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.     

Begging and the risk of predation in nestling birds

Theoretical models of the evolution of begging in nestling passerinesassume that begging is costly, either energetically or in termsof predation. However, few empirical measures of these costsexist. We examined whether nestling begging calls could attractpredators to nests by comparing predation rates at artificialnests with and without playbacks of tree swallow begging calls.Nests were baited with quail eggs and placed in pairs on theground or in modified nest-boxes. Nests with playbacks of beggingcalls were depredated before control nests significantly moreoften in both the ground and nest-box trials, suggesting thatpredators may use begging calls to locate nests. These resultssuggest that the risk of nest predation may be increased becauseof calling by nestlings and provide further support for theassumption that conspicuous begging is costly in terms of predation     

Synchronous provisioning increases brood survival in cooperatively breeding pied babblers

1.  Behavioural synchrony typically involves trade-offs. In the context of foraging, for example, synchrony may be suboptimal when individuals have different energy requirements but yield net benefits in terms of increased foraging success or decreased predation risk.
2.  Behavioural synchrony may also be advantageous when individuals collaborate to achieve a common goal, such as raising young. For example, in several bird species, provisioners synchronize nest-feeding visits. However, despite the apparent prevalence of provisioning synchrony, it is not known whether it is adaptive or what its function might be.
3.  Here, we propose a novel explanation for provisioning synchrony: it increases brood survival by decreasing the number of temporally separate nest visits and accordingly the chance that the nest will be detected by predators. Using cooperatively breeding pied babblers, we showed experimentally that provisioners synchronized nest visits by waiting for another provisioner before returning to the nest. Brood survival increased with provisioning synchrony. Provisioners were more likely to synchronize feeding visits for older nestlings as they were louder and possibly more conspicuous to predators. Finally, provisioners in large groups were more likely to wait for other provisioners and synchronized a higher proportion of all visits than those in smaller groups. Thus, provisioning synchrony may be one mechanism by which large groups increase brood survival in this species.
4.  This study highlights a novel strategy that birds use to increase the survival of young and demonstrates the advantages of coordinated behaviour in social species.     

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.     

Calling at a cost: elevated nestling calling attracts predators to active nests

Begging by nestling birds has been used to test evolutionary models of signalling but theory has outstripped evidence. Eavesdropping predators potentially impose a cost on begging that ensures signal honesty, yet little experimental evidence exists for such a cost at active nests because the use of artificial nests, long playback bouts and absence of parents may have exaggerated costs. We broadcast short periods (1 h) of either nestling vocalizations or background noise at active white-browed scrubwren, Sericornis frontalis, nests. Nestlings called naturally during both treatments, allowing us to test whether elevated calling increases risk, a key but rarely tested assumption of evolutionary models. Predators visited nests exclusively during periods of elevated calling. Furthermore, playbacks affected neither adult visits nor nestling activity, suggesting that calling alone attracted predators. Adults gave alarm calls and nestlings usually called less when predators approached nests. Predation risk to broods is, therefore, likely to fluctuate substantially over short periods of time, depending on nestling hunger and whether adults or young have detected predators. This study confirms a present-day cost of nestling begging, demonstrates that this cost can be incurred over short periods and supports the importance of parent-offspring antipredator strategies in reducing predation risk.     

Nest defence by Magpies(Pica pica) and the brood parasitic Great Spotted Cuckoos(Clamator glandarius) in parasitized and unparasitized nests

Summary Nest defence is a frequent and widespread parental behaviour which enhances brood survival. We have found that in a Spanish Magpie population which is heavily parasitized by the brood parasitic Great Spotted Cuckoo, Magpies defend (1) unparasitized more frequently than parasitized nests, and (2) at the end of the nestling period more frequently than in other stages of the breeding cycle. Great Spotted Cuckoos are brood parasites, which means that their eggs are incubated and their nestlings are raised by members of a host species. Brood parasites are not thought to take care of their own offspring. However, we have found that Great Spotted Cuckoos sometimes scolded us on our regular visits to parasitized magpie nests (but never on those to unparasitized nests). Frequency of nest defence by cuckoos differed significantly among years, being significantly higher at the beginning of the study. Although sporadic observations of adult brood parasites feeding juveniles have been recorded, nest defence has not previously been suggested for any brood parasite.

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Nestverteidigung von durch den Häherkuckuck(Clamator glandarius) parasitierten und unparasitierten Nestern bei Elstern(Pica pica)

Zusammenfassung Nestverteidigung ist ein häufiges und weit verbreitetes elterliches Verhalten zur Erhöhung des Bruterfolges. In einer spanischen Elsterpopulation, die sehr intensiv vom Häherkuckuck parasitiert ist, wurden unparasitierte Nester häufiger verteidigt als parasitierte, und zum Ende der Nestlingsperiode wurden Nestern häufiger verteidigt als zu früheren Phasen des Brutzyklus. Häherkuckucke sind Brutparasiten, deren Eier von den Wirtseltern bebrütet und die Nestlinge von ihnen aufgezogen werden. Solche Brutparasiten kümmern sich im allgemeinen nicht selbst um ihre Nachkommen. Manchmal jedoch haßten Häherkuckuck auf uns, wenn wir parasitierte Nester der Elster kontrollierten, während an unparasitierten Nestern ein solches Hassen niemals erfolgte. Die Häufigkeit dieser Nestverteidugung der Kuckucke variierte zwischen Jahren und war signifikant häufiger zu Beginn unserer Untersuchung. Zwar wurde gelegentlich schon Füttern der Jungvögel durch elterliche Brutparasiten beobachtet, die hier festgestellte Nestverteidung ist bisher aber von keinem Brutparasiten beschrieben.

     

Variation in maternal effects and embryonic development rates among passerine species

Embryonic development rates are reflected by the length of incubation period in birds, and these vary substantially among species within and among geographical regions. The incubation periods are consistently shorter in North America (Arizona study site) than in tropical (Venezuela) and subtropical (Argentina) South America based on the study of 83 passerine species in 17 clades. Parents, mothers in particular, may influence incubation periods and resulting offspring quality through proximate pathways, while variation in maternal strategies among species can result from selection by adult and offspring mortality. Parents of long-lived species, as is common in the tropics and subtropics, may be under selection to minimize costs to themselves during incubation. Indeed, time spent incubating is often lower in the tropical and subtropical species than the related north temperate species, causing cooler average egg temperatures in the southern regions. Decreased egg temperatures result in longer incubation periods and reflect a cost imposed on offspring by parents because energy cost to the embryo and risk of offspring predation are both increased. Mothers may adjust egg size and constituents as a means to partially offset such costs. For example, reduced androgen concentrations in egg yolks may slow development rates, but may enhance offspring quality through physiological trade-offs that may be particularly beneficial in longer-lived species, as in the tropics and subtropics. We provide initial data to show that yolks of tropical birds contain substantially lower concentrations of growth-promoting androgens than north temperate relatives. Thus, maternal (and parental) effects on embryonic development rates may include contrasting and complementary proximate influences on offspring quality and deserve further field study among species.