Comparative manipulation of predation risk in incubating birds reveals variability in the plasticity of responses

The evolution of different parental care strategies is thoughtto result from variation in trade-offs between the costs andbenefits associated with providing care. However, changingenvironmental conditions can alter such fitness trade-offsand favor plasticity in the type or amount of parental care provided. Avian incubation is a form of parental care whereparents face changing environmental conditions, including variationin the risk of nest predation. Because parental activity candraw attention to the location of the nest, a reduction innest visitation rates is a predicted response to an increased,immediate predation risk. Here, we experimentally increasedthe risk of nest predation using model presentations at nestsof five coexisting species that differ in their ambient levelsof nest predation. We examined whether individuals detect changesin nest predation risk and respond by reducing visitation tothe nest. We also tested whether this behavioral response differsamong species relative to differences in their ambient risk of nest predation. We found that males of all species detectedthe increased predation risk and reduced the rate at whichthey visited the nest to feed incubating females, and the magnitudeof this change was highly correlated with differences in therisk of nest predation across species. Hence, as the vulnerabilityto nest predation increases, males appear more willing to trade the cost of reduced food delivery to the female against thebenefit of reduced predation risk. Our results therefore suggestthat nest predators can have differential effects on parentalbehaviors across species. We discuss how the comparative natureof our results can also provide insight into the evolution of behavioral plasticity.     

Nest Crypsis, Reproductive Value of a Clutch and Escape Decisions in Incubating Female Mallards Anas platyrhynchos

In cryptically coloured birds, remaining on the nest despite predator approach (risk‐taking) may decrease the likelihood that the nest will be detected and current reproductive attempt lost. By contrast, flushing may immediately reveal the nest location to the predator. Escape decisions of incubating parents should therefore be optimized based on the risk‐to‐parent/cost of escape equilibrium. Animal prey may assess predation risk depending on a variety of cues, including the camouflage that vegetation provides against the predator. We examined interactive effects of nest crypsis and the current reproductive value of a clutch on flushing distances in incubating mallards (Anas platyrhynchos) approached by a human. Our results were consistent with predictions of parental investment theory: flushing distances were inversely correlated with measures of the reproductive value of the current clutch, namely with clutch size, stage of incubation and mean egg volume. Independently of a reproductive value of a clutch, nest concealment explained a significant portion of the variation in flushing distance among females; individual females tended to increase/decrease flushing distances according to change in nest cover. The results further suggest that vegetation concealment greatly influenced the risk of nest detection by local predators, suggesting that vegetation may act as a protective cover for incubating female. A female's ability to delay flushes according to the actual vegetation cover might thus be viewed as an antipredator strategy that reduces premature nest advertising to visually oriented predators. We argue, however, that shorter flying distances from densely covered sites might be maladaptive in areas where a predator's ability to detect incubating female does not rely on visual cues of nests.     

Parental investment strategies in two species of nuthatch vary with stage-specific predation risk and reproductive effort

Life-history theory predicts that differences in reproductive effort and residual reproductive value among species should result in differences in the level of risk that parents are willing to tolerate to themselves versus their offspring. Specifically, highly fecund and shorter-lived species are expected to place greater value in current offspring than themselves, whereas less fecund and longer-lived species are expected to place greater value in their own survival and future breeding opportunities. Here, we test the prediction that parental investment decisions are correlated with life histories by comparing risk-taking behaviour in two species of nuthatch that differ in reproductive effort: the white-breasted nuthatch, Sitta carolinensis (more fecund, lower survival) and the red-breasted nuthatch, S. canadensis (less fecund, higher survival). We experimentally manipulated stage-specific predation risk by presenting models of an adult predator (hawk) and an egg predator (wren) and measured the willingness of males to feed incubating females on the nest. We found that both species of nuthatch responded to predators by increasing the length of time between visits and aborting more visits to the nest. However, as predicted by their life histories, S. carolinensis displayed a significantly stronger response to the egg predator, whereas S. canadensis responded more strongly to the adult predator. Thus, species can differ in their willingness to tolerate risk to themselves and their young, and such differences appear to be related to differences in investment in current reproduction and the probability of future survival. Copyright 2000 The Association for the Study of Animal Behaviour.     

Within‐season increase in parental investment in a long‐lived bird species: investment shifts to maximize successful reproduction?

