Prenatal learning in an Australian songbird: habituation and individual discrimination in superb fairy-wren embryos

Embryos were traditionally considered to possess limited learning abilities because of the immaturity of their developing brains. By contrast, neonates from diverse species show behaviours dependent on prior embryonic experience. Stimulus discrimination is a key component of learning and has been shown by a handful of studies in non-human embryos. Superb fairy-wren embryos (Malurus cyaneus) learn a vocal password that has been taught to them by the attending female during incubation. The fairy-wren embryos use the learned element as their begging call after hatching to solicit more parental feeding. In this study, we test whether superb fairy-wren embryos have the capacity to discriminate between acoustical stimuli and whether they show non-associative learning. We measured embryonic heart rate response using a habituation/dishabituation paradigm with eggs sourced from nests in the wild. Fairy-wren embryos lowered their heart rate in response to the broadcasts of conspecific versus heterospecific calls, and in response to the calls of novel conspecific individuals. Thus, fairy-wrens join humans as vocal-learning species with known prenatal learning and individual discrimination.     

Differences in the nestling begging calls of hosts and host-races of the common cuckoo, Cuculus canorus

We compared nestling begging calls of four hosts (reed warbler, Acrocephalus scirpaceus; great reed warbler, A. arundinaceus; dunnock, Prunella modularis; and meadow pipit, Anthus pratensis) and the respective host-races of the common cuckoo. Note structure varied between host species, but not between cuckoo host-races, so cuckoos did not vary their call note structure to match that of their hosts' chicks. Call rate increased with age, but there were marked differences between both host species and cuckoo host-races. Dunnock-cuckoos called more rapidly than reed warbler-cuckoos despite growing at the same rate. We suggest this difference reflects how cuckoos tune into the way these host species respond to begging signals from their own young, because dunnock chicks called much more rapidly than reed warbler chicks. Great reed warbler-cuckoos called at a lower rate than reed warbler-cuckoos when young, but at a greater rate when older than 8 days. This could also result from the cuckoo chicks tuning into differences in the way these hosts respond to begging signals. However, great reed warbler-cuckoos grew at a faster rate than the other cuckoo host-races, so they may also call faster to demand higher provisioning rates from this larger host. To test these hypotheses critically, data are needed on how the different host species integrate visual and vocal begging signals from their own broods. We discuss how differences in cuckoo begging might develop, given that cuckoo host-races are restricted to female cuckoo lineages. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Nestling cuckoos,Cuculus canorus,exploit hosts with begging calls that mimic a brood

Nestling cuckoos, Cuculus canorus, eject host eggs or young from the nest and are then raised alone by the hosts. Using reed warblers, Acrocephalus scirpaceus, as hosts, we investigated how the single cuckoo chick can command the same provisioning rate as a whole brood of host young. Large size alone is not sufficient to stimulate adequate provisioning because single blackbird, Turdus merula, or song thrush, T. philomelos, chicks of the same mass as a cuckoo were fed at a lower rate. Our experiments show that the key stimulus is the cuckoo chick''s rapid begging call (''si, si, si, si ...''), which sounds remarkably like a whole brood of host chicks, and which it matched in calling rate. When single blackbird or song thrush chicks were accompanied by loudspeakers that broadcast either cuckoo begging calls or calls of a brood of reed warblers, the hosts increased their provisioning rate to that for a cuckoo chick. We suggest that the cuckoo needs vocal trickery to stimulate adequate care to compensate for the fact that it presents a visual stimulus of just one gape.     

Eggshell characteristics and yolk composition in the common cuckoo Cuculus canorus: are they adapted to brood parasitism?

