|Behav Ecol Sociobiol (1997) 40: 253 — 260 © Springer-Verlag 1997
Jose L. Tella Manuela G. Forero Jose A. Donazar
Juan J. Negro Fernando Hiraldo
Non-adaptive adoptions of nestlings in the colonial lesser kestrel:
proximate causes and fitness consequences
Received: 3 May 1996 / Accepted after revision: 19 January 1997
Abstract We examined the causes, costs and benefits of adoption in the altricial lesser kestrel Falco naumanni. Specifically, we tested the intergenerational conflict hy- pothesis, proposed to explain adoption in some birds. Adoptions involved 76% of the nests and 51% of the nestlings at a mean age of 25 days (12 days before fledging). Nest-switching nestlings were not in poorer body condition, more parasitized or younger than their siblings, and body condition and prey delivery rates of their parents did not differ from those of other parents. Tn the foster nest, adopted nestlings did not benefit from higher feeding rates or a prolongation of the nestling period. They did not have fewer nest-mates or achieve higher rank within the new brood. Thus, adopted nes- tlings did not improve their body condition and survival. Adult lesser kestrels seemed unable to finely discriminate beween their own and alien chicks. Foster parents bore the cost of an increase of prey delivery rates, although it did not affect their survival or subsequent reproductive performance. Therefore, our results do not support the intergenerational conflict hypothesis, and suggest that adoption in this species is non-adaptive. Traditionally, the lesser kestrel bred in cliffs where movement among nest-sites was restricted. Nowadays, about half of the colonies are in tiled roofs which facilitate nest-switching by nestlings. The high rate of adoptions may thus be explained as reproductive errors associated with the re- cent occupation of a new breeding habitat.
Key words Adoptions Tntergenerational conflict
Falco naumanni Offspring recognition Parental care
J.L. Tella ()) M.G. Forero J.A. Donazar J.J. Negro F. Hiraldo
Estacion Biologica de Donana (CSTC), Avda Ma Luisa s.n.,
E-41013 Sevilla, Spain
Fax: 34 95 4621125; e-mail: Tella@cica.es
The care of young by conspecifics other than their bio- logical parents (i.e. alloparental care) has been reported in a number of birds and mammals (Riedman 1982). Tn birds, some nestlings leave their nests apparently to seek adoption by foster parents. Adoption of nestlings is relatively common in semiprecocial species such as co- lonial sea-birds (Graves and Whiten 1980; Pierotti 1988; Morris et al. 1991; Saino et al. 1994; Jouventin et al.
1995), while brood amalgamation often takes place in precocial species such as waterfowl (Eadie et al. 1988; Williams 1994; Larsson et al. 1995). Tn these species adoption usually occurs early in the development of the nestlings and young can receive alloparental care over extended periods (Pierotti 1988). Tn contrast, altricial and semialtricial nestlings need certain flying capabilities to abandon their nest; adoptions usually take place close to fledging and the period of alloparental care is com- paratively short (Bustamante and Hiraldo 1990; Ken- ward et al. 1993; Ferrer 1993; Redondo et al. 1995).
Proximal factors thought to be responsible for nest- switching are sub-standard parental care (Pierotti and Murphy 1987; Saino et al. 1994; Brown et al. 1995) or harassment by nest-mates (Pierotti and Murphy 1987; Redondo et al. 1995). Tn either case, nest-switching nestlings are assumed to be disadvantaged and to derive benefits if they get better care and/or reduced aggression in the foster nest. However, it is not clear why parents tolerate and often feed nestlings that are not their own (Redondo et al. 1995). This has been explained as the result of reproductive errors (Roberts and Hatch 1994) or non-recognition of visitors as strangers (Poole 1982; Holley 1984, 1988; Pierotti 1991; Kenward et al. 1993), sometimes due to unusually high densities (Patterson et al. 1982; Holley 1984; Bustamante and Hiraldo 1990; Donazar and Ceballos 1990; Ferrer 1993).
The cost of rearing unrelated young is highly de- pendent on the developmental strategy of the species. Tn precocial birds, the cost to a foster parent can be neg-
ligible (Patterson et al. 1982; Eadie et al. 1988; Larsson et al. 1995), and a strong selective pressure towards the avoidance of adoptions is not expected (Redondo 1993). Tn contrast, in semiprecocial and altricial birds, a foster parent may incur reproductive costs, including death of its offspring (Bustamante and Hiraldo 1990; Brown et al.
