Protective association and breeding advantages of choughs nesting in lesser kestrel colonies
ABSTRACT Some bird species may breed close to aggressive predators to reduce predation risk by more dangerous, generalist predators. We tested this protective nesting association hypothesis by studying solitary choughs,Pyrrhocorax pyrrhocoraxbreeding within or outside lesser kestrel,Falco naumannicolonies in northern Spain (1993–1994). We found 27 potential predators of choughs and kestrels and confirmed predation on adults and nests of both species by at least eight common predators. We experimentally assessed the defence investment of choughs and kestrels towards a stuffed eagle owl,Bubo buboin 19 buildings shared by both species. Lesser kestrels were more efficient at detecting the predator, and defended more vigorously than choughs. Choughs clearly selected for breeding buildings where lesser kestrel colonies were installed. Breeding success of these choughs was much higher than that of choughs breeding alone, because of a lower percentage of nest failure. Benefits to choughs probably accrued from both the ‘parasitism’ of the kestrels' nest defence and the dilution of predation risk in the colonies. Since lesser kestrels do not prey on choughs, this could be a good example of protective nesting association in birds.
- SourceAvailable from: Marcin Polak[Show abstract] [Hide abstract]
ABSTRACT: Nest predation is one of the most important factors limiting reproductive success, and antipredator behaviour can significantly reduce the loss of avian broods. I carried out field experiments on two sympatric passerines: the barred warbler and the red-backed shrike. Many authors have described the protective nature of nesting association between these species. However, we have little knowledge about the true nature of the relationships between associates. I examined (1) whether barred warblers and red-backed shrikes respond differently to an avian predator, and (2) whether males and females differ in the intensity of nest defence. Decoys of a known nest predator and a non-predatory control species were used to examine the types and relative intensity of parental response. I measured behavioural responsiveness by recording aggressive behaviour toward each model during the nestling period. Barred warblers and red-backed shrikes showed considerable variation in their response. Warblers more vigorously defended their own territories than shrikes. No differences between the sexes in antipredator behaviour in red-backed shrike were found. By contrast, in barred warbler, male was more involved in nest defence. The experimental tests provide evidence that these two species are able to differentiate between a predator and non-predator species.Journal of Ethology 01/2013; 31:1-7. · 1.00 Impact Factor
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ABSTRACT: Breeding close to top predators is a widespread reproductive strategy. Breeding animals may gain indirect benefits if proximity to top predators results in a reduction of predation due to suppression of mesopredators. We tested if passerine birds gain protection from mesopredators by nesting within territories of a top predator, the Ural owl (Strix uralensis). We placed nest boxes for pied flycatchers (Ficedula hypoleuca) in Ural owl nest sites and in control sites (currently unoccupied by owls). The nest boxes were designed so that nest predation risk could be altered (experimentally increased) after flycatcher settlement; we considered predation rate as a proxy of mesopredator abundance. Overall, we found higher nest predation rates in treatment than in control sites. Flycatcher laying date did not differ between sites, but smaller clutches were laid in treatment sites compared to controls, suggesting a response to perceived predation risk. Relative nest predation rate varied between years, being higher in owl nest sites in 2 years but similar in another; this variation might be indirectly influenced by vole abundance. Proximity to Ural owl nests might represent a risky habitat for passerines. High predation rates within owl territories could be because small mesopredators that do not directly threaten owl nests are attracted to owl nest sites. This could be explained if some mesopredators use owl territories to gain protection from their own predators, or if top predators and mesopredators independently seek similar habitats.Oecologia 04/2012; 170(2):507-15. · 3.01 Impact Factor