The developmental rate of cuckoo embryos and their hatching size is greater than that of host species, which may require more nutrient resources in the egg and more intensive gas exchange during development. In the present study, we compared various egg characteristics of a brood parasite, the common cuckoo Cuculus canorus, and its frequent host, the great reed warbler Acrocephalus arundinaceus. As maternally‐derived testosterone is known to enhance growth rate of embryos and hatchlings, cuckoo eggs are expected to contain higher concentration of testosterone than host eggs. In addition, we expected higher concentration of antioxidants in cuckoo eggs to protect embryos from oxidative stress associated with accelerated growth. Our results showed that cuckoo eggs had thicker shells and higher pore density than great reed warbler eggs. Yolk was significantly heavier in cuckoo eggs and contained higher concentrations of carotenoids and vitamin E, however, yolk androgen and immunoglobulin concentrations were lower in cuckoo eggs as compared to great reed warbler eggs. We also examined whether eggshell colour was associated to egg quality, and detected a positive association between blue‐green chroma and yolk antioxidant concentration in both species, suggesting that eggshell colour reflects the antioxidant investment of the female into the eggs. Our results suggest that cuckoo females increase the size, growth rate and competitive ability of their young by providing them with more nutrients and more dietary antioxidants for embryonic development, and not through elevated yolk testosterone or antibody levels. In addition, increased porosity of cuckoo eggshells may allow embryos to develop more rapidly because of a greater capacity of gas exchange.     

Imperfect mimicry of host begging calls by a brood parasitic cuckoo: a cue for nestling rejection by hosts?

Coevolutionary interactions between avian brood parasites and their hosts often lead to the evolution of discrimination and rejection of parasite eggs or chicks by hosts based on visual cues, and the evolution of visual mimicry of host eggs or chicks by brood parasites. Hosts may also base rejection of brood parasite nestlings on vocal cues, which would in turn select for mimicry of host begging calls in brood parasite chicks. In cuckoos that exploit multiple hosts with different begging calls, call structure may be plastic, allowing nestlings to modify their calls to match those of their various hosts, or fixed, in which case we would predict either imperfect mimicry or divergence of the species into host-specific lineages. In our study of the little bronze-cuckoo (LBC) Chalcites minutillus and its primary host, the large-billed gerygone Gerygone magnirostris, we tested whether: (1) hosts use nestling vocalizations as a cue to discriminate cuckoo chicks; (2) cuckoo nestlings mimic the host begging calls throughout the nestling period; and (3) the cuckoo begging calls are plastic, thereby facilitating mimicry of the calls of different hosts. We found that the begging calls of LBCs are most similar to their gerygone hosts shortly after hatching (when rejection by hosts typically occurs) but become less similar as cuckoo chicks get older. Begging call structure may be used as a cue for rejection by hosts, and these results are consistent with gerygone defenses selecting for age-specific vocal mimicry in cuckoo chicks. We found no evidence that LBC begging calls were plastic.     

Hopeless solicitation? Host-absent vocalization in the common cuckoo has no effect on feeding rate of reed warblers

Begging behavior of nestlings can signal both hunger and competitive ability. Studies of begging in evicting avian brood parasites exclude the influence of nestling competition and may provide new insights into the host–parasite conflict and the evolution of signaling. Apart from the begging call, common cuckoo Cuculus canorus nestlings use special vocal displays in the absence of their hosts, termed here host-absent vocalization (HAV). Since these conspicuous calls can increase the risk of predation and require energy, their costs should be balanced by some benefits, such as increased food provisioning. However, there has been no evidence that chicks convey information about their hunger by HAV. We therefore tested experimentally whether cuckoo chicks use HAV as an additional signal to enhance food-delivery rate by their hosts. We used playback of HAV recorded from cuckoo nestlings to determine whether their hosts, reed warblers Acrocephalus scirpaceus, increase their provisioning in response to an apparent increase in HAV. Older chicks spent more time in HAV than younger chicks, suggesting that HAV is not caused by inaccurate discrimination of host arrival stimuli. Negative correlation of HAV with feeding rate and mass gain between the two experiments suggested that hunger was the motivation of HAV. The playback experiment, however, did not prove that HAV affects host provisioning rate. We discuss possible reasons for this result and provide alternative explanations for HAV, such as creating a bond between the hosts and the parasitic young used later in the postfledging care.     