1995). Tn species incurring costs, adoption may still be adaptive due to (1) kin-selection (Waltz 1981), (2) re- ciprocal altruism (Pierotti 1980, 1982, 1988) or (3) di- lution of predation risk on their own offspring (Eadie et al. 1988).
One explanation for adoption is based on the arms race concept. According to the intergenerational con- flict hypothesis of Pierotti and Murphy (1987), nestlings would be selected to abandon their natal nests when they receive deficient parental care. This, in turn, generates a selective pressure for the adults to discriminate against visiting nestlings. Nestlings can be expected to win this
arms race because benefits for them (i.e. survival) are higher than the costs for the foster parents (i.e. partial fitness reduction). Furthermore, selection will act more strongly on traits expressed early on life (e.g., the trait promoting nest-switching in the stressed nestlings) (Pierotti 1991; Redondo et al. 1995). Recent studies have found that this arms race explained the fostering be- haviour of some semi-precocial species (Morris et al.
1991; Saino et al. 1994; Brown et al. 1995), but not in precocial ones (Williams 1994; Larsson et al. 1995). Tn altricial species this hypothesis has only been properly tested — and supported — in the white stork (Ciconia ciconia) (Redondo et al. 1995).
Tn this study we examined the patterns, costs and benefits of adoption in the altricial lesser kestrel (Falco naumanni). This is a colonial falcon in which adoptions have been previously reported (Donazar et al. 1991). We tested the following predictions derived from the inter- generational conflict hypothesis (Pierotti and Murphy
1987): (1) adopted nestlings receive sub-standard pa- rental care prior to nest-switching, (2) nestlings acquire a higher rank in the foster brood or receive better parental care than in their natal nests, and (3) adoptions are costly to the foster parents, which should discriminate against foreing nestlings. Tn addition, we examined the effects of adoptions on fitness, in terms of post-breeding survival of both adults and fledglings, and the subse- quent reproductive performance of adults.
The study was conducted in Los Monegros (north-east Spain), a pseudo-steppe area where lesser kestrels breed exclusively on tiled roofs of abandoned rural constructions (Tella et al. 1996a). Ob- servations were conducted in 1993 in the three largest colonies (34,
34 and 17 pairs, respectively), and in one colony in 1994 (33 pairs). Adoptions were monitored in all pairs breeding in nest-boxes,
which had been installed under the roof of the buildings in 1990.
Nest boxes have an outside opening and a rear one-way mirror and trap-door to check the content of the nests from the inside of the
building to minimize disturbance. Successive inspections of the
nest-boxes never provoked abandonment by the parents, nor nest- switching of nestlings after handling.
Nest-boxes were installed in hollows under the tiles where lesser
kestrels had nested in previous years. Nest-boxes had no effect on: (1) nest site availability, because they simply substituted former
natural hollows, (2) inter-nest distance, (3) levels of parasite in- festation, and (4) breeding success (J.L. Tella, unpublished work).
These results suggest that the installation of nest-boxes did not bias our results.
We monitored 42 (45%) of the 93 nests containing nestlings in the four colonies under study. Focal nests were visited every 6 days
from laying until the first adoption was detected. Those nests produced 133 nestlings (45% of the nestlings produced in the four
colonies, n = 295). During the first visit after hatching nestlings were ranked according to their relative size among brood-mates
and individually marked with numbered cloth bands. At age
2 weeks post-hatching, cloth bands were replaced with metal and
PVC bands. Starting with the first detected adoption in a colony, visits to all focal nests were conducted every 4 days until 8 days
after the last focal nestling fledged.
