mourning Dove Zenaida macroura 79 426 46
Columbidae 47
Ruby-throated
Hummingbird* Archilochus colubris 61 35 20 16 132
Red-headed Woodpecker1Melanerpes
erythrocephalus 17 11 79 44
Red-bellied Woodpecker Melanerpes carolinus 30 60 9
Northern Flicker Colaptes auratus 21 02 5
Picidae 58
Eastern Wood-Pewee* Contopus virens 11 20 4
Least Flycatcher* Empidonax minimus 01 00 1
Eastern Phoebe Sayornis phoebe 20 01 3
Great Crested Flycatcher* Myiarchus crinitus 04 40 8
Eastern Kingbird* Tyrannus tyrannus 107 156 13 6 282
Flycatcher spp. Tyrannidae spp. 20 00 2
Table 1. Total number of observed reorientation migrants throughout the study (2010-2012).
Species are in taxonomic order following American Ornithologist Union (1998). Totals are delineated
by species, study site (Fish Point, Pelee Island, ON; and Point Pelee National Park, ON) and year;
1denotes a species at risk; 2 denotes a vagrant bird species; and * denotes a Neotropical migrant.
Bird families with more than one representative have been identified by their family name and subtotals provided,
e.g. Columbidae. Bird families with only a single representative are separated with a blank space below their
names, e.g. Ruby-throated Hummingbird.
Figure 5. Scarlet Tanagers were observed to
participate in spring reorientation flights less
commonly than previously thought (n = 111).
Photo: Brandon R. Holden. May 2012.
Point Pelee National Park.
76 Ontario Birds Decembe r 2015
Common name Latin name 2010 Fish 2011 Fish 2012 Fish 2012 Point Total
Point Total Point Total Point Total Pelee Total Individuals
Tyrannidae 300
Yellow-throated Vireo* Vireo flavifrons 15 1 0 7
Blue-headed Vireo Vireo solitarius 08 0 0 8
Warbling Vireo* Vireo gilvus 20 21 24 3 68
Philadelphia Vireo* Vireo philadelphicus 09 2 0 11
Red-eyed Vireo* Vireo olivaceus 2 16 00 18
Vireo spp. Vireo spp. 22 28 04 54
Vireonidae 166
Blue Jay Cyanocitta cristata 349 220 101 439 1109
American Crow Corvus brachyrhynchos 20 0 0 2
Crow spp. Corvus spp. 00 0 2 2
Corvidae 1113
Horned Lark Eremophila alpestris 04 1 0 5
Blue-gray Gnatcatcher Polioptila caerulea 31 8 10 36 85
Ruby-crowned Kinglet Regulus calendula 05 0 0 5
Eastern Bluebird Sialis sialis 03 2 0 5
American Robin Turdus migratorius 147 151 52 215 565
Turdidae 570
Gray Catbird Dumetella carolinensis 00 1 0 1
European Starling Sturnus vulgaris 502 362 238 581 1683
American Pipit Anthus rubescens 83 11 16 26 136
Cedar Waxwing Bombycilla cedrorum 33 116 128 482 759
Ovenbird* Seiurus aurocapillus 02 0 0 2
Northern Waterthrush* Parkesia noveboracensis 00 1 0 1
Golden-winged Warbler1*Vermivora chrysoptera 00 1 0 1
Blue-winged Warbler* Vermivora cyanoptera 01 4 1 6
Black-and-white Warbler* Mniotilta varia 0 25 00 25
Prothonotary Warbler1*Protonotaria citrea 11 1 1 4
Volu me 33 Number 3 77
Common name Latin name 2010 Fish 2011 Fish 2012 Fish 2012 Point Total
Point Total Point Total Point Total Pelee Total Individuals
Tennessee Warbler* Oreothlypis peregrina 1 11 6018
Orange-crowned Warbler* Oreothlypis celata 02 1 0 3
Nashville Warbler* Oreothlypis ruficapilla 58 626 119 28 831
Hooded Warbler1*Setophaga citrina 01 0 0 1
American Redstart* Setophaga ruticilla 0 53 3056
Kirtland's Warbler1 2*Setophaga kirtlandii 01 0 0 1
Cape May Warbler* Setophaga tigrina 3 11 6020
Northern Parula* Setophaga americana 0 28 0028
Magnolia Warbler* Setophaga magnolia 0 286 12289
Bay-breasted Warbler* Setophaga castanea 0 32 0032
Blackburnian Warbler* Setophaga fusca 3 68 3276
Yellow Warbler* Setophaga petechia 153 129 166 133 581
Chestnut-sided Warbler* Setophaga
pensylvanica 2 136 10139
Blackpoll Warbler* Setophaga striata 08 0 0 8
Black-throated Setophaga
Blue Warbler* caerulescens 0 33 0033
Palm Warbler Setophaga palmarum 11 268 56 16 351
Pine Warbler Setophaga pinus 01 3 1 5
Yellow-rumped Warbler Setophaga coronata 236 404 1618 19 2277