Cuckoos versus reed warblers: Adaptations and counteradaptations

At study sites in Cambridgeshire, England, the percentage of reed warbler, Acrocephalus scirpaceus, nests parasitized by cuckoos, Cuculus canorus, in 2 years was 22·5% and 9·1%. The warblers rejected cuckoo eggs at 19% of parasitized nests. Parasitized clutches suffered less predation than unparasitized clutches, suggesting that the cuckoo itself was the major predator, plundering nests too advanced for parasitism so that the hosts would re-lay. The cuckoos laid a mimetic egg, parasitized nests in the afternoons during the host laying period, usually removed one host egg, laid a remarkably small egg and laid very quickly. Nests were experimentally parasitized with model eggs to study the significance of this procedure. Experiments showed that host discrimination selects for: (1) egg mimicry by cuckoos (poorer matching model eggs were more likely to be rejected); (2) parasitism during the laying period (mimetic eggs put in nests before host laying began were rejected); (3) afternoon laying (mimetic eggs were less likely to be accepted in the early morning than in the afternoon, when hosts were more often absent from the nest); (4) a small egg (large eggs, typical of non-parasitic cuckoos, were more likely to be rejected); (5) rapid laying (a stuffed cuckoo on the nest stimulated increased rejection of model eggs), and (6) sets a limit to host egg removal by cuckoos (if more than one or two are removed desertion may occur). Mimicry may also be selected for because it reduced the chance that second cuckoos can discriminate the first cuckoo's egg from the host's clutch. Predation did not select for mimicry; nests with a non-mimetic egg did not suffer greater predation than those with a mimetic egg. Host rejection of model eggs did not depend on: (1) stage of parasitism once host egg laying had begun (nevertheless cuckoos were more likely to lay early in the host laying period probably to increase the chance the cuckoo chick hatched); (2) removal of a host egg (however, this reduced the incidence of unhatched eggs so cuckoos may remove a host egg so as not to exceed the host incubation limit). There were two costs of rejection, an ‘ejection’ cost (own eggs ejected as well as the cuckoo egg) and, with mimetic eggs, a ‘recognition’ cost (own eggs ejected instead of the cuckoo egg). Reed warblers did not discriminate against unlike chicks (another species) and did not favour either a cuckoo chick or their own chicks when these were placed in two nests side by side. Possible reasons why the hosts discriminate against unlike eggs but not unlike chicks are discussed.     

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.     

Leishmania in the chick embryo: IV. Effects of embryo age and hatching,and behavior of L. donovani in cultures of chick fibroblasts

On incubation Days 9, 11, 12, 14, or 15, chick embryos were injected intravenously with 4.0 × 10⁶L. donovani amastigotes. Embryos were incubated at 33 C immediately after infection. Numbers of amastigotes found in the liver 1 hr after injection increased as the age of embryo recipients increased. Most 14- or 15-day infected embryos hatched when allowed to do so, but many younger embryos were unable to survive at 33 C. Numbers of amastigotes in the liver of chicks, hatched after infection as embryos, decreased as the cloacal temperature of the chicks increased. Despite a 31 C incubation temperature, chicks exhibited a mean 38.3 C cloacal temperature 1 day after hatching.Chick fibroblast cultures were initiated as explants of embryo brain and infected with amastigotes from hamster spleen. Only amastigotes were seen in cultures kept at 37 C, but extracellular promastigotes and intracellular amastigotes were present in cultures at 33 C. Although promastigotes increased in number in the medium overlay at 33 C, amastigotes decreased in number at 33 C and 37 C. One intracellular amastigote was seen in a culture which had been incubated at 25 C after inoculation with promastigotes.     