For analytical purposes, the date when nest-switching occurred
was taken to be the mid-point between the visit when the foreign nestling was first noticed in a strange nest and the previous visit. A
nestling was considered adopted only if it had been observed in a foster nest in at least two consecutive visits. The length of the eighth
primary feather of the nestlings was used to estimate their hatching date and age (Donazar et al. 1991). Fledging date for each nestling
was estimated by nest monitoring. Given that duration of parental care after fledging is short for lesser kestrels (5 days on average;
Bustamante and Negro 1994), it was not considered in this study. During each nest inspection, all nestlings present were identified,
and their body mass and length of the eighth primary feather were recorded. We also systematically counted the number of the two
main ectoparasites of lesser kestrel nestlings (Carnus hemapterus
and Ornitophila gestroi; Tella 1996). When we found banded non-
focal nestlings in focal nests we also checked their nest of origin and took the same information as in the focal nests. At the focal col-
onies, 169 adult kestrels were trapped in their nests and banded with PVC bands. Body condition was calculated as the residuals
from the regression of body mass on the cube of a measurement of body size (Potti 1993; Mateos and Carranza 1996; Blanco and Tella
in press). We used wing length for adults, and the length of the eighth primary feather for nestlings, the best estimators of body
size for the lesser kestrel according to Tella (1996). Due to seasonal and yearly changes in adult body mass, we obtained the residuals
after controlling for capture date and year with two-way ANOVAs
(Tella et al. 1997). Young were sexed according to plumage
characteristics (Tella et al. 1996b).
Behavioural observations were conducted in all nests in the
main colony using spotting scopes from a distance of 100 m, during
363 h/nest before the first adoption and during 555 thereafter until
all nestlings fledged, over a total of 11 days in 1993 and 26 days in
1994. Surveys were done in two daily periods (0800—1200 hours and
1700—2100 hours), coinciding with the maximum activity of lesser kestrels. We observed about 15 nests simultaneously in each survey,
and covered the whole colony every two consecutive surveys. We identified adult and nestlings by their band numbers, and recorded
instances and the apparent causes of nest-switching of nestlings, as well as prey deliveries and aggression.
The quality of parental care, as well as the costs and benefits of adoptions were examined using the following variables: (1) feeding
rates (prey deliveries/h) 2 days before and 2 days after adoptions took place, in both natal and foster nests of the adoptees; (2) body
condition of parents and nestlings; (3) brood size and rank of the adopted nestling in both its natal and foster nests; (4) gender of
nestlings; (5) prevalence — percentage of infected nestlings — and intensity — number of parasites by infected nestling — of ectopara-
sites; (6) survival of nestlings until fledging. Long-term fitness effects of adoptions for foster parents were examined through post-
breeding survival and reproductive success the following year. Since lesser kestrels are highly philopatric, both adult and fledging
survival were estimated by considering those birds not resighted in
subsequent years in the study area as dead (Tella et al. 1997). Accordingly, we searched banded birds in all 70 colonies in the population in 1994 and 1995, from February to August. Breeding success (clutch size and number of fledgings) of adults surviving until the next year was recorded in 1994 and 1995. For the analyses, we included focal birds as well as other adopted nestlings origi- nated from non-focal nests. All variables were assessed for possible differences among years and colonies. Since no differences were found, we pooled data for further analyses.
Frequency of adoptions
Of 133 focal nestlings 68 (51.35%) switched nests. Considering all nestlings in the study colonies, the pro- portion of male (24.45%, n = 147) and female (25.18%, n = 139) nestlings that switched nests did not differ significantly (x2 = 0.42, df = 1, P = 0.52). Adoptions occurred when the nest-switching nestlings were 25 days old on average (SD = 4.48, range = 16—33 days, n = 68). The duration of foster care was about 12 days (i.e. about one-third of the nestling stage; Bustamante and Negro 1994).
Of the 68 nestlings that switched nests, 42 (62%) completed growth and fledged from their foster nest, 21 (31%) switched to a second foster nest from which they fledged, and 5 (7%) eventually returned to their natal nests. Adoptions affected 32 (76%) of the 42 focal nests. Ten pairs (23.8%) adopted nestlings, 5 (11.9%) had a nestling that moved to another nest, and 17 (40.5%) both produced nest-switching nestlings and adopted others. The number of nestlings adopted in each foster nest ranged from one to ten (Fig. 1). However, the mean number of adoptees simultaneously present in a given
Fig. 1 Number of adopted nestlings in each lesser kestrel nest (n = 42 nests). All adopted nestlings were not always present at the same time
nest was low and similar in all of them (1.4 ± 0.7, n = 27 nests). The duration of the period of adoption was also comparable in most nests (10 ± 5.1 days, n = 27 nests).