Black-throated
Green Warbler* Setophaga virens 11 44 11 0 66
Canada Warbler1*Cardellina canadensis 06 0 0 6
Wilson's Warbler* Cardellina pusilla 07 0 0 7
Warbler spp. Parulidae spp. 148 4277 1378 172 5975
Parulidae 10842
Chipping Sparrow Spizella passerina 40 3 19 1 63
Clay-colored Sparrow Spizella pallida 01 0 0 1
Field Sparrow Spizella pusilla 12 1 0 4
Lark Sparrow2Chondestes grammacus 10 0 0 1
Savannah Sparrow Passerculus
sandwichensis 05 0 0 5
Sparrow spp. Emberizidae spp. 47 12 91 2 152
Emberizidae 226
Summer Tanager2*Piranga rubra 12 0 0 3
Scarlet Tanager* Piranga olivacea 10 101 00111
Northern Cardinal Cardinalis cardinalis 00 1 8 9
78 Ontario Birds Decembe r 2015
Common name Latin name 2010 Fish 2011 Fish 2012 Fish 2012 Point Total
Point Total Point Total Point Total Pelee Total Individuals
Rose-breasted Grosbeak* Pheuticus ludovicianus 1 24 13 5 43
Blue Grosbeak2*Passerina caerulea 00011
Indigo Bunting* Passerina cyanea 255 188 228 117 788
Dickcissel2*Spiza americana 14106
Cardinalidae 961
Bobolink1*Dolichonyx oryzivorus 42 126 33 40 241
Red-winged Blackbird Agelaius phoeniceus 2553 2498 3584 6398 15033
Eastern Meadowlark1Sturnella magna 11002
Yellow-headed Blackbird2Xanthocephalus
xanthocephalus 00011
Rusty Blackbird1Euphagus carolinus 230813
Common Grackle Quiscalus quiscula 949 1400 1574 2288 6211
Brown-headed Cowbird Molothrus ater 388 831 287 143 1649
Orchard Oriole* Icterus spurious 58 24 116 68 266
Baltimore Oriole* Icterus galbula 1014 634 644 491 2783
Meadowlark spp. Sturnella spp. 00011
Blackbird spp. Icteridae spp. 9718 6553 0 215 16486
Icteridae 42686
House Finch Haemorhous mexicanus 0 12 5017
Purple Finch Haemorhous purpureus 01001
Pine Siskin Spinus pinus 0 21 0021
American Goldfinch Spinus tristis 442 188 160 401 1191
Fringillidae 1230
House Sparrow Passer domesticus 2200 4
Small Bird spp. Passeriformes spp. 122 546 00668
Neotropical
(N=42 species) 1614 2614 1359 888 6475
Temperate
(N=38 species) 16055 18214 9409 11524 55202
Total 17669 20828 10768 12412 61677
Discussion
While species richness was high, several
species and families were conspicuously
absent from reorientation flights.
Cath -
arus
thrushes were completely absent,
despite being relatively abundant mig rants
at Fish Point and Point Pelee during all
study years (K. Burrell, pers. obs.). Weid-
ner
et al.
(1992) also found that
Catharus
thrushes rarely participated in diurnal
reorientation flights, accounting for
0.01% of all identified Neotropical
migrants (among a sample size of 24,378).
Catharus
thrushes are largely nocturnal
migrants (Mack and Yong 2000, Lowther
et al.
2001, Rimmer
et al.
2001), and our
results confirm they essentially do not par-
ticipate in diurnal reorientation flights.
Several other species were also ob -
served in lower numbers than expected
based on the senior author’s previous
experience with spring migration and
reorientation flights in the Pelee region
(K. Burrell, pers. obs.). Fewer than expect-
ed Rose-breasted Grosbeaks (
Pheucticus
ludovicianus
;
n
= 43), Scarlet Tanagers
(
Pir anga olivacea
;
n
= 111) (Figure 5),
vireos (
n
= 166), sparrows (
n
= 226), and
tyrant flycatchers (
n
= 300) were noted.
Similar to
Catharus
thrushes, these species
and families are all noted to be primarily
nocturnal migrants (Lanyon 1997, Mid-
dleton 1998, Mowbray 1999, Cimprich
et al.
2000, Wyatt and Francis 2002) and
common in Ontario (Cadman
et al.
2007). It is possible that larger landbird
species which flock, such as blackbirds,
may be better adapted for diurnal migra-
tion and in particular diurnal spring reori-
entation flights than other birds. Birds
that flock are generally better adapted for
identifying predators and alerting other
birds to their presence (Thompson
et al.
1974, Lazarus 1979, Cresswell 1994).