Costs and benefits of variable breeding plumage in the red-backed fairy-wren

The red-backed fairy-wren is a socially monogamous passerine bird which exhibits two distinct types of breeding male, bright males that breed in bright red and black plumage and dull males that breed in dull brown plumage. Most males spend their first potential breeding season in dull plumage and subsequent breeding seasons in bright plumage, but a relatively small proportion of males develop bright plumage in their first breeding season. This study quantifies morphology, behavior, and reproductive success of dull and bright males to assess the adaptive costs and benefits of bright plumage while controlling for age. Older bright males (two years of age or older) attempted to increase their reproductive success via copulations with extrapair females, whereas younger (one-year old) bright males and dull males did not. Thus, older bright males spent less time on their own territories, intruded on neighboring groups with fertile females more frequently, gave more courtship displays, and had larger sperm storage organs than did younger bright males and dull males. Microsatellite analyses of paternity indicate that the red-backed fairy-wren has extremely high levels of sexual promiscuity, and that older bright males had higher within-brood paternity than dull males or younger bright males. Regardless of age, bright males were more attractive to females in controlled mate choice trials than were dull males, and both age classes of bright males obtained higher quality mates earlier in the breeding season than did dull males, when nesting success was higher. In conclusion, although it appears that bright plumage increases access to higher quality mates, age also plays a central role in determining a male's overall reproductive success because of the high levels of sexual promiscuity exhibited by the red-backed fairy-wren.     

Comparative analysis of septic injury-inducible genes in phylogenetically distant model organisms of regeneration and stem cell research, the planarian Schmidtea mediterranea and the cnidarian Hydra vulgaris

Background

Host-parasite interactions are among the most important biotic relationships. Host species should evolve mechanisms to detect their enemies and employ appropriate counterstrategies. Parasites, in turn, should evolve mechanisms to evade detection and thus maximize their success. Females of the European beewolf (Philanthus triangulum, Hymenoptera, Crabronidae) hunt exclusively honeybee workers as food for their progeny. The brood cells containing the paralyzed bees are severely threatened by a highly specialized cuckoo wasp (Hedychrum rutilans, Hymenoptera, Chrysididae). Female cuckoo wasps enter beewolf nests to oviposit on paralyzed bees that are temporarily couched in the nest burrow. The cuckoo wasp larva kills the beewolf larva and feeds on it and the bees. Here, we investigated whether H. rutilans evades detection by its host. Since chemical senses are most important in the dark nest, we hypothesized that the cuckoo wasp might employ chemical camouflage.

Results

Field observations suggest that cuckoo wasps are attacked by beewolves in front of their nest, most probably after being recognized visually. In contrast, beewolves seem not to detect signs of the presence of these parasitoids neither when these had visited the nest nor when directly encountered in the dark nest burrow. In a recognition bioassay in observation cages, beewolf females responded significantly less frequently to filter paper discs treated with a cuticular extract from H. rutilans females, than to filter paper discs treated with an extract from another cuckoo wasp species (Chrysis viridula). The behavior to paper discs treated with a cuticular extract from H. rutilans females did not differ significantly from the behavior towards filter paper discs treated with the solvent only. We hypothesized that cuckoo wasps either mimic the chemistry of their beewolf host or their host's prey. We tested this hypothesis using GC-MS analyses of the cuticles of male and female beewolves, cuckoo wasps, and honeybee workers. Cuticle extracts of Hedychrum nobile (Hymenoptera: Chrysididae) and Cerceris arenaria (Hymenoptera: Crabronidae) were used as outgroups. There was little congruence with regard to cuticular compounds between H. rutilans females and honeybees as well as females of C. arenaria and H. nobile. However, there was a considerable similarity between beewolf females and H. rutilans females. Beewolf females show a striking dimorphism regarding their cuticular hydrocarbons with one morph having (Z)-9-C25:1 and the other morph having (Z)-9-C27:1 as the major component. H. rutilans females were more similar to the morph having (Z)-9-C27:1 as the main component.

Conclusion

We conclude that H. rutilans females closely mimic the composition of cuticular compounds of their host species P. triangulum. The occurrence of isomeric forms of certain compounds on the cuticles of the cuckoo wasps but their absence on beewolf females suggests that cuckoo wasps synthesize the cuticular compounds rather than sequester them from their host. Thus, the behavioral data and the chemical analysis provide evidence that a specialized cuckoo wasp exhibits chemical mimicry of the odor of its host. This probably allows the cuckoo wasp to enter the nest with a reduced risk of being detected by olfaction and without leaving traitorous chemical traces.     