In nest‐building species predation of nest contents is a main cause of reproductive failure and parents have to trade off reproductive investment against antipredatory behaviours. While this trade‐off is modified by lifespan (short‐lived species prioritize current reproduction; long‐lived species prioritize future reproduction), it may vary within a breeding season, but this idea has only been tested in short‐lived species. Yet, life history theory does not make any prediction how long‐lived species should trade off current against future reproductive investment within a season. Here, we investigated this trade‐off through predator‐exposure experiments in a long‐lived bird species, the brown thornbill. We exposed breeding pairs that had no prior within‐season reproductive success to the models of a nest predator and a predator of adults during their first or second breeding attempt. Overall, parents reduced their feeding rate in the presence of a predator, but parents feeding second broods were more risk sensitive and almost ceased feeding when exposed to both types of predators. However, during second breeding attempts, parents had larger clutches and a higher feeding rate in the absence of predators than during first breeding attempts and approached both types of predators closer when mobbing. Our results suggest that the trade‐off between reproductive investment and risk‐taking can change in a long‐lived species within a breeding season depending on both prior nest predation and renesting opportunities. These patterns correspond to those in short‐lived species, raising the question of whether a within‐season shift in reproductive investment trade‐offs is independent of lifespan.     

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.     

Increasing the perceived predation risk changes parental care in female but not in male Great Tits Parus major

In birds, little is known about how the presence of predators alters parental food distribution decisions among nestlings. We found that experimentally increasing perceived predation risk changed parental care in female but not in male Great Tits Parus major. Females fed the lightest and average nestlings at similar rates under control conditions when predation risk was not manipulated but ignored the lightest nestling under increased perceived predation risk. Moreover, females reduced the duration of nest visits greatly after encountering a model predator, suggesting that the perception of predators may facilitate brood reduction mechanisms.     

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.     

Effects of a food supplementation experiment on reproductive investment and a post-mating sexually selected trait in magpies Pica pica

Food availability is an important factor affecting breeding success in birds. Food supplementation experiments in birds have in general focused on the effects on reproductive success in terms of female investment (laying date, clutch size, egg size), however, it is also known that the estimation of mate quality based on sexually selected signals influences female reproductive investment. In the particular case of magpies, females use nest size, a post-mating sexually selected signal, to assess male's likelihood to invest in reproduction, and accordingly adjust reproductive investment (clutch size). Then, the possible effects of food supplementation on female reproductive investment could be mediated by other variables related to parental quality, such as nest size in magpies. In the present study, we explore if higher food availability in a magpie territory affected both male sexually selected traits (i.e. nest size) and female reproductive investment (laying date, egg size, clutch size). We performed a food supplementation experiment in which we experimentally increased food availability in several magpie territories, keeping others as controls. In food-supplemented territories, males built significantly larger nests and females significantly increased egg size by 4.1% compared to control females. Results suggest that the continuous provisioning of protein rich food allowed magpie females to increase egg size. However, laying date and clutch size did not differ between control and food-supplemented magpie pairs. Food availability also affected the relationship between female reproductive investment and nest size. In control territories, females decreased their egg size in response to a larger nest, whereas a tendency for the opposite relationship was revealed in food-supplemented territories. We discuss the possibility that magpie females adopt different strategies for reproductive investment according to food availability.     

Factors Affecting Sleep/vigilance Behaviour in Incubating Mallards

Vigilance is a behavioural tactic that allows individuals to control their surroundings and to assess predation risk. In contrast, sleep is unique behavioural state with widely hypothesized restorative and energy‐saving functions, but reducing attentiveness and increasing susceptibility to predation. Sleeping birds resolve this conflict by interrupting sleep with short periods of eye opening (termed ‘scans’) during vigilant sleep. Miscellaneous environmental factors and sleeping postures may affect the perception of risk and corresponding vigilance level. Here, we investigated the influence of nest vegetation concealment, time of day and sleeping postures on the sleep/vigilance trade‐off in incubating Mallards (Anas platyrhynchos). We found that incubating females increased their vigilance with increasing nest vegetation cover facing the vigilant eye during both the day and the night periods; however, mean nest vegetation concealment did not affect female vigilance. Females also reduced their total vigilance along with scan frequency during the night period, while displaying the opposite pattern during the daylight. The rest‐sleeping position was preferred more during the night compared with the daylight period, and females were more vigilant in this position at night. Our data show that the nest vegetation concealment regardless of visual abilities during different light conditions, time of day and sleeping posture play an underlying role in antipredator vigilance during sleep in this cryptic ground‐nesting bird.     

Sexual pigmentation and parental risk‐taking in yellow warblers Setophaga petechia