Involvement in spring reorientation
flights through the Pelee region of fami-
lies and species from different wintering
areas varied. Although we observed more
individuals of temperate-wintering species
than Neotropical-wintering species, num-
ber of species was similar between the two
groups (Table 1). Wood warblers and car-
dinals and allies were the most abundant
Neotropical-wintering migrants during
spring reorientation flights, while black-
birds were the most abundant temperate-
wintering migrants. Based on their flight
ecology, nocturnal migrants, such as wood
warblers, are expected to be less prone to
engage in diurnal flight events in com-
parison to diurnal migrants, such as black-
birds (Van Doren
et al.
2015). Our results
confirmed this, as we found that the high-
est number of reorienting birds was black-
birds. However, wood warblers still
accounted for 17.5% of all observed mig -
rants (
n
= 10,842); supporting the results
of Wiedner
et al.
(1992) that wood war-
blers engage frequently in this migration
phenomenon, despite the general tenden-
cy of nocturnal migrants to be less prone
to engaging in reorientation.
Distinct differences were noted
between Fish Point and Point Pelee during
surveys in 2012. Temperate-wintering
migrants outnumbered Neotropical-win-
tering migrants by a substantial margin at
Point Pelee, while the opposite was true at
Fish Point. Additionally, as Point Pelee has
a larger amount of immediately available
vegetative cover in comparison to Fish
Point and Pelee Island, our results suggest
Volu me 33 Number 3 79
that increased vegetative land cover may
result in increased number of birds in the
study site, thus increasing density among
migrants and increasing the likelihood for
increases in the number of migrants to be
counted. In particular, the larger amount
of wetlands at Point Pelee may help
account for the relatively high abundance
of blackbirds. Water crossing is also a dif-
ference that is likely to affect responses
between study sites. Point Pelee is on the
Ontario mainland 45km from the US
mainland while Fish Point is on Pelee
Island, 21km and 24km from the US and
Canadian mainland shorelines, respec-
tively.
While spring reorientation flights are a
regularly observed phenomenon, the
implications and repercussions of these
flights are not clearly understood. It is pos-
sible that birds engaging in this form of
flight do so to take advantage of propitious
weather to the south because of inclement
weather. Impacts associated with migra-
tion delays may have negative impacts on
the life-cycles of birds most readily seen
through delays reaching suitable territories
and/or engaging in breeding opportuni-
ties. Monitoring programs (e.g., the Cana-
dian Migration Monitoring Network) and
short-term studies such as ours allow
researchers and conservationists the abili-
ty to monitor migratory bird populations
unobtrusively. The study of spring reori-
entation flights warrants more research to
determine their relationship with weather
events, potential differences in life-cycle
impacts of migration delays among long-
and short-distance migrant groups and to
determine how far reorienting birds trav-
el in the opposing direction before resum-
ing normal migration orientation.
Acknowledgments
We thank the Pelee Island Bird Observa-
tory for logistical support in 2010 and
2011 and J. Vandermuelen for extensive
field work in 2012. We also thank Parks
Canada and Ontario Parks for permission
to conduct research on-site.
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Volu me 33 Number 3 81
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Article
Full-text available
Many passerines that typically migrate at night also engage in migratory flights just after sunrise. These widely observed ‘‘morning flights’’ often involve birds flying in directions other than those aimed toward their ultimate destinations, especially in coastal areas. Morning flights have received little formal investigation, and their study may improve our understanding of how birds orient themselves during and after nocturnal movements and how they use stopover habitat. We studied autumn morning flights in the northeastern United States to identify associations between the number of birds undertaking morning flights and the magnitude of nocturnal migratory movements, nocturnal winds, and local topography. Our analyses included observations of more than 15,000 passerines at 7 locations. We found positive relationships between morning flight size and nocturnal migration density and winds aloft: Significantly more birds flew following larger nocturnal movements, quantified from weather surveillance radar and recordings of nocturnal flight calls, and after stronger nocturnal crosswinds. We also found consistent differences in morning flight size and direction among sites. These patterns are consistent with migrants engaging in morning flight as a corrective measure following displacement by nocturnal winds and to search for suitable stopover habitat.
