Parental Care In Birds Research Articles

Parental provisioning is weakly age‐dependent and heritable in a small passerine

Parental care in birds encompasses many behaviours, from selecting nest‐sites and supplying eggs with nutrients to incubation and offspring provisioning. Unlike the early stages, where offspring are passive receivers of care, chicks actively solicit care after hatching. This may lead to either parent–offspring conflict or coadaptation, or both, if there is genetic variance in both parental provisioning and offspring begging. However, given that parental provisioning is highly responsive to brood size and age, its genetic determination is questioned. In this study, we used a multigenerational pedigree and 11 years of provisioning data to dissect the variation in this trait in the Collared Flycatcher Ficedula albicollis, and to examine whether offspring provisioning changes as parents age. We found that the parental provisioning rate was weakly repeatable (r = 0.088–0.213) and heritable (h2 = 0.052–0.158). Higher values were obtained when calculated based on within‐individual averages rather than individual measurements, and after adjusting for fixed effects. Older parents provisioned offspring at a lower frequency than younger ones. An additive genetic component in provisioning indicates potential for the evolution of parental care, parent–offspring coadaptation and indirect genetic effects on offspring traits. Future studies employing new tracking technologies, including accelerometers, radars and radio‐frequency identification readers, may help to determine whether senescence or increased experience causes the lower rate of offspring provisioning observed in older parents.

Sex differences in avian parental care patterns vary across the breeding cycle

Parental care in birds consists of elaborate forms across stages, including nest building, incubation, and offspring provision. Despite their evolutionary importance, knowledge gaps exist in the extent to which parents contribute disproportionately to these forms and factors that are associated with variations in care patterns between sexes. Here, we analyzed 1533 bird species and discovered remarkable variability in care patterns. We show that parental care should not be viewed as a unitary trait but rather as a set of integrated features that exhibit considerable temporal and sex-specific variation. Our analyses also reveal moderate consistency in care patterns between breeding stages, pointing towards shared intrinsic factors driving sex-specific care. Notably, we found that species experiencing strong sexual selection on males or species facing paternity uncertainty display a tendency towards female-biased care. This work advances our understanding of the temporal variations in sex-specific contributions to avian parental care and their potential evolutionary drivers.

Parental care in birds

Parental care, a usefully imprecise catch-all term for behaviors performed by breeding adults that benefit their offspring, is a popular research area among behavioral ecologists. Across Class Aves, it takes many forms, ranging from warming the eggs during incubation - such that the embryo develops within and eventually escapes from its protective shell - to extensive post-hatching assistance, especially by providing food but also by protecting young from weather and predators (Figure 1). In this primer, I will address the evolutionary forces likely to have shaped the peculiar avian habit of involving both parents in substantial post-fertilization investments, rather than just one. For this, I will focus on the costs, benefits, and complex social dynamics associated with elaborate parental care to show that what we may regard as the simple nuclear family (mom, dad, and a few kids) is anything but simple.

Brain Distribution and Sexually Dimorphic Expression of Amylin in Different Reproductive Stages of the Zebra Finch (Taeniopygia guttata) Suggest Roles of the Neuropeptide in Song Learning and Social Behaviour.

The expression of the recently identified neuropeptide, amylin, is restricted in rodents to the postpartum preoptic area and may play a role in the control of parental behaviours and food intake. These processes are substantially different between bird and rodent parents as birds do not lactate but often show biparental care of the offspring. To establish the presence and role of amylin in the bird brain, in the present study, we investigated the distribution of amylin in brains of adult male and female zebra finches in three different reproductive stages (i.e. paired without young, incubating eggs or provisioning nestlings) and in unpaired control birds living in same sex flocks. Amylin mRNA was identified in the hypothalamus of zebra finch by RT-PCR, which was also used to produce probes for in situ hybridisation. Subsequently, in situ hybridisation histochemistry was performed in brain sections, and the labelling signal was quantified and compared between the groups. Amylin showed a much wider brain distribution than that of rodents. A strong and, in some regions, sexually dimorphic label was found in the striatum and several brain regions of the social behavioural network in both males and females. Many regions responsible for the learning of birdsong also contained amylin-positive neurons, and some regions showed sex differences reflecting the fact that vocalisation is sexually dimorphic in the zebra finch: only males sing. Area X (Ar.X), a striatal song centre present only in males, was labelled in paired but not unpaired male. Ar.X, another song centre, the lateral part of the magnocellular nucleus of the anterior nidopallium (lMAN) also contained amylin and had higher amylin label in paired, as opposed to unpaired birds. The wider distribution of amylin in birds as compared to rodents suggests a more general role of amylin in social or other behaviours in avian species than in mammals. Alternatively, parental care in birds may be a more complex behavioural trait involving a wider set of brain regions. The sex differences in song centres, and the changes with reproductive status suggest a participation of amylin in social behaviours and related changes in the singing of males.