Proximal factors inducing nest-switching
On roofs, nestlings are able to leave the nest, walk over the roof and potentially switch nests. Actual nest- switching was observed for 21 nestlings; 20 were adopted and the remaining 1 returned to its natal nest. The main cause inducing adoptions was the nestlings following non-parent adults delivering prey. During the last
15 days of the nestling period, we recorded 917 feedings outside the nest boxes. On 160 (17.4%) of these occa- sions alien nestlings approached the adult delivering prey, 37 (4%) pirated food, 9 (0.9%) entered foster nests and were adopted, and only one was mobbed away by adults. Seven adoptions started when nestlings outside their natal nests were attacked by adults and rushed to shelter in other nests. Another nestling rushed in panic to a new nest when a peregrine falcon (Falco peregrinus) attacked a stock dove (Columba oenas) on the same roof. Three nestlings left their natal nests and inspected new ones, where they were adopted. No adults or predators were present in these three cases.
Tn 48 nests where the identity of both parents was known, there was no adult mortality before the nestling had abandoned the nest. Before nest-switching, the body condition of the parents of adoptees did not differ from that of parents whose nestlings did not move; feeding rates did not differ between nests abandoned by nestlings and others where all nestlings stayed (Table 1). Tn ad- dition, nest-switching nestlings were not in poorer body condition than their siblings (Table 2); 15 (47%) of 32 nest-switching nestlings were in better condition than the mean of their siblings. There was no tendency for the smallest (i.e. lowest rank) nestling in a brood to leave the nest. On the contrary, in only 19 cases was it the youngest nestling that moved (27.9%), versus 49 occa- sions on which an older nestling moved.
We were unable to determine whether sibling ag- gression triggered nest-switching in the lesser kestrel. However, several factors make this unlikely. The oc- currence of nest-switching is independent of both brood size in the natal nest, and rank of the switching nestling. Flying exercises are also unlikely causes of nest-switch- ing since most nestlings were too young to perform ex- ercises when they switched nests.
1.13 ± 5.07
0.14 ± 4.22
(n = 18)
(n = 15)
F = 0.56, P = 0.47
—1.02 ± 3.55
0.36 ± 2.08
(n = 16)
(n = 19)
F = 0.18, P = 0.68
2.79 ± 1.38
2.49 ± 1.17
(n = 21)
(n = 12)
Z = —0.56, P = 0.57
Table 1 Body condition of the parents and feeding rates (number of feedings/h) before adoptions in A nests with nest- switching nestlings, B nests without nest-switching nest- lings. Values are mean ± SD
Table 2 Body condition and
A B Test
parasite load in nest-switching
nestlings (A) and their siblings
(B) 2 days before departure. Prevalence refers to the percen- tage of infected nestlings, and intensity to the number of ec- toparasites by infected nestling. Values are mean ± SD or per- centage
Body Cond. 1.01 ± 15.70 —1.35 ± 15.39 Wilcoxon test
(n = 32) (n = 32) Z = —0.23, P = 0.81
Prevalence 8.06% 12.29% x2 = 0.76, P = 0.38
C. hemapterus (n = 62) (n = 122)
Tntensity 4.40 ± 3.49 2.20 ± 1.33 U-test
C. hemapterus (n = 5) (n = 15) Z = —1.30, P = 0.19
Prevalence 12.90% 15.57% x2 = 0.23, P = 0.63
O. gestroi (n = 62) (n = 122)
Tntensity 1.12 ± 0.33 1.26 ± 0.44 U-test
O. gestroi (n = 8) (n = 19) Z = 0.74, P = 0.46
Another reason to move might be that nest-switching nestlings were suffering from higher ectoparasitic infes- tations than their siblings. However, the prevalence and intensity of the two main haematophagous ectoparasites were low and no differences were found between nes- tlings that moved and those that did not (Table 2).
Benefits for a moving nestling in the foster nest
The adopted nestling may benefit by improved food provisioning in at least three ways: (1) obtaining a higher feeding rate; (2) prolonging its nestling stage by joining a less-developed brood; (3) obtaining a higher rank in the foster brood. Tn all cases, an improvement in body condition and survival is expected.