Response to a Dangerous Enemy: Should a Brood Parasite be Mobbed?

Models of a long-tailed cuckoo and a song thrush (as a control) were presented 1.5 to 3 m from the nests of whiteheads, a species that the cuckoo is known to parasitize. Tests were conducted early in incubation (days 3 to 8) and during the nestling period. Most breeding females responded to the cuckoo by remaining inconspicuous or hiding. Other members of the cooperatively breeding group mobbed the cuckoo if they saw it. This difference in behaviour resulted in differences in overall response between incubation and nestling periods; the cuckoo was only occasionally mobbed during incubation because birds other than the female rarely approached the nest, whereas the cuckoo was almost always mobbed after eggs hatched. Cuckoos benefit from group breeding by whiteheads because chicks receive more food if reared by larger groups. I suggest that inconspicuous behaviour by the female in the presence of a cuckoo results in at least two effects: first, females can determine what the intentions of the cuckoo are, particularly if it intends to lay in the nest; second, no cues about size of breeding group are given to the cuckoo. Comparisons are drawn with the recent study of PAYNE et al. (1985) on splendid wrens and Chrysococcyx cuckoos. Despite similar social behaviour, splendid wrens and whiteheads behaved differently in the presence of a cuckoo. Mobbing may not always be the most appropriate tactic when faced with a brood parasite.     

Increased yolk testosterone facilitates prenatal perceptual learning in Northern bobwhite quail (Colinus virginianus)

Prenatal learning plays an important role in the ontogeny of behavior and birds provide a useful model to explore whether and how prenatal exposure to hormones of maternal origin can influence prenatal learning and the development of behavior. In this study we assessed if prenatal exposure to yolk testosterone can influence auditory learning in embryos of Northern bobwhite quail (Colinus virginianus). We experimentally enhanced testosterone concentrations in bobwhite quail eggs prior to incubation. The embryos from these T-treated eggs as well as control embryos that had received the vehicle-only or were non-treated were exposed to an individual bobwhite hen's maternal call for 120 min over the course of the day prior to hatching. All chicks were tested at 24 h following hatching for their auditory preference between the familiar bobwhite maternal call versus an unfamiliar bobwhite maternal call. T-treated chicks spent significantly more time in proximity to the familiar call compared to the unfamiliar call and also showed shorter latencies to approach the familiar call than control birds. Increased emotional reactivity, i.e. propensity to express fear responses, was also found in T-treated chicks. Baseline heart rates recorded in a second group of T-treated embryos and control embryos did not differ, which suggests no effect of yolk testosterone on baseline arousal level. To our knowledge this is the first demonstration of the influence of prenatal exposure to testosterone on auditory learning.     

Reed warblers guard against cuckoos and cuckoldry

Previous studies have shown that reed warblers, Acrocephalus scirpaceus, are more likely to reject a cuckoo, Cuculus canorus, egg if they have seen a cuckoo at their nest. This suggests that they would benefit from watching out for cuckoos. We tested whether presentations of a cuckoo mount near the nest (to simulate nest inspection) led to increased nest attendance by the warblers. Cuckoo presentations at completed nests before laying, when males guarded their females closely, led to desertion at 40% of nests before any eggs were laid (there were no desertions after presentations of a jay,Garrulus glandarius , a nest predator). In the remaining cases, there was no effect of the cuckoo on nest attendance before laying began, but a marked increase in male nest attendance (compared with jay and no-presentation controls) on the days the first and second eggs were laid. Cuckoo presentations at the one-egg stage led to the same increase in male nest attendance as did the prelaying presentations. Increased male nest attendance at the one-two-egg stage was not at the expense of mate guarding, because this declined anyway when laying began, and it did not lead to increased paternity loss compared with controls. Overall, 15% of broods had one or two extrapair young (6% of all young extrapair). We conclude that male reed warblers do increase nest guarding in response to cuckoos, but only after their females have begun egg laying, when there are less likely to be costs in lost paternity. Females did not increase nest guarding, perhaps because they need to spend more time foraging during the egg-laying period. Our results suggest that cuckoos should be secretive not only when they lay but also when they monitor host nests beforehand. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.      

