Parental provisioning, nestling departure decisions and prefledging mass recession in Cassin's auklets

We investigated how parental provisioning and nestling departure behaviour interact to produce prefledging mass recession in Cassin's auklets, Ptychoramphus aleuticus. Under our hypothesis, auklet parents are reluctant provisioners and should be increasingly likely to terminate or reduce provisioning of their single nestling as it matures. For the nestling, remaining in the nest presents a risk of losing mass (if the parent does not provision) but also a possibility of additional provisioning. As the nestling matures and becomes increasingly capable of independence, the benefits of remaining in the nest decrease and nestlings should be increasingly likely to fledge. Nestlings also should be more likely to fledge when the expectation of additional provisioning is low. Data on parental provisioning (based on growth increments) and nestling departure were consistent with our hypothesis. Older and heavier nestlings had smaller growth increments than younger and lighter nestlings, suggesting that parents were reluctant to provision old and heavy nestlings. Older and heavier nestlings also were increasingly likely to fledge. We hypothesized that nestlings might use the intensity of mass loss to gauge parental reluctance to provision, and accordingly, nestlings were increasingly likely to fledge after experiencing a greater amount of mass loss. Copyright 1999 The Association for the Study of Animal Behaviour.     

Sex‐Specific Parental Care in Response to Predation Risk in the European Roller,Coracias garrulus

Sublethal effects of predation constitute an important part of predation effects, which may modulate prey population and community dynamics. In birds, the risk of nest predation may cause a reduction in parental activity in the care of offspring to reduce the chance of being detected by predators. In addition, parents may modify their parental food allocation preferences within the brood in response to predation risk. Our aim in this study was to evaluate the effects of risk of nest predation on parental care and within‐nest food allocation in the European Roller (Coracias garrulus), an asynchronously hatching bird. We manipulated brood predation risk by placing a snake model near the nests that simulates the most common nest predator in the Mediterranean region. Our results show that males but not females increased their provisioning rate when they were exposed to the model and that despite this, nestlings’ body mass decreased in response to this temporary increase in predation risk. We did not find evidence that parents changed their food allocation strategy towards senior or junior nestlings in their nests in response to predation risk. These results show that the European roller modifies parental care in response to their perception of predation risk in the nest and a sex‐specific sensitivity to the threat, which suggests a different perception of offspring reproductive value by parents. Finally, our results show that changes in parental behaviour in response to nest predation risk might have consequences for nestling fitness prospects.     

Parent birds assess nest predation risk and adjust their reproductive strategies

Avian life history theory has long assumed that nest predation plays a minor role in shaping reproductive strategies. Yet, this assumption remains conspicuously untested by broad experiments that alter environmental risk of nest predation, despite the fact that nest predation is a major source of reproductive failure. Here, we examined whether parents can assess experimentally reduced nest predation risk and alter their reproductive strategies. We experimentally reduced nest predation risk and show that in safer environments parents increased investment in young through increased egg size, clutch mass, and the rate they fed nestlings. Parents also increased investment in female condition by increasing the rates that males fed incubating females at the nest, and decreasing the time that females spent incubating. These results demonstrate that birds can assess nest predation risk at large and that nest predation plays a key role in the expression of avian reproductive strategies.     

Parental Care Syndromes in House Sparrows: Positive Covariance Between Provisioning and Defense Linked to Parent Identity

Parents of many species provide multiple forms of care to their offspring. In many birds, parents often provision offspring with food and defend them from predators and/or nest‐site competitors. We tested how these two forms of parental care covary in a wild population of house sparrows (Passer domesticus). Using a behavioral reaction norm approach, we found that nestling provisioning exhibited between individual differences and positively covaried with a measure of nest defense: the propensity to attack a heterospecific nest box competitor, the European starling (Sturnis vulgaris). This result would seem to support parental investment theory and suggests that high‐provisioning parents have high‐value offspring, which they will defend more vigorously than low‐provisioning parents. In addition, we found that parents with nestlings that hatched earlier in the season and grew faster approached a model starling more frequently and tended to be more likely to strike the starling. However, we also found that although brood value explained significant variation in both nestling provisioning and nest defense, it did not eliminate the positive, between‐individual relationship between provisioning and defense. This suggests that some of the correlation between provisioning and defense is tied to individual identity and hence may be a behavioral syndrome in which differences between individuals in underlying attributes produce correlated behaviors.     