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ABSTRACT: Invasive species can take advantage of resources unexploited by natives (opportunism hypothesis) or they can exploit the same resources but more aggressively or efficiently (competition hypothesis), thus impacting native species. However, invasive species tend to exploit anthropogenic habitats that are inefficiently used by natives such as urban environments. Focusing on the ring-necked parakeet (Psittacula krameri), one of the most invasive birds worldwide, we combined observations of interspecific aggressions, species-specific cavity-nest preferences and the spatial distribution of the native cavity-nesting vertebrate community to determine the invasion process as well as its potential impacts on native species in a Mediterranean city. Our results support the competition hypothesis, suggesting that ring-necked parakeets are outcompeting native species sharing nest-site preferences. Parakeets initiated and won most interspecific aggressions, which were directed towards competitors but also towards predators. This behaviour could explain the spatial arrangement of natives, with most bird species breeding close to parakeets possibly to take advantage of their effective antipredatory behaviour. However, temporal and spatial patterns of segregation suggest that a threatened bat species is negatively affected by parakeets. This demonstrates that common species gain benefits and threatened ones (in this study, a bat and possibly a falcon) lose nest sites due to invaders. Therefore, the conservation status of the native species that pay the costs of competition with invaders should be considered. This scenario of winners and losers may, however, shift towards more losers if the ring-necked parakeet population continues to grow, thus requiring close monitoring and control/eradication programs to avoid further impacts.PLoS ONE 01/2014; 9(6):e100593. · 3.53 Impact Factor
Anim. Behav., 1997, 54, 335–342
Protective association and breeding advantages of choughs nesting in lesser
GUIL L ER MO BL A NCO* & J OSE u L . T EL L A †
*Departamento de Biologı ´ a Animal, Universidad de Alcala ´de Henares
†Estacio ´n Biolo ´gica de Don ˜ana (C.S.I.C.), Sevilla
(Received 10 J une 1996; initial acceptance 26 J uly 1996;
final acceptance 18 October 1996; MS. number: 5266)
Abstract. Some bird species may breed close to aggressive predators to reduce predation risk by more
dangerous, generalist predators. We tested this protective nesting association hypothesis by studying
solitary choughs, Pyrrhocorax pyrrhocorax, breeding within or outside lesser kestrel, Falco naumanni,
colonies in northern Spain (1993–1994). We found 27 potential predators of choughs and kestrels and
confirmed predation on adults and nests of both species by at least eight common predators. We
experimentally assessed the defence investment of choughs and kestrels towards a stuffed eagle owl,
Bubo bubo, in 19 buildings shared by both species. Lesser kestrels were more efficient at detecting the
predator, and defended morevigorously than choughs. Choughs clearly selected for breeding buildings
where lesser kestrel colonies were installed. Breeding success of these choughs was much higher than
that of choughs breeding alone, because of a lower percentage of nest failure. Benefits to choughs
probably accrued from both the ‘parasitism’ of the kestrels’ nest defence and the dilution of predation
risk in the colonies. Since lesser kestrels do not prey on choughs, this could be a good example of
protective nesting association in birds.
?1997 The Association for the Study of Animal Behaviour
Nest predation is a major factor limiting breeding
success in birds and, consequently, has played a
central role in the evolution of nesting strategies
(Martin 1992; Suhonen et al. 1994). Individuals
should adaptively choosenest sitesthat reducethe
risk of predation to young and adults; a variety of
relevant parental behaviourshaveevolved, includ-
ing those related to the construction and location
of nests, nest guarding and breeding associations
(Bulmer 1984).These strategies may
together in bird species that select nest sites in
association with more aggressive species to gain
breeding advantages (Collias & Collias 1984).
Nesting associations of birds with social and
aggressive insects are not rare in the tropics
(Hindwood 1959; McCrae & Walsh 1974) and
may protect birds from mammalian and reptilian
predators (Janzen 1969). Similarly, grebes, gulls,
ducks, terns, waders, pigeons and passerines
breeding within colonies of aggressive species or
close to raptor nests may benefit by the exclusion
generalist predators such as corvids (Veen 1977;
Go ¨tmark & Andersson 1980; Slagsvold 1980;
Wiklund 1982; Burger 1984; Paine et al. 1990;
Bogliani et al. 1992; Ueta 1994; Norrdahl et al.
1995). However, in most of the cited studies two
circumstances may obscure an ideal ‘protective
nesting association’: (1) gulls and raptors, that is,
the most usual putative protectors, also prey to
varying degrees on the eggs, chicks and even on
their adult associates, and (2) the associated
species may nest together because of their similar
breeding requirements or nest-site limitation
(Dona ´zar et al. 1996). Nesting birds associated
with colonial species could also dilute the pre-
dation risk, which is one of the major advantages
of coloniality (Wittenberger & Hunt 1985). The
balance between the cost of nesting close to a
potential predator and the benefits of decreased
predation by more dangerous predator species
probably determines whether the association lasts
(Y oung & Titman 1986; Go ¨tmark 1989; Norrdahl
et al. 1995).