Distribution of Experimentally Increased Costs of Parental Care Among Family Members Depends on Duration of Offspring Care in Biparental Birds

Biparental care systems are a valuable model to examine conflict, cooperation, and coordination between unrelated individuals, as the interactions between the parents determines their fitness. Temporarily handicapping one parent induces a higher cost of providing care and is a widespread experimental technique for testing coordinated responses to changes in the costs of parental care in birds. However, dissimilarity in experimental designs of handicapping studies has hindered interspecific comparisons of the patterns of cost distribution between parents and their offspring. Here we apply a comparative approach by handicapping a parent at nests of five altricial bird species using the same experimental treatment. Across species, handicapped parents reduced their nest visitation rate, indicating increased costs of parental care for the manipulated parent. Unexpectedly, the partners of handicapped individuals did not compensate for the reduction in care, and the increased costs were subsequently passed to their offspring. The strength of this effect was mediated by the total duration of offspring care; in species with long care periods, the offspring were passed a greater share of the additional cost. This effect was evident in both changes to nest visitation rates and the body mass gain of the nestlings. Surprisingly, these responses were independent of life history pace (i.e., adult survival and fecundity). While most studies of the costs of parental care focus on the trade-off between current and future reproduction or survival (intra-individual trade-offs), our study highlights that a greater attention to inter-generational trade-offs is warranted, particularly in species with prolonged parental care. Moreover, our findings demonstrate that parental care decisions may be weighed more against physiological workload constraints than against future prospects of reproduction, supporting evidence that avian species may devote comparable amounts of energy into survival, regardless of life history pace.

Nest attendance by tropical and temperate passerine birds: Same constancy, different strategy.

Parental care in birds varies among species and geographic regions. Incubation behavior influences embryonic development rate and varies substantially among species.We studied attendance at the nest by videoing nests or collecting data from the literature for 112 species in north temperate and lowland tropical sites, then associated patterns of incubation on‐ and off‐bouts with species and environmental traits.Songbirds nesting at low elevations incubate their eggs for an average of 74.1% (±12.9 SD, n = 60 species) of the time in temperate regions and 71.0% (±12.2 SD, n = 52 species) in tropical regions during daylight hours, and 84.3% (±8.2 SD) and 85.3% (±6.2 SD), respectively, of each 24‐hr cycle.While these attendance percentages do not differ significantly between latitudes, our data also show that lowland tropical songbirds make fewer visits to the nest and, consequently, have longer on‐bouts and off‐bouts during incubation. This pattern in attendance reflects a latitudinal contrast in parental care strategy, where lowland tropical birds reduce visits to the nest by increasing on‐ and off‐bout lengths while maintaining the same proportion of time spent incubating their eggs (constancy).Similar constancy across latitude suggests that tropical and temperate birds may be similarly constrained to maintain elevated egg temperatures for normal embryo growth.The different attendance strategies adopted in each region may reflect differences in ambient temperature, adult foraging time, and nest predation rate. Consistently warm ambient temperatures likely allow tropical birds to take longer off‐bouts, and thereby to reduce activity around the nest, compared to temperate birds.

Neuronal activation in zebra finch parents associated with reintroduction of nestlings.