Parental allocation of food to nestling tree swallows: the influence of nestling behaviour, sex and paternity

Parents are expected to invest more in young that provide the greatest fitness returns. The cues that parents use to allocate resources between their offspring have received much recent attention. In birds, parents may use begging intensity, position in the nest or nestling size as cues to provision the most competitive young or those most likely to survive. It may also benefit parents to invest in young differentially by sex or relatedness if the fitness returns of sons and daughters differ or broods are sired by multiple males. We examined the allocation of food to tree swallow, Tachycineta bicolor, nestlings in relation to their begging behaviour, size, sex and paternity. Provisioning by parents was not related to nestling size, sex or paternity. The begging behaviour of nestlings did not differ with respect to sex or paternity. Both parents were more likely to feed nestlings that begged first or were closer to the nest entrance, suggesting that parents allocate food resources in response to cues that nestlings control. As a consequence, brood reduction was facilitated by biased provisioning within the brood in addition to the nestling size hierarchies created by hatching asynchrony. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Sex-dependent risk taking in the collared flycatcher, Ficedula albicollis, when exposed to a predator at the nestling stage

An increased mortality rate is a cost of parental care, and can be high during the provisioning phase of altricial nestlings. When a parent stops feeding the nestlings temporarily after seeing a predator, it can reduce its own predation risk, but the suspension of parental care may also reduce its offspring's chances of surviving. We modelled this situation by exposing a stuffed sparrowhawk near collared flycatcher nests and removing it when both parents had seen it. We measured the time (return time) between the removal and when each parent entered the nestbox. The parents' risk taking and the return time are assumed to be inversely related. We studied which brood variables the parents take into account when deciding how much risk they are willing to take during the provisioning period. Males took more risk for older and better-quality nestlings and earlier broods. The females' behaviour was opposite to that of the males: they took significantly less risk for older and better-quality offspring and visited the nestbox later for earlier broods. The males' behaviour supported the reproductive value hypothesis, that risk taking is related to brood value and survival chances, whereas the females' behaviour supported the harm to offspring hypothesis, that risk taking is related to the broods' vulnerability. Copyright 2000 The Association for the Study of Animal Behaviour.     

Reduced nestling growth of East African Stonechats Saxicola torquata axillaris in the presence of a predator

We investigated nestling growth of tropical East African Stonechats Saxicola torquata axillaris to evaluate the effects of nest predation, predator presence and food availability. We provided some Stonechat pairs with supplemental food, while others in a similar habitat served as a control. Concomitantly, we assessed the presence of Fiscal Shrikes Lanius collaris in supplemental fed and unsupplemented territories. Fiscal Shrikes prey on adult Stonechats and nestlings. We found that nestling growth was considerably reduced in Stonechat pairs that shared their territory with a Shrike. This effect was greater in nestlings of pairs that did not receive supplemental food. The reduction in nestling growth rates was significantly correlated with a reduced rate of visiting by the parents. Behavioural observations further suggested that parents reduced their feeding visits to the nest presumably to minimize their own predation risk, rather than predation risk of their brood. Our experiments show that the lower reproductive investment in tropical Stonechats can be attributed to risk-sensitive behaviour of the parents, especially when food is in limited supply.     

Darwin's Finch Begging Intensity Does Not Honestly Signal Need in Parasitised Nests

Parental care should be selected to respond to honest cues that increase offspring survival. When offspring are parasitised, the parental food compensation hypothesis predicts that parents can provision extra food to compensate for energy loss due to parasitism. Chick begging behaviour is a possible mechanism to solicit increased feeding from attending parents. We experimentally manipulated parasite intensity from Philornis downsi in nests of Darwin's small ground finch (Geospiza fuliginosa) to test its effects on chick begging intensity and parental food provisioning. We used in‐nest video recordings of individually marked chicks to quantify nocturnal parasite feeding on chicks, subsequent diurnal chick begging intensity and parental feeding care. Our video analysis showed that one chick per brood had the highest parasite intensity during the night (supporting the tasty chick hypothesis) and weakest begging intensity during the day, which correlated with low parental care and rapid death. We observed sequential chick death on different days rather than total brood loss on a given day. Our within‐nest video images showed that (1) high nocturnal larval feeding correlated with low diurnal begging intensity and (2) parent birds ignored weakly begging chicks and provisioned strongly begging chicks. Excluding predation, all parasite‐free chicks survived (100% survival) and all parasitised chicks died in the nest (100% mortality). Weak begging intensity in parasitised chicks, which honestly signalled recent parasite attack, was not used as a cue for parental provisioning. Parents consistently responded to the strongest chick in both parasitised and parasite‐free nests.     

Plasticity of parental care under the risk of predation: how much should parents reduce care?