Correspondence: G. Blanco, Departamento de Biologı ´ a
Animal, Universidad de Alcala ´ de Henares, 28871
email@example.com). J. L. Tella is at the Estacio ´n
Biolo ´gica de Don ˜ana (C.S.I.C.), Avda. MaLuisa s.n.,
41013 Sevilla, Spain.
? 1997 The Association for the Study of Animal Behaviour
The chough, Pyrrhocorax pyrrhocorax, is a
medium-sized (300–360g) insectivorous corvid
speciesthat nestsinsidecavitieson cliffsor similar
artefacts. In Spain, large populations of choughs
breed exclusively as solitary pairs inside aban-
doned buildings (Tella et al. 1993; Blanco et al.
1997), showing a non-random distribution of
nests that depends on the availability of buildings
(Blanco et al. 1997). These old buildings are
sometimes shared with lesser kestrels, Falco nau-
manni, a small falcon (130–170g) that breeds
colonially in the tiled roofs (Tella et al. 1993,
1996a). Lesser kestrels and choughs do not prey
on each other (Cramp 1980); nor do they compete
for nesting sites as they have different require-
ments (Tella et al. 1993).
Our aims in this study wereto determinewhich
species prey on breeding adults and nests of
choughs and lesser kestrels and then to test pre-
dictions arising from a ‘protective nesting associ-
ation hypothesis’ (Collias & Collias 1984), where
choughs would be the associates and lesser kes-
trels the protectors. If choughs and lesser kestrels
share the same predators, we should expect that,
in the presence of a common predator near a
shared nesting site, breeding choughs should
benefit from the defensive parental effort of
several pairs of colonial lesser kestrels. If this is
thecase, wepredict that (1) thedefensiveeffort of
lesser kestrels (i.e. the number and intensity of
attacks) should behigher than that of choughs, (2)
choughs would select buildings occupied by col-
onies of lesser kestrels and (3) choughs nesting
within lesser kestrel colonies would have a net
fitness benefit, that is, will breed moresuccessfully
than solitary choughs.
ME T H ODS
Study Area, Censuses and Nest-site Availability
The study was conducted in Los Monegros
(41?20?N, 0?11?W, northeastern Spain), an arid
plain mostly devoted to dry cereal crops. In this
area choughs and lesser kestrels breed exclusively
in abandoned farmhouses (Tella et al. 1993;
Blanco et al. 1997). During 1993 and 1994 we
censused nests and colonies of both species, by
systematic inspections of all buildings present in
about 225km2. We calculated the availability of
unoccupied nest sites by counting the buildings
suitablefor each species, taking into account their
respective nest-site requirements (i.e. the aban-
doned status of the building, and the presence of
tile roofs and holes or beams inside the buildings
wherelesser kestrelsand choughsplacetheir nests,
respectively; see Tella et al. 1993 for more details
on nest-site requirements).
We recorded the occurrence of predators of
both species from 1993 to 1995 during the course
of intensive observations of nests and colonies
with telescopes (Tella et al. 1996a; Forero et al.
1996; unpublished data). The observations were
made over more than 2000h distributed through-
out the day but intensively from 0800 to 1200
hours and from 1700 to 2100 hours. We did not
make intensive observations at night but oppor-
tunistically recorded nocturnal predatorswhen we
captured choughs and lesser kestrels at roost in
their nesting sites. We regarded as potential
predators those species that are known to catch
prey of similar or bigger size than lesser kestrel
and chough eggs, nestlings or adults, and that
were observed inside and around occupied build-
ings. In addition, we tried to determine the pred-
ator in each case of predation by marks, tracks
and faeces found on eggs, nests and prey remains.
We also searched for prey remains in nests, bur-
rows and perching sites of potential predators in
the study area from 1986 onwards (Tella 1991;
Tella & Blanco 1993; Tella & Man ˜osa 1993; Oro ´
& Tella 1995; Tella et al. 1996a; unpublished
data). We classified predators into five taxonomic
categories (reptiles, rodents, carnivorous mam-
mals, raptors i.e. diurnal and nocturnal birds of
prey, and corvids) to assess their potential to prey
on eggs, nestlings, fledglings and adults.
Nest Defence Experiments
We conducted an experiment using a stuffed
eagle owl, Bubo bubo, a natural predator of
choughs and lesser kestrels in the study area, to
assess the defensive behaviour of both species.