The duration of the nestling stage in nests with adoptees (35.4 ± 4.7 days, range 23—43, n = 27) did not differ from that in nests with no adopted nestlings (34.7 ± 6.1 days, range 26—44, n = 9) (U = —0.40, P = 0.68). Moreover, 20 nestlings moved to nests where their nestling period was actually shorter than that of their siblings in the natal nest, 14 had a nestling period of a similar length, and only 10 had a longer nestling period (Sign test, Z = 1.20, P = 0.23, n = 43).
Only 24 (44.4%) nest-switching nestlings moved to foster nests which, two days before the adoption oc- curred, showed higher feeding rates than their natal nests; in 19 (35.2%) nests, feedings were lower and in
11 (20.4%) nests were equal. (Sign test, Z = 0.91, P = 0.36, n = 54). After adoptions took place, provi- sioning rates in foster nests (3.89 ± 3.93 prey/h, n = 39) were not significantly higher than in the natal nests of the adoptees (3.57 ± 3.02 prey/h, n = 39) (Sign test Z = 0, P = 1, n = 39). Regarding the new rank of the adoptee in the foster brood, only 12 (22%) nestlings were adopted in nests with fewer nest-mates, 32 (58%) gained more nest mates and 11 (20%) were adopted in brood of the same size. Additionally, 18 (28%) out of 64 lowered their previous rank in the foster brood, 18 (28%) kept the same rank, and 28 (43%) achieved a higher rank (Sign test, Z = 0.44, P = 0.66, n = 63).
On average, adopted nestlings did not improve their body condition in their foster nests. On the contrary, two days after the adoption started, 19 (70%) adopted nestlings were in poorer condition and only 8 (30%) were in better condition than their siblings remaining in
their natal nests (Wilcoxon test, Z = 2.56, P = 0.01). Nonetheless, nestling survival did not differ between adopted and non-adopted nestlings (x2 = 0, df = 1, P = 1, n = 184), nor their survival until the next breeding season (x2 = 0.81, df = 1, P = 0.37, n = 166) (Fig. 2).
Tmmediate costs of nest-switching for the moving nestling
Adult lesser kestrels may steal nestlings and practise cannibalism (Negro et al. 1993); and thus they can po- tentially inflict serious injuries on intruder nestlings. Of
917 observed visits of adults to their nests while foreign nestlings were nearby, the adults attacked foreign nest- lings 94 (10.2%) times. However, no injuries were de- tected.
Nest-switching nestlings are also exposed to preda- tors when they walk over the roofs. Although we have not recorded predation attempts during our observa- tions in focal colonies, up to 17 avian species prey upon nestling and fledgling lesser kestrels in this population (Tella et al. 1996a; Blanco and Tella, in press).
We observed 16 male and 15 female parents of nest- switching nestlings feeding their offspring in their new
Fig. 2 Survival of adopted and non-adopted nestlings until fledging, as well as that of the survivors ( juveniles) until the subsequent breeding season
adoptive nests. Two males and two females were ob- served feeding their nestlings when they fell from the roof to the ground and stayed there for some days. These qualitative observations suggest some degree of individual nestling recognition independent of circum- stantial cues such as nest-hole recognition. However, at least 11 males and 13 females also fed alien nestlings when they apparently tried to feed their own nestlings in foster nests (61.6% of 171 feedings), and attacked their own offspring 30 times (17.6% of feedings, n = 171). Similarly, the parents occasionally attacked their own offspring (10.8% of 485 feedings) when they were out- side the nest. On the other hand, when alien nestlings were inside their foster nests during feedings (n = 82 feedings by 13 males and 10 females) they were never attacked by foster parents.
Costs for the foster parents
The brood size of abandoned nests before adoptions, (3.31 ± 1.27 nestlings, n = 55) was similar to that of nests with adopted nestlings (3.32 ± 1.86, n = 66) (U = 0.05, P = 0.96). Although some nests both lost and received nestlings (see above), this interchange resulted in fewer nestlings in natal nests (2.24 ± 2.26, n = 45) than in the receiving ones (4.45 ± 2.40, n = 69) (U = 4.89, P = < 0.0001). As a consequence, feeding rates generally increased in a higher proportion when an adoption took place (+1.49 ± 3.82 feedings/h, n = 24) compared to nests where the number of nestlings remained constant (+0.45 ± 1.67 feedings/h, n = 27), while feedings diminished after a nestling left (—0.90 ± 2.45 feedings/h, n = 31) (Kruskal-Wallis test, Z = 10.85, P = 0.004). These relative changes of feeding rates were closely related to the changes in brood size (rs = 0.40, P = 0.0003, n = 83) as a result of nest- switching of nestlings.