Predation can be an important agent of natural selection shaping parental care behaviours, and can also favour behavioural plasticity. Parent birds often decrease the rate that they visit the nest to provision offspring when perceived risk is high. Yet, the plasticity of such responses may differ among species as a function of either their relative risk of predation, or the mean rate of provisioning. Here, we report parental provisioning responses to experimental increases in the perceived risk of predation. We tested responses of 10 species of bird in north temperate Arizona and subtropical Argentina that differed in their ambient risk of predation. All species decreased provisioning rates in response to the nest predator but not to a control. However, provisioning rates decreased more in species that had greater ambient risk of predation on natural nests. These results support theoretical predictions that the extent of plasticity of a trait that is sensitive to nest predation risk should vary among species in accordance with predation risk.     

Predator density influences nest attendance of Yellow‐headed Blackbirds Xanthocephalus xanthocephalus

Nest attendance behaviour in birds is a function of the careful balance between the risk of nest predation and the needs of the parents and nestlings. This attendance must be carefully regulated, as increased parental activity at the nest increases nest predation risk. We tested the long‐standing hypothesis that nest predation risk influences parental behaviour by evaluating the influence of local Marsh Wren Cistothorus palustris density on the off‐bout frequency of Yellow‐headed Blackbirds Xanthocephalus xanthocephalus. Marsh Wren density was negatively correlated with Yellow‐headed Blackbird off‐bout frequency during the morning (05:00–10:00 h) and evening (16:00–21:00 h), suggesting that Yellow‐headed Blackbirds alter their nest attendance behaviour in response to a perceived increased risk of nest predation. We suggest that Yellow‐headed Blackbirds are sensitive to nest predation risk and alter their behaviour accordingly to increase overall fitness, although future research is needed to evaluate the influence of Marsh Wren nest predation on the reproductive success of Yellow‐headed Blackbirds.     

Anthropogenically-Mediated Density Dependence in a Declining Farmland Bird

Land management intrinsically influences the distribution of animals and can consequently alter the potential for density-dependent processes to act within populations. For declining species, high densities of breeding territories are typically considered to represent productive populations. However, as density-dependent effects of food limitation or predator pressure may occur (especially when species are dependent upon separate nesting and foraging habitats), high territory density may limit per-capita productivity. Here, we use a declining but widespread European farmland bird, the yellowhammer Emberiza citrinella L., as a model system to test whether higher territory densities result in lower fledging success, parental provisioning rates or nestling growth rates compared to lower densities. Organic landscapes held higher territory densities, but nests on organic farms fledged fewer nestlings, translating to a 5 times higher rate of population shrinkage on organic farms compared to conventional. In addition, when parental provisioning behaviour was not restricted by predation risk (i.e., at times of low corvid activity), nestling provisioning rates were higher at lower territory densities, resulting in a much greater increase in nestling mass in low density areas, suggesting that food limitation occurred at high densities. These findings in turn suggest an ecological trap, whereby preferred nesting habitat does not provide sufficient food for rearing nestlings at high population density, creating a population sink. Habitat management for farmland birds should focus not simply on creating a high nesting density, but also on ensuring heterogeneous habitats to provide food resources in close proximity to nesting birds, even if this occurs through potentially restricting overall nest density but increasing population-level breeding success.     

Nest predation and habitat change interact to influence Siberian jay numbers

We studied Siberian jays, breeding in northern Sweden, to examine the potential for interactions between nest predation and reduced vegetation heterogeneity around nest sites to cause a decrease in jay numbers. Parent behaviour and nests are highly cryptic in the species. Our 12-year data showed, however, that nests had a probability of only 0.46 to be successful and produce at least one nestling. Nest predation was intense and a main cause of nest failure. All predators that could be identified were visually oriented hunters, mostly other corvids able to colonize taiga forest only close to human settlements. Consistent with the idea that predators used visual cues, nest predation increased with parental activity, which suggests that predators used parental provisioning trips to locate nests. Furthermore, a reduction in daily nest survival rates with decreasing amount of nesting cover was more pronounced in areas with high corvid activity as predicted when cover mediates the hunting efficiency of visual oriented predators. Declining temperatures interacted with the effects of habitat characteristics to further reduce daily nest survival rates suggesting that parents were not able to increase nest visitation rates to satisfy the higher energy demands of their nestlings without endangering the nest. Our results identify a mechanism through which predation and human-induced reduction in nesting cover on a larger scale may interact to cause a reduction in Siberian jay numbers larger than expected from habitat loss alone.     