Adult‐directed predation risk imposes important behavioral constraints on parents and might thus alter relationships between costly sexual ornaments and parental performance. For instance, under low predation risk, highly ornamented individuals might display better parental performance than others, as predicted by ‘good parent’ models of sexual selection. However, under high risk of predation, highly ornamented individuals might abandon parental effort if conspicuous to predators, or if social partners are more willing to take parental risks when paired with highly ornamented mates. We experimentally elevated perceived adult‐directed predation risk near nests to explore how carotenoid‐ and phaeomelanin‐based pigmentation in both sexes relate to parental risk‐taking for offspring in the yellow warbler Setophaga petechia. Compared to other males, males with more intense carotenoid‐based pigmentation maintained higher levels of paternal effort under predation risk at highly concealed nests, but reduced nestling provisioning rate more at exposed nests. Further, when faced with predation risk, females with more phaeomelanin‐based pigmentation reduced nestling provisioning rate less than other females, regardless of nest concealment. Females displayed higher parental effort across treatments when paired to males with more colorful carotenoid pigmentation. However, birds did not reduce parental effort under risk less when paired to a highly ornamented mate, suggesting that predation risk did not accentuate differential allocation. Males did not take fewer parental risks than females. Results indicate that nest concealment modifies parental risk‐taking by males with colorful carotenoid‐based pigmentation, and suggest that female melanin‐based pigmentation may indicate boldness and greater a propensity to take parental risks.     

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.     

Is hissing behaviour of incubating great tits related to reproductive investment in the wild?

Anti-predator behaviour of breeding animals is a complex trait that depends on current reproductive investment as well as individual differences in risk-taking propensities. In response to nest predation, many bird species produce specific sounds, such as the hissing calls in incubating great tits (Parus major), that may provoke an acoustic startle response in the predator. However, it is still unclear whether the propensity of incubating females to produce hissing calls towards nest predators depends on the reproductive investment. With our 3-year study, we show that response type (females that do not hiss versus females giving hissing calls) to a potential nest predator, the woodpecker, is a repeatable trait. We found no differences in the studied reproductive traits between hissing and non-hissing birds. Interestingly, among hissing birds, fast-responding females started egg-laying earlier than slow-responding ones. Among non-hissing birds, heavier birds initiated clutches earlier. We also revealed that hissing birds breed in areas with decreased nest-box occupancy, suggesting either that they potentially select different areas to breed or that territory size is larger as a result of hissing birds being more aggressive. These findings demonstrate that response type is not related to the early reproductive value of the brood across distinct behavioural groups. However, our results do suggest that non-hissing and hissing females differ in terms of individual quality or dominance or personality related aspects.     

Increased nest defence of upland‐nesting ducks in response to experimentally reduced risk of nest predation

Parent birds should take greater risks defending nests that have a higher probability of success. Given high rates of mammalian nest predation, therefore, parents should risk more for nests in areas with a lower risk of mammalian predation. We tested this hypothesis using nest defence data from over 1300 nests of six species of dabbling ducks studied in an area where predation risk had been reduced through removal of mammalian predators. When predator removal reduced nest predation, the ducks increased risk taking as predicted. Also as predicted, risk taking varied inversely with body size, an index of annual survival, among species. For ducks to vary nest defence in response to variation in predation risk they must be able to assess the risk of nest predation. Because ducks modified nest defence in the breeding season immediately following predator removal, ducks may be able to assess predator abundance indirectly (e.g. by UV reflection from urine) rather than by seeing or interacting directly with the predators.     

Introduced mammalian predators induce behavioural changes in parental care in an endemic New Zealand bird

The introduction of predatory mammals to oceanic islands has led to the extinction of many endemic birds. Although introduced predators should favour changes that reduce predation risk in surviving bird species, the ability of island birds to respond to such novel changes remains unstudied. We tested whether novel predation risk imposed by introduced mammalian predators has altered the parental behaviour of the endemic New Zealand bellbird (Anthornis melanura). We examined parental behaviour of bellbirds at three woodland sites in New Zealand that differed in predation risk: 1) a mainland site with exotic predators present (high predation risk), 2) a mainland site with exotic predators experimentally removed (low risk recently) and, 3) an off-shore island where exotic predators were never introduced (low risk always). We also compared parental behaviour of bellbirds with two closely related Tasmanian honeyeaters (Phylidonyris spp.) that evolved with native nest predators (high risk always). Increased nest predation risk has been postulated to favour reduced parental activity, and we tested whether island bellbirds responded to variation in predation risk. We found that females spent more time on the nest per incubating bout with increased risk of predation, a strategy that minimised activity at the nest during incubation. Parental activity during the nestling period, measured as number of feeding visits/hr, also decreased with increasing nest predation risk across sites, and was lowest among the honeyeaters in Tasmania that evolved with native predators. These results demonstrate that some island birds are able to respond to increased risk of predation by novel predators in ways that appear adaptive. We suggest that conservation efforts may be more effective if they take advantage of the ability of island birds to respond to novel predators, especially when the elimination of exotic predators is not possible.     