The increased parental effort did not seem to affect the post-breeding survival of parents, which was the same for both foster and non-foster parents (males: x2 = 0.01, df = 1, P = 0.93, n = 36 females; x2 = 0.31, df = 1, P = 0.57, n = 48; pooled: x2 = 0.14, df = 1, P = 0.79, n = 84) (Fig. 3). Furthermore, the breeding success of surviving foster adults the following year did not differ from that of surviving non-foster adults (Table 3).
A nestling is expected to leave its natal nest and stay in a
Fig. 3 Post-breeding survival of foster and non-foster parents until the subsequent breeding season
obtain a higher rank, extended parental care or higher survival in the new nest.
Tf adoptees did not receive improved parental care, why did they move? Adopted nestlings seemed to reach their foster nests in a fortuitous way. Nestling lesser kestrels usually rush to the opening of their nests when the parents deliver prey. Most observed adoptions started during feedings, when neighbouring nestlings attempted to steal prey. Rushing at the adults is a common behaviour in fledgling falconiforms (Poole
1982; Kenward et al. 1993; Donazar and Ceballos 1990). Other nestlings moved to foster nests as a result of at- tacks from adult kestrels or predators. The adopted nestlings may not have returned to their natal nests be- cause they did not know the way back or because it would mean re-exposure to attacks and predation. Tn fact, the five nestlings that did return to their natal nests were in foster nests that were closer to their original nests (52 ± 33.3 cm, n = 5) than nestlings who did not return (109.8 ± 66.6 cm, n = 32) (U = — 2.17, P = 0.029).
Even though adopted nestlings did not derive obvious benefits from adoption, an increase in their inclusive fitness could still be possible. The abandonment of their natal nests, with the subsequent brood reduction, would permit an improvement in the condition and survival of their parents and siblings (Dijkstra et al. 1990). How- ever, the survival of the adults in nests where adopted nestlings originated did not differ significantly from that of adults in nests that had no brood reduction, and was no higher than that of foster adults with increased
Table 3 Breeding performance of fostering and non-fostering adults in the subsequent breeding season
foster nest only if the obtained benefits exceed the costs. Tn the lesser kestrel, the potential costs involved in nest-
Fostering adults Non-fostering
switching were an increased risk of predation during moving and initial mild aggression by the foster parents. The potential benefits were not apparent; nestlings did not move to nests with lower brood size, not did they
Clutch size 4.33 ± 0.97
(n = 15)
Fledglings 2.28 ± 0.93
(n = 25)
4.37 ± 1.30
(n = 8)
2.25 ± 0.96
(n = 12)
Z = 0.00
P = 1.0
Z = 0.15
P = 0.88
broods (x2 = 0.15, df = 2, P = 0.93). Mortality of
even if this occurs in different breeding seasons, the
nestlings until fledging was negligible <(
5%) for both
absence of clear benefits for the moving nestlings makes
adopted and non-adopted nestlings. Tn sum, indirect kin-selected benefits were not a likely ultimate cause for the adoptions we observed.
The question that remains is why foster adults tol- erated alien young. Adults that fostered alien nestlings had to provision an enlarged brood during a relatively long period, potentially affecting the survival of foster parents (Dijkstra et al. 1990; but see Korpimaki and Rita, in press). Nonetheless, the increase in feeding rates may be partially due to the true parents of the adopted nestlings feeding them along with the foster parents. Other unmeasured negative fitness effects might occur for the foster parents, such as immune system depression and a rise in parasite loads (Sheldon and Verhulst 1996). However, if they do occur, these effects must be subtle because the survival of foster parents was not reduced in the following year compared to other adults. A reduc- tion of future reproductive output (Dijkstra et al. 1990) is not expected, since the breeding success of foster parents in the year following that of adoption was the same as that of other parents.
The high rate of nestling adoptions found in the lesser kestrel can hardly be explained in terms of the inter- generational conflict hypothesis (Pierotti and Murphy
1987). Even though there were no apparent long-term costs for the foster parents, there were no obvious ben- efits for the moving nestling. The situation that we ob- served in the lesser kestrel may be explained by one of the following hypotheses:
1. Adoptions occur because foster parents would obtain benefits in terms of dilution of predation risk on their own offspring (Eadie et al. 1988). However, this hypothesis is unlikely because predation occurs mainly in early breeding stages, affecting complete broods (au- thors, unpublished work).