Recent studies of the brain mechanisms of parental behaviors have mainly focused on rodents. Using other vertebrate taxa, such as birds, can contribute to a more comprehensive, evolutionary view. In the present study, we investigated a passerine songbird, the zebra finch (Taeniopygia guttata), with a biparental caring system. Parenting-related neuronal activation was induced by first temporarily removing the nestlings, and then, either reuniting the focal male or female parent with the nestlings (parental group) or not (control group). To identify activated neurons, the immediate early gene product, Fos protein, was labeled. Both parents showed an increased level of parental behavior following reunion with the nestlings, and no sexual dimorphism occurred in the neuronal activation pattern. Offspring-induced parental behavior-related neuronal activation was found in the preoptic, ventromedial (VMH), paraventricular hypothalamic nuclei, and in the bed nucleus of the stria terminalis. In addition, the number of Fos-immunoreactive (Fos-ir) neurons in the nucleus accumbens predicted the frequency of the feeding of the nestlings. No difference was found in Fos expression when the effect of isolation or the presence of the mate was examined. Thus, our study identified a number of nuclei involved in parental care in birds and suggests similar regulatory mechanisms in caring females and males. The activated brain regions show similarities to rodents, while a generally lower number of brain regions were activated in the zebra finch. Furthermore, future studies are necessary to establish the role of the apparently avian-specific neuronal activation in the VMH of zebra finch parents.

Evolution of parental activity at the nest is shaped by the risk of nest predation and ambient temperature across bird species.

Incubation is an important component of parental care in birds, and species differ widely in their incubation rhythm. In this comparative study, we focused on factors responsible for those differences. As hypothesized by A. Skutch, increased parental activity at the nest increases the probability of nest depredation. High risk of nest predation should therefore lead to the evolution of lower frequency of parental activity at the nest. We thus expected to find a negative relationship between frequency of nest visits and the risk of nest depredation. Using a large dataset of 256 species of passerines breeding worldwide, we found that the frequency of nest visits decreased as the risk of nest depredation increased and that this effect was strongest in tropical species. Further, foraging bouts were longer in species experiencing warmer ambient temperatures during incubation and those with domed nests. Incubation bouts were longer and frequency of nest visits was lower in species with higher body mass. Our results support the view that natural selection favors lower frequency of nests visits in species under higher risk of nest predation and demonstrate the importance of other factors (temperature, geographic space, nest type, and body mass) in shaping the evolution of incubation rhythm.

Physiology, activity and costs of parental care in birds.

Parental care is assumed to be costly in that it requires sustained, high-intensity activity sufficient to cause costs of reproduction (decreased survival and future fecundity of parents). Costs of reproduction are, in turn, thought to have a physiological basis where intense activity causes a decrease in parental condition. However, attempts to identify the physiological basis of costs of reproduction have produced mixed results. Here, I argue that in birds, the central idea that parental care represents sustained, high-intensity work might be incorrect. Specifically: (a) the duration of intense activity associated with chick-rearing might be quite limited; (b) flight, the most obvious sustained, high-intensity activity, might only represent a small component of an individual's overall activity budget; (c) some (high-quality) individuals might be able to tolerate costs of intense activity, either owing to their physiological state or because they have access to more resources, without perturbation of physiological homeostasis; and (d) individuals might utilise other mechanisms to modulate costs of activity, for example, mass loss, again avoiding more substantial physiological costs. Furthermore, I highlight the important fact that life-history theory predicts that reproductive trade-offs should only be expected under food stress. Most birds breed in spring and early summer precisely because of seasonal increases in food abundance, and so it is unclear how often parents are food stressed. Consequently, I argue that there are many reasons why costs of reproduction, and any physiological signature of these costs, might be quite rare, both temporally (in different years) and among individuals.

Lowering prolactin reduces post-hatch parental care in male and female zebra finches (Taeniopygia guttata)

