Brood reduction in response to manipulated brood sizes in the common swift (Apus apus)

Following a brood size manipulation experiment on the common swift (Apus apus) in which different levels of parental effort were created, brood reduction occurred in all five nests manipulated to four chicks and in two of the five manipulated to three young. This provided an opportunity for looking in detail at the parental investment decisions concerning allocation strategies between parent and young during the process of brood reduction. The data recorded here were analysed on a visit-by-visit basis regarding changes in parental and chick body masses, the mass of prey delivered and the estimated mass of parental self-feeding. The results show that food delivery rates did not increase in proportion to the number of chicks in the broods. This reduction in delivery rates per chick resulted in lower chick masses and ultimately in the death of one or more chicks in the larger broods. The resulting low parental body masses for adult birds feeding larger broods suggested that these parents could not have raised all their young without risking their own survival. There was a tendency for parents from nests that experienced brood reduction to feed themselves instead of allocating most of the food gathered to the young. After brood reduction, parents regained lost body mass and their young fledged at masses only slightly lower than normal. The differential allocation decisions regarding parental self-feeding which resulted in brood reduction were largely responsible for keeping the parents in good enough condition to care for the surviving nestlings. Therefore, this study clearly demonstrates that brood reduction can operate through the differential allocation of food between parent and young in a way that can have adaptive consequences for the survival of parents and their young. Correspondence to: T.L.F. Martins     

Is reduced clutch size a cost of parental care in Eastern Phoebes (Sayornis phoebe)?

What is the cost of parental care in birds? Previous studies using observational and experimental techniques on nest building and clutch sizes in a small migrant flycatcher, the Eastern Phoebe (Sayornis phoebe), led to contradictory results that did not show a consistent cost of current reproductive effort on residual reproductive output. The data presented here indicate that different elements of parental behaviors are indeed costly because they reduce various aspects of phoebes' subsequent reproductive performance. Experimental removal of old nesting structures at previously used breeding sites reduced but did not eliminate the chance of phoebes' settlement in the subsequent year. Comparing sites at which phoebes did and did not build new nests showed that nest builders completed their first clutches later, had lower probabilities of second breeding attempts, and more often lost their nesting attempt due to fallen nest structures than nest reusers. There was, however, no significant effect of nest building on the clutch sizes and rates of cowbird parasitism of first nesting attempts. Overall, sites with newly built nests had lower seasonal reproductive effort than sites with reused nests. I also examined phoebes' relative residual reproductive output in a separate breeding season when nest building was not experimentally manipulated. When controlled for confounding variables this analysis indicated that in those phoebes that did breed for a second time, the relative decrease of the sizes of first to presumed second clutches was greater at sites where first breeding attempts consisted of more total nestlings. These data are consistent with the hypothesis that parental care is costly in Eastern Phoebes and support predictions of trade-offs between the nest building, brood care, and residual egg-investment components of reproduction.     

Nest-maintenance effort and health status in chinstrap penguins, Pygoscelis antarctica: the functional significance of stone-provisioning behaviour