The experiment was conducted during the final
nestling and first flight stagesof lesser kestrelsand
choughs, respectively. At this time, the offspring
valuein terms of parental fitness is expected to be
high according to parental investment theory
(Andersson et al. 1980; Clutton-Brock 1991) and,
therefore, parental effort in nest defenceshould be
Animal Behaviour, 54, 2
high (Montgomerie & Weatherhead 1988). We
placed thecovered dummy on theground, in open
country offering full visibility, at about 20m from
19 different buildings simultaneously occupied by
lesser kestrels and choughs. After removing the
cover, we observed from a hide 100–150m away,
thus avoiding any disturbance. We recorded the
timeelapsed until a chough or kestrel approached
and alarm-called or attacked the dummy for the
first time and then watched for a further 10min
(see Tolonen & K orpima ¨ki 1995 for a similar
methodology). We recorded the number of indi-
viduals involved and the number and intensity of
attacksin each defencetrial. Thedefenceintensity
was classified into five categories which mainly
considered the distance to which each defender
dived at the owl: closest distance approached was
(1) more than 2m, (2) >1–2m, (3) 0.2–1m, (4)
<0.2mwithout physical contact with theowl, and
(5) physical contact with the owl.
During 1993 we collected breeding data for
choughs breeding outsideand within lesser kestrel
colonies. The visits to chough nests were distrib-
uted beforeand during thelaying season to deter-
mine laying date and complete clutch sizes. We
determined the laying date of nests found during
the laying period by backdating using a laying
interval of 1 day per egg (personal observation).
We revisited every nest monitored during the
nestling stage to assess the percentage of total
failures and the number of fledglings. We did not
record partial predation of clutches/broods. We
defined breeding success as the mean number of
fledglings/monitored nests, and fledging success as
the mean number of fledglings/successful nests
with at least one fledgling. Since breeding per-
formance could be influenced by parental quality
(Tella et al. 1994; unpublished data), we tried
to capture the maximum number of breeding
choughs before laying to measure their size and
body condition. Sex was determined through dis-
criminant functions (Tella & Torre 1993). Wing
length was used as a measureof body sizeand the
residualsfromtheregression of masson thecubed
wing length as an index of body condition, separ-
ately for each sex. Ageof birdswasdetermined on
the basis of plumage characteristics (Blanco et al.
1996) and by known dates of ringing in previous
years. All statistical comparisonsweremadeusing
Mann–Whitney U-tests or t-tests depending on
R E SUL T S
A total of 27 confirmed or potential predator
species for both choughs and lesser kestrels was
present in the study area. At least 11, 16, 17, and
7 predator species of eggs, nestlings, fledglings
and adults, respectively, were shared by choughs
and lesser kestrels, and predation on both species
wasconfirmed for eight predator species(TableI).
Reptiles and mammals dominated among the
predators on eggs of choughs and lesser kestrels,
corvids being the only birds involved. Similar
resultswerefound for thenestling stageexcept for
the predation by several raptor species. The
highest number of predator specieswasreached in
the fledging stage because of the increase in the
number of raptors preying on both species.
After the behavioural monitoring and regular
inspection of about 700 lesser kestrel nests
between 1993 and 1995, no remains or signs of
predation of choughs by kestrels were found.
Lesser kestrels detected the predator in all the
19 defence trials while choughs apparently did so
in only two trials (Fisher’s exact test: P<0.00001).
Choughs took longer to detect the predator than
lesser kestrels (Table II), and the number of
lesser kestrels involved was significantly higher
(Mann–Whitney U-test: U=5.23, N1=N2=19,
P<0.0001; Table II). Choughs attacked the
dummy directly in only one defence trial and the
frequency of dives was lower than the mean
frequency of attacks by lesser kestrels, which
attackedtheowl in18of 19trials(TableII). Inthe
other defence trial where choughs detected the
dummy, they only gave alarm calls. Furthermore,
the defence intensity of lesser kestrels was almost
twice that of the only pair of choughs that
attacked the predator (Table II).
Availability of Nest Sites and Breeding
Each pair of choughs bred solitarily in a differ-
ent building, except in two buildings where two
Blanco & Tella: Protective association of choughs
and three pairs of choughs bred close together.
Both cases coincided with lesser kestrel colonies.