Kea Nestor notabilis mothers produce nest-specific calls with low amplitude and high entropy

Vocal behaviour of nesting altricial birds is subject to selection pressure from several sources. Offspring beg to attract parents’ attention, thus increasing the chances of being fed, but also increasing the chances of being detected by predators. Research on passerines has shown that parents may reduce the risk of nest predation by alarm calling to warn nestlings to be quiet, and by producing food calls which solicit begging when parents are present to defend the nestlings. Both nestlings and parents may reduce the risk of predator detection by producing calls of low amplitude and high entropy which are acoustically difficult to locate. Although extensive research has been undertaken on nesting passerine vocalizations, little is known about parrots in this regard, and studies are needed to determine whether parrots show similar adaptations. We investigated the calling behaviour of Kea Nestor notabilis mothers during the nesting period to determine whether maternal vocalizations were adapted in a way that could increase the chance of brood success. A microphone was installed inside the nest to record calls produced both inside the nest and in the direct vicinity. Our prediction was that calls outside the nest would be easy to locate and could function as alarm calls to alert conspecifics or distract the predator, whereas calls inside the nest would be difficult to locate and could serve to communicate with nestlings without alerting predators. Our results accorded with these predictions. Calls produced outside the nest were loud and tonal, and corresponded to previously described Kea alarm calls. Calls produced inside the nest, however, were high-entropy and low-amplitude calls, and formed a distinct structural category. We thus provide the first evidence that a parrot species has a vocal category for communication inside the nest, and that calls within this category are structured in a way that could reduce the risk of nest predation.     

Effect of alarm calling by male Red‐winged Blackbirds on nestling begging and female provisioning behavior

ABSTRACT Nestling begging and parental provisioning can attract nest predators and reduce reproductive success, so parents and their offspring might be expected to respond adaptively by minimizing predator‐attracting cues when predators threaten nests. Male Red‐winged Blackbirds (Agelaius phoeniceus) are well known for their antipredator alarm calls that contain information about the approach of potential nest predators. We examined the begging behavior of nestlings and the provisioning behavior of females in response to antipredator alarm calls of males to test the adaptive response hypothesis. Playback experiments provided no evidence that alarm calls function to switch off vocal begging; nestlings were equally likely to beg vocally during playback and control periods. Video recordings showed that male alarm calling had no significant effect on inappropriate vocal begging (in the absence of an adult), but significantly reduced the incidence of spontaneous calling (in the absence of begging). Adult females responded to male antipredator alarm calls by delaying their provisioning visits. In addition, although having no significant effect on use of nest‐arriving calls by females, male alarm calling significantly reduced their use of nest‐leaving calls. We conclude that nestling and female Red‐winged Blackbirds respond to male alarm calls in ways that might reduce the risk of predation, but nestlings beg vocally when females arrive to feed them, regardless of male alarm calling, perhaps to avoid a competitive disadvantage with broodmates.     

Predator-induced plasticity in nest visitation rates in the Siberian jay (Perisoreus infaustus)

Bird nestlings may be at risk not only from starvation but alsofrom predators attracted to the nest by parental feeding visits.Hence, parents could trade reduced visitation rates for a lowerpredation risk. Here, through field data and an experiment,we show plasticity in daily patterns of nest visitation in theSiberian jay, Perisoreus infaustus, in response to predatoractivity. In high-risk territories, jay parents avoided goingto the nest at certain times of the day and compensated by allocatingmore feeding effort to periods when predators were less active.Such modifications in provisioning routines allowed parentsin high-risk habitat to significantly lower the risk of providingvisitation cues to visually oriented corvid nest predators.These results indicate that some birds modify their daily nestvisitation patterns as a fourth mechanism to reduce predator-attractingnest visits in addition to the clutch size reduction, maximizationof food load-sizes, and prevention of allofeeding suggestedby Skutch.     

Migrants in tropical bird communities: the balanced breeding limitation hypothesis

Explanations for the ecological integration of migratory and non-migratory (resident) insectivorous birds in the tropics have been complicated by the paradox that arthropod abundances are low when bird abundances reach their annual peak. The breeding currency hypothesis and the nest predation hypothesis both account for this paradox by postulating that residents are held below the non-breeding season carrying capacity, which frees resources available for migratory insectivores. The breeding currency hypothesis suggests residents are limited by food suitable for nestlings, whereas the nest predation hypothesis emphasizes the primacy of high rates of nest predation. However, theoretical arguments suggest that food availability and predation risk interact strongly to limit breeding birds. We use graphical analyses to extend the breeding currency hypothesis to incorporate effects of nest predation. This yields a more synthetic and realistic model for the integration of migrant and resident insectivores in the tropics – the balanced breeding limitation hypothesis.     