Stone provisioning is a nest-maintenance activity accomplished by pygoscelid penguins after reliefs during the incubation/brooding period. The functional significance of this behaviour has been mainly explained as a parental strategy preventing nest flooding under detrimental weather conditions. In addition, and in the light of recent studies, this behaviour could also fit into the sexual selection process. In this study, we tested the first idea, that is, whether stone provisioning is a nest-maintenance behaviour to increase egg/nestling survival by lowering the risk of nest flooding, and can thus be considered a form of parental care. Additionally, we investigated if the effort invested by parents in nest maintenance is constrained by physiologically limiting resources. The effort of stone collection and the perceived risk of nest flooding were experimentally manipulated during the incubation and early brooding phases in a chinstrap penguin, Pygoscelis antarctica, colony. Three groups of nests were established. After weighing, control nests were left unmanipulated. In a second group of nests (reduced group), only one-half of the initial weight of stones was returned to the nests. In a third group of nests (snow-added group), we both reduced nest weight by a half and added snow outside the nest bowl over 6 consecutive days. Ten days after manipulation, the difference in nest weight between initial and final conditions was significantly related to treatment: penguins increased stone provisioning in the reduced group (44% of half-reduced nests), but drastically more in the reduced and snow-added group (123% of half-reduced nests), while the weight of control nests was unchanged compared to premanipulation conditions. The intensity of stone provisioning was affected by nest date, peaking about hatching time and shortly after, and declining with advancing chick age. These results suggest that stone provisioning is a mechanism that has evolved to prevent egg or chick mortality by nest flooding. The haematocrit, but not leukocyte numbers as expressed by the buffy coat layer, varied with the experimental conditions. Penguins investing more time in nest maintenance had a lower haematocrit, suggesting a physiological trade-off probably mediated by competition between the time devoted to nest maintenance versus foraging activities. The amount of stones collected and the haematocrit were positively related to the number of neighbour nests, so those individuals surrounded by more nests seemed to obtain benefits in the availability of nest material and energy savings. This study indicates that stone-provisioning behaviour is a nest-maintenance activity evolved to improve thermal nest characteristics potentially increasing offspring survival, and competing in time and energy with other reproductive activities. Stone provisioning in penguins should therefore be regarded as a form of parental care and an important part of individual reproductive effort in species breeding in harsh environments. Furthermore, nest size and nest-maintenance effort should be considered reproductive traits indicative of parental quality and thus could also be involved in the post-mating sexual selection process.     

Asymmetric costs favor female desertion in the facultatively polyandrous northern flicker (Colaptes auratus): a removal experiment

Previous studies of biparental care in birds have focused on the males contribution and experimental removal of males. Woodpeckers, with a high level of paternal care including nocturnal incubation and brooding by males, offer a meaningful system in which to examine the importance of care by females. I studied the reproductive performance of 17 widowed male and 8 widowed female northern flickers (Colaptes auratus) relative to biparental (control) pairs. Of these single parents all widowed shortly after hatching, only one female abandoned its nest. Single parents boosted their provisioning rates to achieve 83% the provisioning rate of control broods, but reared significantly fewer young and young of poorer quality. However, single males, with 85% of the reproductive success of controls, were more successful than females with 43% the success of controls. Among widowed birds, a not significant lower survival was observed, but the chance of re-pairing with the same partner in a subsequent year was only 16% in the natural population, so long-term costs of desertion may be small. Although females seem to have the incentive and ability to desert, a lack of available males may constrain opportunities. This study demonstrates that when parental care roles are reversed in altricial birds, asymmetric benefits can favor female and not male desertion.     

Parental investment, adult sex ratios, and sexual selection in a socially monogamous seabird

Although most birds are monogamous, theory predicts that greater female parental investment and female-biased adult sex ratios will lower the polygyny threshold. This should result in polygynous mating, unless obligate biparental care or the spatial and temporal distribution of fertilizable females constrains a male’s ability to take advantage of a lowered polygyny threshold. Here we present data on the extent of male sexually dimorphic plumage, adult sex ratios and breeding season synchrony in three populations of a socially monogamous seabird, the brown booby Sula leucogaster. For one of these populations, San Pedro Mártir Island, we also present data on differences in male and female parental investment, mortality and probability of pairing. The extent of plumage dimorphism varied among populations. Sex ratios were female biased in all populations. On San Pedro Mártir Island, parental investment was female biased, females failed more often than males to find a mate, but there was no polygyny. We suggest that on San Pedro Mártir: (1) a period of obligate biparental care coupled with a relatively synchronous breeding season constrained the ability of males to take advantage of a high environmental polygamy potential and (2) the resulting socially monogamous mating system, in combination with the female-biased adult sex ratio, caused females to be limited by the availability of males despite their greater parental investment. Received: 18 November 1999 / Accepted: 24 January 2000     