Choughs bred in 33 of 36 and 36 of 39 buildings
with lesser kestrel colonies in 1993 and 1994,
respectively. There were, however, more than 150
unused but suitable buildings for both species in
each breeding season. Therefore, considering the
number of suitable nest sites used and unused
by each species and by both species (Fig. 1), we
found a clear selection by choughs for lesser
kestrel colonies in 1993 (?2
and 1994 (?2
correction in both cases).
1=94.90, P<0.00001; with Y ate’s
Benefits of Association with Lesser Kestrels
Choughs that associated with lesser kestrel col-
onies laid clutches of similar size and at similar
dates as non-associated pairs (Table III). How-
ever, breeding success was significantly higher for
choughs within lesser kestrel colonies because of
lower total nest failure. Predation was the main
causeof failure, being confirmed in at least 78% of
these cases. On average, choughs that associated
with lesser kestrels produced more than double
the fledglings per pair than did their non-
associated conspecifics. However, the number of
Table I. Number of potential and confirmed predator species on eggs, nestlings, fledglings and adult choughs (CH)
and lesser kestrels (LK )
Eggs Nestlings FledglingsAdults
CH LKBothCH LKBothCH LKBothCHLK Both
1711 11 1116 1616 14
In bold, confirmed predators of both species.
*Elaphe scalaris, Malpolon monspessulanus, Lacerta lepida.
†Rattus rattus, Elyomis quercinus.
‡Martes foina, Vulpes vulpes, Felis catus, Canis canis.
§Milvus milvus, Milvus migrans, Circaetus gallicus, Buteo buteo, Hieraaetus pennatus, Aquila chrysaetos, Neophron
percnopterus, Circus aeruginosus, Circus pygargus, Falco peregrinus, Falco tinnunculus, Tyto alba, Athene noctua,
**Pica pica, Corvus monedula, Corvus corone, Corvus corax.
Table II. Defence of parent choughs and lesser kestrels towards a model of a common predator
RangeN % of trials
Detection time (min)
Number of birds attracted
Animal Behaviour, 54, 2
fledglings produced per successful nest did not
differ between choughs associated and non-
associated with lesser kestrels (Table III). This
suggests that the results are not explained by
differencesin parental or territory quality between
the two groups. Furthermore, all parents were
adults (more than 3 years old), and no differences
werefound in sizeor body condition beforelaying
between associated and non-associated choughs
We found a clear selection by choughs for
nesting within lesser kestrel colonies. This associ-
ation cannot beexplained by similar choiceof nest
sites (Dona ´zar et al. 1996) since these species’
nest-site requirements do not overlap (Tella et al.
1993) and because there were plenty of suitable
but unoccupied nest sites. Neither do habitat
features appear to be a causal factor because the
high habitat homegeneity of thestudy area (>90%
occupied by dry cereal crops, Tella et al. 1996b)
precludes any difference in habitat and predator
assemblagecomposition between theforaging ter-
ritories of choughs that did and did not nest
within lesser kestrel colonies. The availability of
thethreemain biotopes used by foraging choughs
(unpublished data), assessed in circles of 3km
radius centred at 11 lesser kestrel colonies in 1994
(J. L. Tella, M. G. Forero, F. Hiraldo & J. A.
Dona ´zar, unpublished data), showed low coef-
ficients of variation between plots: cereal crops
(CV =13.9), ploughed fields (CV =13.9) and field
margins (CV =13.7). In thewholearea covered by
Percentage of buildings used by choughs
1994 1994 1993
Figure1. Percentageof suitablenest sitesused by nesting
choughs in relation to the presence of lesser kestrel
colonies. Numbers above the bars are sample sizes.
Table III. Breeding and parental data of choughs breeding within and outside lesser kestrel colonies
Total nest failure
Data are means? except for total nest failure which is expressed as a percentage.
‡Chi-square test with Y ate’s correction.
Blanco & Tella: Protective association of choughs
these plots, 30 and 24 pairs of choughs nested
respectively. There were many more chough pairs
than kestrel colonies, and most kestrel colonies
were occupied by choughs.
In the study area, lesser kestrels arrive from
Africa and settle in the colonies in late February,
that is, 1 month before choughs begin laying.
From arrival until laying (in early May), lesser
kestrels spend most of the time in the colonies
(Dona ´zar et al. 1992; Negro et al. 1992). There-
fore, choughs may choose to breed together with
lesser kestrels at thestart of each breeding season.
Furthermore, lesser kestrel colonies are usually in
the same buildings from year to year and so
choughs could maintain their nest sites from one
year to the next.