Great Spotted Cuckoo Nestlings but not Magpie Nestlings Starve in Experimental Age‐Matched Broods

Nestlings of non‐evicting avian brood‐parasites have to compete for food with foster parents' own nestlings. The outcome of these competitive contests is determined mainly by body size differences between parasitic and host nestlings. As part of the coevolutionary arms race between brood parasites and their hosts at the nestling stage, it has been reported that some host foster parents discriminate against parasitic chicks and are reluctant to feed them. Here, by experimentally creating size‐matched broods of different composition (only magpie Pica pica chicks, only great spotted cuckoo Clamator glandarius chicks or mixed broods), we show that great spotted cuckoo chicks starved in 20.2 per cent (17 of 84) of the parasitized magpie nests even in absence of size asymmetries, while in none (0 of 72) of the nests a magpie chick starved. As far as we know, this is the first record of non‐evictor brood parasitic nestlings starving without being smaller than their host nestmates in a frequently used host species. Nest composition had no effect on chick starvation. The cuckoo nestling starved even in two of the nests occupied by only one cuckoo chick. Our results could be explained by (1) magpies being reluctant to feed cuckoo chicks; (2) parasitic chicks receiving lower‐quality food items or cuckoo nestlings being sensitive to some particular component of the diet (e.g. cereal grains); and (3) the existence of cuckoo chick discrimination ability by magpie foster parents.     

Egg Eviction Imposes a Recoverable Cost of Virulence in Chicks of a Brood Parasite

Background

Chicks of virulent brood parasitic birds eliminate their nestmates and avoid costly competition for foster parental care. Yet, efforts to evict nest contents by the blind and naked common cuckoo Cuculus canorus hatchling are counterintuitive as both adult parasites and large older cuckoo chicks appear to be better suited to tossing the eggs and young of the foster parents.

Methodology/Principal Findings

Here we show experimentally that egg tossing imposed a recoverable growth cost of mass gain in common cuckoo chicks during the nestling period in nests of great reed warbler Acrocephalus arundinaceus hosts. Growth rates of skeletal traits and morphological variables involved in the solicitation of foster parental care remained similar between evictor and non-evictor chicks throughout development. We also detected no increase in predation rates for evicting nests, suggesting that egg tossing behavior by common cuckoo hatchlings does not increase the conspicuousness of nests.

Conclusion

The temporary growth cost of egg eviction by common cuckoo hatchlings is the result of constraints imposed by rejecter host adults and competitive nestmates on the timing and mechanism of parasite virulence.     

Development and regulation of heart rate in embryos and hatchlings of gulls (Larus schistisagus and Larus crassirostris) in relation to growth

We compared the developmental patterns of mean heart rate in Larus crassirostris and L. schistisagus embryos and chicks with other avian species of different hatchling developmental modes. We proposed that, since mean heart rate is inversely related to fresh egg mass in all birds, larger species reached a higher fraction of their hatchling mean heart rate by the end of the early phase of incubation and that heart rate contributions to supplying the increasing metabolic demands during mid and late incubation phases were less important than in smaller avian species. Mean heart rate was essentially independent of age throughout the mid-incubation phase (33% of normalised incubation until pipping), but increased with time during early (L. schistisagus only investigated) and late-incubation phases in both species. The O₂ pulse of L. schistisagus embryos and chicks increased linearly with yolk-free body mass (log-log) after the early-phase of incubation until shortly before pipping, but was independent of body mass in the periods before and after. We conclude that a high heart rate in this first period is probably more important for increasing the circulation of nutrients to the embryo at a stage when extra-embryonic circulation to the yolk sac is limited by the size of the growing area vaculosa. Furthermore, large increases in mean heart rate during the late-incubation phase are probably important for increasing the cardiac output to hatching levels with onset of endothermy. However, mean heart rate is stable over the mid-incubation while O₂ pulse increases, suggesting that increases in stroke volume and other circulatory adjustments may be entirely responsible for the largest increases in O₂ transport during incubation of large avian species. Accepted: 18 May 2000     

Corticosterone levels in host and parasite nestlings: Is brood parasitism a hormonal stressor?