Assured fitness returns favor sociality in a mass-provisioning sweat bee,Megalopta genalis (Hymenoptera: Halictidae)

Assured fitness returns models for the evolution of sociality emphasize the selective value of ensuring that offspring receive adequate parental care to reach maturity. If a member of a social group dies, it can accrue returns on investment in offspring through the efforts of surviving social partners. We provide evidence that in the mass-provisioning, facultatively social sweat bee Megalopta genalis, adult presence in the nest throughout brood development provides protection from ant predation. Nests with adults present were well protected, and brood in nests with adults removed suffered higher predation. Females in observation nests showed effective defensive behavior against experimentally introduced ants, and bees in natural nests repulsed naturally occurring ant raids. Megalopta nest architecture and behavior are such that the brood of several cooperating females can be defended with little additional cost relative to solitary nesting. The benefits of cooperative defense may favor group living in mass provisioning bees. Our observations and experiments suggest that parental care throughout brood development can be adaptive in mass provisioning species, supporting the predictions of assured fitness returns models.     

Reproductive performance in lesser snow geese: are two parents essential?

Summary The roles of male and female lesser snow geese (Anser caerulescens) in offspring care are well documented, but we know little about flexibility in these roles and how essential they are for offspring survival. We asked whether uni-parental care was adequate, which sex was required at various stages of the reproductive cycle, and what the costs and consequences were of variable amounts of parental care. We found that two parents were important in acquiring nest sites and producing clutches. Widows losing mates during the latter part of laying or in early incubation experienced similar rates of success in hatching clutches to paired females, but males losing mates during incubation experienced total nest failure. Partial clutch loss, hatch loss, intraspecific nest parasitism, duration of incubation periods and gosling weights at hatch did not differ for pairs or widows. In 1983 at La Pérouse Bay, Manitoba, Canada, widows and pairs had similar incubation behaviour, but widows were harassed more and spent more time in full alert posture while on their nests. In 1952 and 1953 at Southhampton Island, Northwest Territories, widows were significantly lighter in body weight prior to hatch than paired females. Thus while widows can bring clutches to hatch successfully, the loss of mates may result in additional physiological costs. Although all possibilities have not been tested experimentally, it appears that costs to uni-parental care up to hatch are not significant. Data collected so far on consequences of uni-parental care provided during the post-hatch period are equivocal, as in one year survival of goslings from single parent broods seemed poor and in another it did not appeart to differ from pairs. The minimum amount of parental care required to raise snow goslings from hatch to recruitment has yet to be determined.     

Brood sex ratios, female harem status and resources for nestling provisioning in the great reed warbler (Acrocephalus arundinaceus)

The theory of parental investment and brood sex ratio manipulation predicts that parents should invest in the more costly sex during conditions when resources are abundant. In the polygynous great reed warbler, Acrocephalus arundinaceus, females of primary harem status have more resources for nestling provisioning than secondary females, because polygynous males predominantly assist the primary female whereas the secondary female has to feed her young alone. Sons weigh significantly more than daughters, and are hence likely to be the more costly sex. In the present study, we measured the brood sex ratio when the chicks were 9 days old, i.e. the fledging sex ratio. As expected from theory, we found that female great reed warblers of primary status had a higher proportion of sons in their broods than females of lower (secondary) harem status. This pattern is in accordance with the results from two other species of marsh-nesting polygynous birds, the oriental reed warbler, Acrocephalus orientalis, and the yellow-headed blackbird Xanthocephalus xanthocephalus. As in the oriental reed warbler, we found that great reed warbler males increased their share of parental care as the proportion of sons in the brood increased. We did not find any difference in fitness of sons and daughters raised in primary and secondary nests. The occurrence of adaptive sex ratio manipulations in birds has been questioned, and it is therefore important that three studies of polygynous bird species, including our own, have demonstrated the same pattern of a male-biased offspring sex ratio in primary compared with secondary nests. Received: 1 June 1999 / Received in revised form: 10 January 2000 / Accepted: 12 February 2000     