Resource allocation between growth and endothermy allows rapid nestling development at low feeding rates in a species under high nest predation

Nestling development is among the most energy‐demanding periods of a bird’s lifetime and altricial species require extensive parental energy investment in the form of feeding and heating. In the present study I analyze the relation and trade‐offs between nestling growth, development of thermoregulation and feeding rate in blackcap Sylvia atricapilla, a species suffering from high nest predation. Nestlings were characterized by rapid growth but they achieved only 80% of adult mass prior to fledging. Body mass showed highest relative growth rate before nestlings achieved homeothermy. The onset of endothermy, indicated in day 7, coincided with 90% of nestling fledgling weight, indicating that the two processes are separated in time. A strong negative correlation between feeding rate and growth rate demonstrates that blackcap nestlings develop their bodies under relatively low feeding rates and more feeding is needed for maintenance of body temperature than for body growth. The study indicates high cost of endothermy for parents – endothermic nestlings received over 100% more feedings than ectothermic ones. The findings are discussed in the light of adaptation of the species to high predation risk.     

Azure‐winged Magpies Cyanopica cyanus trade off reproductive success and parental care by establishing a size hierarchy among nestlings

It is common in birds that the sizes of nestlings vary greatly when multiple young are produced in one nest. However, the methods used by parents to establish size hierarchy among nestlings and their effect on parental provisioning pattern may differ between species. In the Azure‐winged Magpie Cyanopica cyanus, we explored how and why parents controlled the sizes of nestlings. Asynchronous hatching was the main cause of size hierarchy within the brood, although the laying of larger eggs later in the laying sequence reduced this effect. Parents with asynchronous broods produced more eggs and fledged more nestlings than those with synchronous broods but their brood provisioning rates, food delivery per feeding bout and feeding efficiency did not differ. We performed a cross‐fostering experiment to synchronize some asynchronous broods. Provisioning rates of asynchronous broods were lower than those of synchronized broods, but the daily growth rates and fledging body mass of their nestlings were not different. Our findings indicate that parents of asynchronous broods can achieve higher reproductive success than those of synchronous broods based on the same parental care, and the same reproductive success as those of synchronized broods based on less parental care. It appears that parent birds can better trade off reproductive success and parental care by establishing a size hierarchy among nestlings.     

Manipulation of hunger levels affects great spotted cuckoo and magpie host nestlings differently

Brood parasitic nestlings usually exhibit an exaggerated begging behaviour, which is mainly attributed to reduced inclusive fitness costs since they typically share the nest with unrelated individuals. However, energetic costs also constrain begging expression and accordingly a relation between food requirements and intensity of begging behaviour could also exist in brood parasites, just as in nesting bird species. Here, we tested this hypothesis in the great spotted cuckoo Clamator glandarius and its main host, the magpie Pica pica, by studying the effect of an appetite enhancer, cyproheptadine hydrochloride, on nestling provisioning and development (size, body mass and cell‐mediated immune response). To study nestling provisioning, neck‐collars were meticulously placed around nestling necks allowing normal respiration but avoiding the ingestion of food delivered by adult magpies during ca 2.5 h. Loss in body mass during neck‐collar trials was used as a proxy for energetic begging costs, while the amount of food received during these trials and growth during the whole nestling period were used as variables reflecting short‐ and long‐term effects of the experimental treatment. During neck‐collar trials, we found that experimental nestlings of both species received more food than control nestlings. However, experimental magpies, but not cuckoos, lost more body mass than control nestlings. These results suggest a short‐term beneficial effect of an escalated begging behaviour in both species that would be energetically cheaper for cuckoos than for magpies. We found positive long‐term effects of the appetite enhancer only in magpies (in terms of tarsus and wing length at fledging, but not in terms of immune response and body mass); suggesting that exaggerated begging would be beneficial for hosts only. We discuss the possible effect of begging behaviour on the risk of predation and on inclusive fitness, but also the possibility that our results may be explained by some kind of limitation in the capability of food assimilation by parasitic species.     

Nestling presence affects the anti-predator response of adult superb fairy-wrens (Malurus cyaneus)

Nest predation accounts for the majority of nesting failure, and hence there has been strong selection on behaviour to reduce nest predation, including patterns of nest defence. Here, we test risk-taking behaviour to experimentally placed predators in the presence or absence of nestlings. We use the superb fairy-wren (Malurus cyaneus) to test the prediction that vigilance will be comparable with and without nestlings, but that alarm vocalisations will increase when nestlings are present. We found support for both predictions. Adult vigilance of predators was comparable with and without nestlings. Alarm vocalisation intensity was predicted by predator type and nestling presence, and was highest to the snake model in the presence of nestlings. These results point to selection for differentiated nest defence response in relation to predator type and the presence or absence of nestlings.