Protective Nesting and Breeding Advantages
Aggressive birds can deter predators and
increase nesting success by reducing losses to
predation (Martin 1992). Our results show that
lesser kestrels are effective in detecting and
attacking predators by means of communal
defence. The protection afforded by kestrels
from generalist predators on eggs and nestlings
of both species presumably explains the higher
breeding success of choughs breeding in lesser
kestrel colonies (instead of different parental or
territory qualities), in agreement with other
breeding associations with raptors (Wiklund
1982; Paine et al. 1990; Ueta 1994; Norrdahl
et al. 1995). However, 46% of the confirmed
predation events on eggs or nestling lesser kes-
trels (N=152 nests; J. L. Tella, J. A. Dona ´zar &
F. Hiraldo, unpublished data) were caused by
black rats, Rattus rattus, at night, thus prevent-
ing any communal defensive reaction. The same
data show that the percentage of predated nests
decreases as colony size increases. Thus one of
the main benefits to the chough seems to be in
diluting the risk of predation when nesting in the
same buildings as several pairs of lesser kestrels.
Additionally, after fledging, choughs may benefit
from the exclusion from territories and nest sites
of a large number of raptor species that prey
upon fledglings and adults. In fact, lesser kestrels
frequently chase raptors detected around the
colony (Cramp 1980; personal observation), so
choughs may be warned of their presence
(Wittenberger & Hunt 1985).
The fact that choughs scarcely responded to
the dummy could be an artefact of using a
stuffed rather than a live owl (Shalter 1978;
Gard et al. 1989), although the behaviour of the
lesser kestrels suggests otherwise. (Although,
even if the choughs had attacked the owl, they
would still have benefited from the greater
number of kestrels also attacking it.) We believe
this to be unlikely, however, as in more than
2000h of obervations from March to July 1993–
1994 in 16 lesser kestrel colonies (Negro et al.
1996; Tella et al. 1996a, 1997, in press; unpub-
lished data) where choughs also nested, we rarely
observed choughs attacking potential predators
(data not quantified). On the other hand, we
cannot discount the possibility that the different
life histories of these two species might explain
their differential defensive effort. Choughs could
give priority to their own survival instead of that
of their eggs and chicks, avoiding the risk of
attacking the predator, because it may improve
their overall reproductive fitness (i.e. the number
of young in future breeding attempts, Linde ´n &
Møller 1989), while the comparatively short-lived
lesser kestrels could risk attacking because of
their reproductive strategy whereby nestling pro-
duction is emphasized (Hiraldo et al. 1996).
Thus, these species could adjust their defensive
responses according to their expected fitness
costs and benefits (Montgomerie & Weatherhead
1988). Although demographic parameters of
lesser kestrels are well known (Hiraldo et al.
1996; unpublished data), very few are known for
choughs, so further studies are needed to test
this hypothesis. Alternatively, the widely dis-
persed nests of choughs in buildings, compared
with cliff-nesting choughs which usually nest at
higher densities (Blanco et al. 1997), could pre-
vent any communal defence and explain the low
defensive activity of choughs in the present
Theassociation reported herediffersfromother
breeding associations described. Since choughs
did not participatein nest defence, mutual benefits
(e.g. Wiklund 1982) are not expected. Choughs
‘parasitize’ nest defence provided by the raptor
(sensu Ueta 1994), but also probably benefit from
thedilution of predation risk. In contrast to other
birds that associatewith
(Wiklund 1982; Paine et al. 1990; Ueta 1994;
Norrdahl et al. 1995), choughs do not suffer
predation by the protector species. Thus, this
Animal Behaviour, 54, 2
could be a good example of a protective nesting
association in birds.
ACK NOWL E DGME NT S
Field work on lesser kestrels was carried out with
J. A. Dona ´zar, F. Hiraldo and M. G. Forero, and
breeding data of choughs were partially obtained
with C. Sa ´nchez and I. Torre. R. Lo ´pez, M.
Villarroel, I. Sa ´nchez and M. Pomarol also helped
in the field. We thank F. Hiraldo, J. A. Dona ´zar,
J. Potti and an anonymous referee for their valu-
able comments on the manuscript. Lesser kestrel
work was financed by the CICY T projects PB90-
1021 and PB93-0040. Both authorsbenefited from
FPIpredoctoral fellowships of the Spanish
Ministerio de Educacio ´n y Ciencia.
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