2. Adoptions might have developed in a context of high inbreeding. Adults would derive indirect benefits from adopting related nestlings (i.e. through kin selec- tion). Lesser kestrels are extremely philopatric and the population in Monegros is relatively isolated (authors, unpublished work). Tn addition, if kestrels engaged in extra-pair copulations or if nest parasitism through egg dumping occurred, foster parents could enhance direct fitness by adopting their own offspring (Saino et al.
1994). However, a concurrent study using DNA finger- printing revealed a low incidence of extra-pair fertiliza- tions (3.4%) and brood parasitism (7.4%) in the same colonies (Negro et al. 1996). The probability of adopting a closely related nestling were thus very low. Tn addition, the same study showed large inter-individual genetic differences. Thus, high inbreeding is unlikely (Kuhnlein et al. 1990).
Tn any case, it is not clear why an adult should incur any costs in helping any young that does not appear to need that help. The same reasoning may be used to discount the occurrence of reciprocal altruism. Although mutual exchange of nestlings was observed (10.5%), and
the existence of this mechanism unlikely.
3. Adoptions are successful because lesser kestrels have not developed a fine-tuned kin-recognition mech- anism. Tn species where the risk of alloparental care is high, mechanisms to discriminate and expel intruders are usually well developed (Redondo 1993). Species phylogenetically related to the lesser kestrel are solitary and only use circumstantial cues (through the presence of offspring inside the nest hole) to recognize their kin (Dijkstra et al. 1990; Wiebe and Bortolotti 1994; Kor- pimaki and Rita in press). This also seems to occur in some colonial species where the risk of adoption is low due to the relative isolation of nests, such as the cliff- nesting kittiwake (Rissa trydactila) (Roberts and Hatch
1994). Tn this context, a negative response by the adults would be maladaptive due to the potentially high cost (Beecher et al. 1991; Redondo 1993). After all, even a very discriminting adult still risks mistakenly expelling or injuring one of their own nestlings (Knudsen and Evans 1986; Hebert 1988).
Before the existence of towns and villages, lesser kestrels may have nested in cliffs. However, at present only 4% of the Spanish colonies are in cliffs. The re- maining colonies are found in human constructions,
50% in wall holes and 44.8% on roofs (Gonzalez and Merino 1990). Building walls are similar to cliffs in the sense that nests are not usually in contact (Negro and Hiraldo 1993). Tn these colonies nest-switching is mainly limited to the fledging period and adoptions are not frequent (Donazar et al. 1991; authors, unpublished work). However, in colonies located on tiled roofs the young can easily walk between nests and engage in adoptions at an early age. Tt is not known for how long the lesser kestrel has been breeding on tiled roofs, but in the evolutionary history of this species, nesting in human dwellings is certainly a recent event.
To conclude, we believe that the high rate of non- adaptive adoptions of lesser kestrels resulted from re- productive errors related to the evolutionarily recent occupation of a new substratum for breeding. Tn this context, proximal factors causing nest-switching arise from the tendency of any young raptor to solicit food from adults passing close to their nests, as well as a tendency to run to any available hole to avoid conspe- cific attacks or predation. Tt would be difficult for adults to evolve response to adoption, because of the costs of misidentification of kin. As a balance between the risk of adoption and of rejection of kin, selection should favour acceptance by lesser kestrels of all nestlings that are in the nest due to the low cost of rearing alien nestlings.
Unless new selective pressures appear non-adaptive adoptions will persist. However, if the species suffered chronic food stress, costs for foster parents would in- crease and they would be expected to develop finer off- spring recognition systems. On the other hand, there would be selection for chicks to seek better parental care. Tn this hypothetical scenario an arms race may
develop fitting the intergenerational conflict hypothesis. The fact that this conflict occurs in some species but not others could be related to the particular stage of their evolutionary histories at the time they were studied. Tt is possible that the current adoptive behaviour of some species is ultimately derived from changes in food or breeding substratum availability promoting nest-site clumping.
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Communicated by M.A. Elgar