Parasite chicks from non-evictor species usually try to monopolize host parental care, thereby increasing considerably the level of food competition in the nest. Here, we propose that brood parasitism is an important stressor for host and parasite nestlings and explore this hypothesis in the non-evictor great spotted cuckoo (Clamator glandarius) and its main hosts, the same-sized black-billed magpie (Pica pica) and the larger carrion crow (Corvus corone). We experimentally created 3-nestling broods of different brood compositions (only cuckoo chicks, only host chicks, or cuckoo and host chicks together) and measured baseline corticosterone levels of nestlings along their developmental period (early, middle and late). We found that brood parasitism increased corticosterone levels in magpie nestlings in the mid and late nestling period compared to those raised in unparasitized nests. Interestingly, carrion crow nestlings from parasitized nests only increased their corticosterone levels in the mid nestling period, when the competition for food with the cuckoo nestling was highest. Our results suggest that brood parasitism could be a potential physiological stressor for host nestlings, especially during the developmental stages where food requirements are highest. Conversely, cuckoo nestlings could be physiologically adapted to high competition levels since they did not show significant differences in corticosterone levels in relation to brood composition.     

Prolactin and parental behavior in Adélie penguins: effects of absence from nest, incubation length, and nest failure

Adélie penguin (Pygoscelis adeliae) males and females, nesting in Antarctica, alternate attendance at the nest with absences of many days to forage at sea. We investigated the importance of tactile input from egg and chicks on prolactin levels by observing nest attendance patterns and obtaining blood samples (1) during the first nest exchange of the incubation stage, (2) from birds whose incubation period was artificially increased or decreased by about 10 days, and (3) from birds whose nests had failed. Prolactin levels in females after 8 to 11 days of absence from the breeding colony did not differ from those in incubating males and did not change after females resumed incubation. Moving eggs between nests resulted in nests in which chicks hatched after about 26, 36 (normal), or 46 days. Duration of incubation did not affect prolactin levels in the parents measured during incubation, at the pip stage, hatch stage, or early brood stage. Adults first left their chicks unguarded on about the same calendar date, regardless of chick age. However, chicks from long incubation nests averaged 8 days younger when they were left unguarded than chicks from control or short-incubation nests. In females, there was no effect of nest failure on prolactin levels. In males, prolactin levels were slightly lower after nest failure than in males tending nests. Testosterone was significantly higher in males after nest failure than in males still tending nests. Prolactin is elevated in Adélie penguins as part of the program of cyclical hormonal changes that accompany the lengthy reproductive season and is relatively independent of tactile input. Sustained prolactin secretion is probably required for the maintenance of parental behavior in offshore feeding species that must be absent from the nest for many days at a time.     

Impaired flight ability during incubation in the pied flycatcher

During the breeding season, many female passerine birds increase in body mass before egg laying, maintain a relatively high body mass during incubation, and then drop back to the original level during the chick-rearing period. The post-hatching reduction in body mass, which can be as large as 10–20%, has been suggested to represent an adaptive mass loss to reduce wing loading, thereby increasing parental flight efficiency when chicks have hatched and have to be fed. Here we present the first study of changes in flight ability from incubation to chick rearing in birds. Wild female pied flycatchers Ficedula hypoleuca flew more slowly during incubation than during chick rearing; a 7% reduction in body mass after the chicks had hatched was associated with a 10% increase in vertical take-off speed. Furthermore, the flight muscle size of the females tracked the reduction in wing load, suggesting that muscle size was adaptively reduced when no longer needed. Since incubation-feeding by males reduces the time females have to spend outside the nest foraging, our results suggest that in addition to increasing female nutritional status and reducing incubation time, incubation-feeding will also reduce predation risk during the period when females face impaired flight ability.