Parental conflict and brood desertion by females in blue-headed vireos

We investigated sexual conflict over parental care in blue-headed vireos (Vireo solitarius) and documented the first example of unvarying unisexual brood desertion in passerines. Females at all nests (N = 24) that were monitored closely near fledgling, deserted their broods on or near the day of fledging leaving males alone to complete parental care of young. No males deserted. This observational evidence was confirmed with radiotracking of females (2004, 2007) and both pair members (2008). Radiotracked females began visiting distant males 1–4 days before young left the nest, subsequently paired with males 355–802 m away, and laid first eggs in new nests less than 5 days after deserting. In contrast, females suffering nest predation did not desert and renested with the same male. We suggest equal parental care (nest building, incubation, feeding) in the sexes, genetic monogamy, and an adult sex ratio biased towards males has led to female control of brood desertion in this species. Unisexual desertion may be more important in altricial birds than generally realized and we discuss prerequisites to predict its occurrence. One is genetic monogamy, which may be a female tactic that reduces the likelihood of males evolving counter-adaptations to female desertion.     

Preferential allocation of food by magpies Pica pica to great spotted cuckoo Clamator glandarius chicks

Adult magpies Pica pica provide parasitic great spotted cuckoo Clamator glandarius nestlings with a diet very similar to that fed to their own chicks. In both naturally and experimentally parasitized nests, great spotted cuckoo chicks were fed at a higher rate than magpie chicks in the same nest. This preferential allocation of food by magpie parents to great spotted cuckoo chicks is consistent with the supernormal stimulus hypothesis, because this result implies that cuckoo chicks provide stronger stimuli for parental care than host chicks. Great spotted cuckoo chicks receive most of the food brought to the nest by the foster parents, because they exploit a series of stimuli which jointly (or sometimes individually) operate as a supernormal stimulus. This hypothesis predicts that if any stimulus is masked, the efficiency of the cuckoo in eliciting parental care will decrease. Here, we analyze experimentally the effects of two of these stimuli, preferential feeding of large nestlings and of nestlings with conspicuous palatal papillae. Firstly, when we experimentally introduced one medium-sized (7–9 days) cuckoo chick into an unparasitized magpie nest where the largest magpie chick was 12–15 days old, the cuckoo did not receive significantly more food than the average or the largest magpie chick. Secondly, when unparasitized nests were experimentally parasitized with a cuckoo chick that had its gape painted to mimic that of magpie chicks, the parasitic cuckoo received less food than the average magpie chick.     

Defending young biparentally: female risk-taking with and without a male in the burying beetle, <Emphasis Type="Italic">Nicrophorus pustulatus</Emphasis>

A positive correlation between the parental effort of a male and female should promote stable biparental care. Risk-taking (as assessed by injuries) against infanticidal intruders by Nicrophorus pustulatus females was expected to be low when females had a low probability of successful defense of the young. I tested the hypothesis that when the presence of a male partner increased the probability of successful defense from low to moderate that female risk-taking would increase. Single females and pairs with first instar larvae were confronted by potentially infanticidal male and female conspecific intruders. Male intruders routinely took over nests from unpaired females (30 of 36 trials). Unpaired females and male intruders were injured infrequently, indicating less intense fights despite the high probability of infanticide. A resident female defending against a male intruder was injured more often when paired than unpaired, suggesting greater risk-taking. A male parent that delays desertion, therefore, receives fitness benefits not only from his own defense of the young, but from greater female defense against male intruders as well. It is hypothesized that the threat of infanticidal takeovers by males promotes extended biparental care in burying beetles. When the intruder was female, on the other hand, a female parent on her own had a moderate probability of successfully defending the brood (22 of 36 trials). The presence of a male partner against female intruders almost guaranteed successful defense (35 of 36 trials) and female intruders did not appear to contest pairs vigorously. Against female intruders the presence of a male partner did not significantly change injury rates of the defending female.     