Parental care is a widespread phenomenon observed in many diverse taxa. Neuroendocrine systems have long been thought to play an important role in stimulating the onset of parental behavior. In most birds with altricial young, circulating prolactin (PRL) levels are low during non-breeding times and significantly increase during late incubation and early post-hatch chick care. Because of this pattern, PRL has been suggested to be involved in the initiation of parental care in birds, but rarely has this hypothesis been causally tested. To begin testing the hypothesis, we inhibited the release of endogenous PRL with bromocriptine (BR) on the 3days prior to hatching in incubating parents and the first 2days of post-hatch care, when PRL was found to be highest in zebra finches. Nest temperatures were recorded during all 5days and parental behavior was recorded on days 1–2 post-hatch. In addition to hormonal systems, reproductive experience may also influence parental care; therefore, we tested age-matched inexperienced and experienced pairs in each group. BR either eliminated or drastically reduced chick brooding and feeding behavior, resulting in decreased nest temperatures on days 1 and 2 post-hatch. Experienced control birds fed chicks more than inexperienced birds and control females fed more than males. Chick feeding behavior was positively correlated in control male-female pairs, but not in BR pairs. This is one of the few causal studies to demonstrate that PRL is necessary for post-hatch care in a biparental songbird, and is the first to show this effect in zebra finches.

Comparative studies need to rely both on sound natural history data and on excellent statistical analysis.

We certainly agree with Schradin [1] that comparative studies need to use the best information available—as well as clear definitions, up-to-date statistical techniques and common sense. We have divided our response into two sections: the first dealing with Schradin's critique of our 2017 paper and the classification of the social organization of shrews; the second dealing with three general issues that his commentary raises. The paper that Schradin is commenting on [2] asks whether cooperative breeders (which we define as species where non-breeding helpers assist breeders to raise their young) in mammals are more commonly found in arid environments than monogamous species (defined as species where breeding pairs of males and females remain together for more than one season) from which cooperative breeders appear to be derived [3]. It shows that, as in birds, cooperative breeders tend to live in relatively arid habitats compared to monogamous species. As we describe below, these conclusions are unaffected by differences between our classification of the social organization of shrews and the classification suggested by Schradin. Schradin's first criticism is that the categorizations of shrews that we have used in this analysis were based on one paper on a single species. This is incorrect, for our categorization of shrews was based on more than 70 separate sources (see electronic supplementary material, appendix S1). When we published our dataset [4], we were required to reduce the associated reference list by the editors of the journal and, to comply, we removed around 75% of our 2000+ sources, often listing a single reference per species or group of species, and frequently using generic reviews for particular groups where these were available: for example, for many rodent species, we simply listed Wolff and Sherman's Rodent Societies [5], and …

Parental care and external sexual characters in the Warbling Doradito (Pseudocolopteryx flaviventris)

Fil: Cardoni, Daniel Augusto. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina

Prolactin is related to individual differences in parental behavior and reproductive success in a biparental passerine, the zebra finch (Taeniopygia guttata)

Variation in parental care can lead to important fitness consequences. The endocrine system is known to regulate physiological and behavioral reproductive traits that are important contributors to lifetime reproductive success. However, the hormonal basis of variation in avian parental care is still not well understood. Plasma prolactin (PRL) concentrations are generally high during post-hatch parental care in birds, and may be a candidate mechanism that regulates variation in parental care and other reproductive success outcomes. Here we analyze the relationship between PRL, parental behavior (chick brooding and feeding) and reproductive success outcomes (clutch size, number of chicks hatched, and chick survival) for the first time in the zebra finch (Taeniopygia guttata). Birds were given cabergoline, a dopamine agonist traditionally used to lower prolactin in mammals, or vehicle in their food. Cabergoline had no effect on prolactin concentrations, but across both groups we found that PRL is positively correlated with parental behavior, number of chicks hatched, and chick survival, but not clutch size. Results from this study will inform hypotheses and predictions for future manipulation studies which test for a causal role for PRL in parental traits.

Factors Driving Variation in Biparental Incubation Behaviour in the Reed WarblerAcrocephalus scirpaceus