Parental care of a cowbird host: caught between the costs of egg-removal and nest predation

Avian brood parasites reduce host fitness through the addition of parasitic eggs and the removal of host eggs. Both parasitic egg-addition and host egg-removal may be important sources of selection on host behaviour, creating fitness trade-offs with selection imposed by nest predation. However, the relative costs hosts suffer from egg-addition and host egg-removal and the responses to these costs are largely unstudied. Through experimental manipulations and observations, we demonstrate that increased nest attentiveness by female yellow warblers (Dendroica petechia) reduces the cost of brood parasitism by reducing egg-removal by brown-headed cowbirds (Molothrus ater). However, female attentiveness does not reduce the addition of parasitic eggs. Experimentally parasitized females respond to the threat of egg-removal by increasing nest attentiveness. Increased attentiveness, however, reduces time for females to gather food and requires males to visit the nest more often to feed incubating females. This increased activity in turn increases the risk of nest predation. Thus, brood parasitism (specifically egg-removal) and nest predation produce conflicting selection on incubation strategies, as parasitized hosts are caught between the costs of egg-removal by brood parasites, and the costs of increased nest predation if the female spends more time on the nest to reduce egg-removal.     

The bright incubate at night: sexual dichromatism and adaptive incubation division in an open-nesting shorebird

Ornamentation of parents poses a high risk for offspring because it reduces cryptic nest defence. Over a century ago, Wallace proposed that sexual dichromatism enhances crypsis of open-nesting females although subsequent studies found that dichromatism per se is not necessarily adaptive. We tested whether reduced female ornamentation in a sexually dichromatic species reduces the risk of clutch depredation and leads to adaptive parental roles in the red-capped plover Charadrius ruficapillus, a species with biparental incubation. Males had significantly brighter and redder head coloration than females. During daytime, when visually foraging predators are active, colour-matched model males incurred a higher risk of clutch depredation than females, whereas at night there was no difference in depredation risk between sexes. In turn, red-capped plovers maintained a strongly diurnal/nocturnal division of parental care during incubation, with males attending the nest largely at night when visual predators were inactive and females incubating during the day. We found support for Wallace''s conclusion that reduced female ornamentation provides a selective advantage when reproductive success is threatened by visually foraging predators. We conclude that predators may alter their prey''s parental care patterns and therefore may affect parental cooperation during care.     

Habitat selection responses of parents to offspring predation risk: an experimental test

The ability of nest predation to influence habitat settlement decisions in birds is widely debated, despite its importance in limiting fitness. Here, we experimentally manipulated nest predation risk across a landscape and asked the question, do migratory birds assess and respond to variation in nest predation risk when choosing breeding habitats? We examined habitat preference by quantifying the density and settlement date of eight species of migratory passerines breeding in areas with and without intact nest predator communities. We found consistently more individuals nesting in areas with reduced nest predation than in areas with intact predator assemblages, although predation risk had no influence on settlement or breeding phenology. Additionally, those individuals occupying safer nesting habitats exhibited increased singing activity. These findings support a causal relationship between habitat choice and nest predation risk and suggest the importance of nest predation risk in shaping avian community structure and breeding activity.     

Why do grebes cover their nests? Laboratory and field tests of two alternative hypotheses

Egg predation is a common feature influencing the reproductive success of open nesting birds. Evolutionary pressure therefore favours building cryptic, inconspicuous nests. However, these antipredatory pressures may be in conflict with thermoregulatory constraints, which select for dry nest material maintaining optimum temperature inside a nest cup during the absence of incubating parents. Here we examined possible trade-offs between nest crypsis and thermoregulation in Little Grebes (Tachybaptus ruficollis), which lay their eggs in floating nests built from wet plant material. As this species regularly covers its eggs with nest material, we experimentally examined (1) the rates of egg predation on covered and uncovered artificial nests and (2) possible thermoregulatory costs from nest covering by comparing temperature and relative humidity changes inside the nest cup. Results revealed that covering clutches is beneficial in terms of deterring predators, because uncovered eggs were more vulnerable to predation. Moreover, covering clutches also had thermoregulatory benefits because the mean temperature and relative humidity inside nest cups covered by dry or wet materials were significantly higher for covered compared to uncovered treatments. Covering clutches in Little Grebes therefore does not pose thermoregulatory costs.     

Does Threat to the Nest Affect Incubation Rhythm in a Small Passerine?

Nest predation is a crucial factor influencing breeding success in birds. One possible way to protect nests is to modify parental activity in the vicinity of the nest. Here, we provide experimental evidence for an adjustment of incubation pattern during periods when there is an increased risk of nest predation in a small passerine. We compared the behaviour of incubating meadow pipit (Anthus pratensis) females during presentations of stuffed dummies of a nest predator (the magpie Pica pica) and a harmless intruder (the crossbill Loxia curvirostra) and during an undisturbed control incubation period. Females significantly decreased their activity in the presence of the nest predator. Specifically, after being flushed out, they returned to the nest later and moved to and from the nest less than when the harmless intruder was present. These results document the ability of birds to assess the nest predation risk and adjust their appropriate incubation strategy.     

Mothers vigilantly guard nests after partial brood loss: a cue of nest predation risk in a paper wasp

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