Helpers-at-the-nest have the same provisioning rule as parents: experimental evidence from play-backs of chick begging

Despite the success of kin selection in explaining helping-at-the-nest among communally breeding birds, we know almost nothing about how helpers regulate their chick-feeding effort. This is especially interesting given how much we now know about parental provisioning `rules-of-thumb' and the evolution of chick begging as an honest signal of `need'. This study explores the provisioning rules of helpers and parents in Arabian babblers (Turdoides squamiceps), using tape play-backs to supplement chick-begging signals and increase apparent brood demand. In all eight groups tested, both helpers and parents fed older, noisier broods at higher rates. Total provisioning rates to nests increased during begging play-back days compared to control days. Absolute provisioning rates by helpers and the scale of their responses to play-backs were statistically indistinguishable from those of parents. In both helpers and parents, increases in nest visits during play-backs were associated with reductions in foraging distance from the nest and increases in size of prey delivered. Older birds of both sexes delivered slightly larger prey items, possibly reflecting differences in foraging ability due to experience. These results are consistent with the idea that, like the parents, helpers-at-the-nest in Arabian babblers provision nestlings as part of a strategy of investment, irrespective of helper age, dominance status or sex. In this species, high relatedness within groups may provide parents and helpers with similar kin-selected fitness benefits, although the mutualistic advantages to helpers from simply augmenting group sizes cannot be ruled out. Received: 17 June 1997 / Accepted after revision: 28 February 1998     

Red-winged blackbird parental investment following brood parasitism by brown-headed cowbirds: is parentage important?

Parental investment by red-winged blackbirds (Agelaius phoeniceus) in response to natural and experi‐mental parasitism by brown-headed cowbirds (Molothrus ater), and in response to freeze-dried, female cowbird mounts presented near redwing nests during the egg-laying period was measured. Two measures of redwing parental investment were used: nest defense effort toward a model predator, and rate of feeding nestlings. There were no significant differences in levels of parental investment among unparasitized nests, naturally parasitized nests, or experimentally parasitized nests. Similarly, parental investment did not differ between redwings that were exposed to the cowbird mount and those that were not exposed to the mount, or among redwings exposed to the cowbird mount at different distances from the nest. This suggests that red-winged blackbirds do not recognize when they have been parasitized, and hence do not associate parasitism with a decrease in their parentage, or that parentage is not an important predictor of parental investment in this species. Received: 24 January 1997 / Accepted after revision: 7 June 1997     

Yolk hormones and sexual conflict over parental investment in the pied flycatcher

Female birds might be able to manipulate the parental effort of their male partner through elevated transfer of hormones to the eggs, since these hormones affect many chick traits that males might use as cues for adjusting the level of their investment. We experimentally studied whether female pied flycatchers Ficedula hypoleuca could manipulate male investment via yolk androgens. There is much more variation in yolk androgen levels between females than within clutches, and in order to change the androgen levels of the eggs, we swapped whole clutches between nests. To estimate the androgen levels of the clutch, we measured the androgen content of a single egg per clutch. Females did not succeed in manipulating male effort using yolk androgens, since there was no relationship between the division of parental care within a pair and either original or foster egg androgen levels. One of these relationships should have occurred if females were manipulating males. The proportion of feeding visits by the male was higher when the male was old (55%) than when he was young (45%) and females laid eggs with higher androgen levels when mated with a young male. Young males did not exhibit any responses to yolk androgen levels either, which indicates that females cannot exploit their effort more than that of old males. We suggest that females may allocate yolk androgens to adjust the growth trajectories of the chicks to poor growing conditions when mated with young males that are poor providers or occupying a poor territory.     