Article
(1) In this paper we describe a simulation model of feeding behaviour of birds in flocks. The model is built using a variety of hypotheses for which some evidence can be found in the literature. The model, which is primarily concerned with exploring the way in which flocking effects feeding success of individuals, contains three major behavioural components: (i) movements--both individual hopping movements and integrated flock flights; (ii) prey preference resulting from experience (to account for phenomena such as `searching image' formation); and (iii) social learning: the ability of one individual to learn the characteristics of a prey type by observing captures made by nearby individuals. (2) These have all been measured in the laboratory or field for one species or another of bird and we guessed at reasonable parameter values by looking at data in the literature. (3) Initially we tested the model by looking to see if we could produce a simulated flock that behaved in a fairly realistic fashion. The model flocks seemed to show movement patterns approximating to those of real flocks, which suggests that there are no major behavioural components about which we are unaware. We carried out a sensitivity analysis on the model to find out the effect of varying one input parameter at a time by a small amount, and measuring the effect of these variations on the rate of capture of a bird and the probability of a bird making no captures in 20 min. (= `risk'). (4) Both feeding rate and risk were sensitive to `giving up time' and capture rate was also sensitive to prey detectability. This means that our subsequent conclusions on the feeding efficiency of individuals in flocks of different sizes may only apply to the particular values of prey detectability and giving up time that we put into the model. (5) We used the model to carry out two `experiments'. In the first we measured the capture rate and feeding risk of birds which showed flocking behaviour and in birds which were not allowed to flock (the parameters involved in flocking were set to zero). The birds were allowed to forage for 20 simulated minutes in each experiment, and the experiments were carried out with various food distributions. The results showed that (a) there was no difference in the capture rate of flocking and non-flocking birds, (b) non-flocking birds had a significantly higher risk of doing badly, and (c) both capture rate and risk increased for flocking and non-flocking birds with clumping of the food. (7) In the second experiment we investigated the relationship between flock size, clumpedness of the food and feeding success. In this experiment flock size was fixed by putting birds into flocks of various sizes from one to sixteen. Again we measured both risk and capture rate. (8) The results showed that (a) risk decreases with increasing flock size for a wide range of degrees of food clumping (risk decreases appreciably up to flock sizes of about ten to twelve) and (b) on the whole small flocks (less than five) do best in terms of capture rate, but the `best' flock size in these terms varies with degree of clumping of the food and with the size of clumps. These results suggest that the `best' flock size in terms of risk is not the same as the best flock for maximizing feeding success. We suggest that for small birds, minimizing risk might be more important than maximizing success. (9) In conclusion, the model suggests the following points (a) that we can reasonably well describe the feeding and movement patterns of a flock on the basis of current knowledge, (b) that minimizing risk is an important consequence of flocking and (c) that several features of the degree of clumping of food influence feeding success.
Article
Anti-predator behaviour in overwintering redshanks on an estuary in south-east Scotland was studied in the context of a very high mortality rate due almost entirely to predation by raptors. Attacks on redshank flocks of different sizes and by different species of raptor were observed frequently. Flocking reduced an individual redshank's probability of being killed by sparrowhawks, Accipiter nisus, and peregrines, Falco peregrinus. Larger flocks were preferentially attacked, but an attack was significantly more likely to succeed on a smaller flock. Within a larger flock a redshank was less at risk through the 'dilution' effect, vigilance effects (which were a direct consequence of flock size) and probably also the 'confusion' effect. A large redshank flock was less likely to fly immediately than a small flock on appearance of a sparrowhawk. Redshanks did not gain any foraging benefits within larger flocks; the number of swallows per unit time remained approximately constant while the number of unsuccessful picks at the ground increased with flock size. Reduced individual risk of predation appeared to be the main reason for flocking.
Article
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Article
During 4 seasons of study, small numbers of birds flew overland to the NNE-E, counter to the main SW migration, intermittently throughout the autumn (31 July – 16 November). Radars also detected overwater reverse migration (RM) from New England to Nova Scotia and from Nova Scotia toward Newfoundland. RM occurred at all hours of the day and night, especially when few birds were migrating SW. Most but not all cases of RM occurred with following SW winds. RM was more common with cloud and/or poor visibility than in fair weather, but was not restricted to cloudy occasions. Mean tracks were correlated with wind direction, but were not consistently downwind. Tracks tended to be closer to downwind in early than in late autumn, and on clear than on overcast nights. Dispersion in tracks was not discernibly related to weather variables, time of day or night, or magnetic disturbance. Eleven hypotheses concerning the reasons for reverse migration in autumn are evaluated; cases of RM recorded in this study are attributable to at least three of these hypotheses (late summer dispersal, hurricanes, dawn reorientation toward coast) and possibly to several others.
Article
Two tests are described, using the weaverbird quelea, of the proposed early warning function of flocking in birds (i.e. larger flocks detect a predator sooner). In experiment 1 flocks detected a goshawk flying over their cage with a greater probability than single birds. In experiment 2 the probability of detection of a brief, artificial, alarm stimulus increased with flock size in the same way, but at a lower absolute level, as that predicted by a simple model of detection. Possible reasons for the discrepancy (non-independence of detection probabilities; flock size influences each individual's probability of detection or response threshold) are evaluated. As flock size increased the type of response elicited changed from taking wing to flight intention movements to orienting responses, a trend which can be understood functionally in terms of the reduced risk of capture in larger flocks once the flock has been detected by a predator.