Offspring recognition and the influence of clutch size on nest fostering among male sand gobies, Pomatoschistus minutus

When parental care is costly, parents should avoid caring for unrelated young. Therefore, it is an advantage to discriminate between related and unrelated offspring so that parents can make informed decisions about parental care. In the present study, we test the hypothesis that male sand gobies (Pomatoschistus minutus) recognize and differentially care for their own offspring when given a choice between a nest with sired eggs and a second nest with eggs sired by an unrelated male. The sand goby is a species with exclusive and costly paternal care. Male parasitic spawnings (e.g., sneaking) as well as nest takeovers by other males are common. Our results show that nests containing sired eggs were preferred and received significantly more care, as measured by nest building and nest occupancy, than nests with foreign eggs even when males cared for both nests. These findings suggest that males respond to paternity cues and recognize their own clutches. Relative clutch size also had a significant effect on male parental care. When sired clutches were larger than foreign clutches, males preferred to care for their own nest. In the few cases where males chose to take care of foreign nests, the foreign clutch was larger than their own clutch. Taken together, our results provide evidence that both paternity cues and clutch size influence parenting decisions among male sand gobies.     

Paternal care in the European starling,Sturnus vulgaris: nestling provisioning

The extent to which male birds in polygynous species with biparental care assist in nestling feeding often varies considerably between nests of different mating status. Both how much polygynous males assist and how they divide their effort between nests may have a profound effect on the evolution of mating systems. In this study we investigated how males in the facultatively polygynous European starling Sturnus vulgaris invested in their different nests. The amount of male assistance affected the quality of the offspring. Polygynous males invested as much as monogamous males, but divided their effort asymmetrically between nests, predominantly feeding nestlings of first-mated (primary) females. Although females partly compensated for loss of male assistance, total feeding frequency was lower at primary females’ nests than at monogamous females nests. Secondary females received even less assistance with nestling rearing, and the extent to which males assisted decreased with the length of the interval between the hatching of the primary and secondary clutches. These results are contrasted with those from a Belgian populations of starlings with a much more protracted breeding season and thus greater opportunities for males to attract additional mates during the nestling rearing period. The results show that both the “defence of male parental investment model” and the “asynchronous settlement model” have explanatory power, but that their validity depends on the potential length of the breeding season. Received: 21 July 1995/Accepted after revision: 13 July 1996     

Delayed juvenile dispersal and monogamy, but no cooperative breeding in white-breasted mesites (Mesitornis variegata)

Although cooperative breeding is known from only about 9 % of bird species, it has received substantial attention because individuals foregoing their own reproduction to help others represent a long-standing evolutionary puzzle. We studied group formation, breeding system, spatial distribution and several life-history traits of white-breasted mesites (Mesitornis variegata). Based on field observations across 3 years, we found that white-breasted mesites live in year-round stable pairs, and that groups are formed by juvenile philopatry. As other family-living birds, M. variegata exhibit a slow pace-of-life, characterized by high annual adult survival, low productivity, long chick dependence and extended parental care. However, although reproduction is monogamous and juveniles showed interest in their parents’ nests, we found no evidence of cooperative breeding. We suggest that slow life-histories, extended parental care and year-round territoriality predispose juvenile mesites to delay dispersal. However, adult intolerance toward older juveniles may prevent them from adopting a cooperative lifestyle. Comparisons with other species of mesite indicate that monogamy and delayed juvenile dispersal are necessary, but not sufficient for the evolution of cooperative breeding in this family of birds, and that particular ecological and social conditions have facilitated the transition from pair-living to a type of group that may represent a stepping stone in the evolution of cooperative breeding in mesites and other birds.     

Effects of paternal care on reproductive success in the polygynous spotless starling Sturnus unicolor

For males of socially polygynous avian species like the spotless starling, there may exist a trade-off between investing in paternal care and controlling several nests. To determine how the intensity of paternal care affects reproductive success per brood sired or expressed as the total number of young raised in all nests controlled by the same male, it is necessary to manipulate paternal care. Testosterone (T) has been shown to depress the tendency for males to care for their young, and induces them to acquire more mates. The effects of paternal care on reproductive success were studied by treating certain male starlings with exogenous T and others with the antiandrogen cyproterone acetate (CA), and comparing the parental behavior of T- and CA-males throughout the breeding season with that of controls. CA-males fed their chicks more during the first week after hatching than T-males, with controls feeding at intermediate rates, both on a per nest basis and as total effort for all nests controlled by the same male. Paternal feeding rates during the first week of chick life had a significant positive effect on the number of fledged young. The hormone treatment significantly affected the number of chicks raised per nest, CA-males having a higher breeding success per nest than T-males, and controls showing intermediate levels of success. There was no significant effect of treatment on total reproductive success attained by males throughout the season. In the polygonous spotless starling, the intensity of paternal care of young affects reproductive success per nest positively but not on a seasonal basis. Received: 6 February 1999 / Received in revised form: 30 June 1999 / Accepted: 11 July 1999     

Divorce in common murres (Uria aalge): relationship to parental quality

Behavioral precursors of 12 divorces were examined in 30 color-banded pairs of common murres (Uria aalge) over six breeding seasons. Common murres are long-lived seabirds that typically return each year to the same mate and nest site in dense colonies. At least one parent is present continuously from egg lay to chick fledging. Murres, therefore, have considerable opportunities to compare their mates parental behavior with that of several familiar neighbors. Previous reproductive success was lower for divorcing birds than for reuniting pairs. As predicted by the better option hypothesis, there were clear choosers (seven females and five males) that initiated divorce by moving to a new birds site or by courting a new partner at their current site. Choosers improved their reproductive success after the divorce, whereas their previous partners, the victims did not. Yearly divorce rates (average 8.2% per year) were significantly correlated with yearly mortality rates. Divorces appear to be opportunistic: pairs divorced after varying numbers of reproductive failures with the immediate precursor usually being the disappearance (death) of a murre from a successful neighboring site. In contrast to the delays experienced by victims, choosers formed new pairs quickly and laid their eggs no later than reuniting pairs. Prior to the divorce, victims fed their chicks less often than choosers, and some engaged in other behaviors that compromised egg or chick survival. These observations suggest that deficiencies in parental behavior were precursors to the divorce. This report is one of the first cases where reproductive failure of divorcing pairs has been linked to deficits in the parental behavior of the subsequent divorce victim.Communicated by C. Brown     

Evolution of Nesting Height in an Endangered Hawaiian Forest Bird in Response to a Non‐Native Predator

Abstract: The majority of bird extinctions since 1800 have occurred on islands, and non‐native predators have been the greatest threat to the persistence of island birds. Island endemic species often lack life‐history traits and behaviors that reduce the probability of predation and they can become evolutionarily trapped if they are unable to adapt, but few studies have examined the ability of island species to respond to novel predators. The greatest threat to the persistence of the Oahu Elepaio (Chasiempis ibidis), an endangered Hawaiian forest bird, is nest predation by non‐native black rats (Rattus rattus). I examined whether Oahu Elepaio nest placement has changed at the individual and population levels in response to rat predation by measuring nest height and determining whether each nest produced offspring from 1996 to 2011. Average height of Oahu Elepaio nests increased 50% over this 16‐year period, from 7.9 m (SE 1.7) to 12.0 m (SE 1.1). There was no net change in height of sequential nests made by individual birds, which means individual elepaios have not learned to place nests higher. Nests ≤3 m off the ground produced offspring less often, and the proportion of such nests declined over time, which suggests that nest‐building behavior has evolved through natural selection by predation. Nest success increased over time, which may increase the probability of long‐term persistence of the species. Rat control may facilitate the evolution of nesting height by slowing the rate of population decline and providing time for this adaptive response to spread through the population.