Biparental incubation is a common pattern of parental care in birds. Within species with biparental incubation, the contribution of each sex can vary widely. Many studies have addressed the factors that influence variation in female incubation behaviour, but the underlying causes of within-species variation in male incubation behaviour remain poorly understood. In this paper we analyse incubation behaviour in the Reed Warbler, a small, predominantly socially monogamous passerine nesting in reed beds. We examined the impact of time of day, weather conditions (ambient temperature, wind speed, rainfall) and progress of the breeding season on male and female incubation behaviour in 81 pairs of Reed Warbler breeding in the Barycz Valley (SW Poland). We found that females had on average higher nest attentiveness (total time spent incubating per hour) than their partners (47% vs. 29%) but mean incubation bout length (a single, uninterrupted stay at the nest) did not differ significantly between the sexes (9 min vs. 7 min). The two parents responded differently to changing environmental parameters. Female nest attentiveness was unaffected by date, time of the day, advancement of incubation and weather conditions, while males spent more time on the nest at higher wind speeds and lower temperatures. In contrast, male incubation bout length was not affected by these factors, whereas female bout length increased throughout the breeding season and was longer at lower temperatures. Incubation recesses (periods where both parents were off the nest) were longer during favourable weather conditions (at high temperatures and low wind), probably because cooling of the eggs takes much longer under such conditions and parents can spend more time foraging. A comparison of our results with those from other populations revealed important betweenpopulation and sex-specific differences in nest attentiveness expressed as the percentage of male/female time spent on the nest. We suggest that betweenpopulation variation may result from differences in habitat quality and/or food resources affecting the necessity of male contribution to parental care, variation in breeding synchrony and densities influencing male engagement in extra-pair copulations, differences in predation levels, or methodological differences.

Manipulation of parental effort affects plumage bacterial load in a wild passerine.

It has been suggested that plumage microorganisms play an important role in shaping the life histories of wild birds. Some bacteria may act as pathogens or cause damage to feathers, and thereby reduce individual fitness. Intense parental care in birds can result in a reduction of self-maintenance and preening behavior in parents and therefore might affect the dynamics of microbiota living on their feathers. However, experimental evidence of this relationship is virtually absent. We manipulated the parental effort of wild breeding pied flycatcher (Ficedula hypoleuca) females by modifying their brood size or temporarily removing male partners. We expected that experimentally decreasing or increasing parental effort would affect feather sanitation in females and therefore also bacterial density on their plumage. In accordance with this hypothesis, manipulation affected the density of free-living bacteria: females with reduced broods had the lowest number of free-living bacteria on their feathers, while females left without male partners had the highest. However, manipulation did not have a significant effect on the densities of attached bacteria. Our results provide experimental evidence that a trade-off between self-maintenance and parental effort affects plumage bacterial densities in birds.

Increased STAT5 signaling in the ring dove brain in response to prolactin administration and spontaneous elevations in prolactin during the breeding cycle

Prolactin acts on target cells in the central nervous system (CNS) to stimulate behavioral changes associated with parental care in birds, but the signaling mechanisms that mediate these actions have not been characterized. In mammals, the Janus Kinase 2–Signal Transducer and Activator of Transcription 5 (JAK2–STAT5) signaling pathway mediates many of the actions of prolactin. To assess the importance of this pathway in prolactin-sensitive target cells in the avian brain, we measured changes in activated (phosphorylated) STAT5 (pSTAT5) in the forebrain of female ring doves sampled as plasma prolactin levels change during the breeding cycle and in prolactin-treated, non-breeding females. The anatomical distribution of cells exhibiting pSTAT5 immunoreactivity in dove brain closely paralleled the distribution of prolactin receptors in this species. The density of pSTAT5 immunoreactive (pSTAT5-ir) cells was highest in the preoptic area, the suprachiasmatic, paraventricular, and ventromedial hypothalamic nuclei, the lateral and tuberal hypothalamic regions, the lateral bed nucleus of the stria terminalis, and the lateral septum. Mean pSTAT5-ir cell densities in these eight brain areas were several fold higher in breeding females during late incubation/early post-hatching when plasma prolactin levels have been observed to peak than in non-breeding females or breeding females sampled at earlier stages when prolactin titers have been reported to be lower. Similar differences were observed between prolactin-treated and vehicle-treated females in all three of the forebrain regions that were compared. We conclude that JAK2–STAT5 signaling is strongly activated in response to prolactin stimulation in the ring dove brain and could potentially mediate some of the centrally-mediated behavioral effects of this hormone.

Ecological, evolutionary, and conservation implications of incubation temperature‐dependent phenotypes in birds

Incubation is a vital component of reproduction and parental care in birds. Maintaining temperatures within a narrow range is necessary for embryonic development and hatching of young, and exposure to both high and low temperatures can be lethal to embryos. Although it is widely recognized that temperature is important for hatching success, little is known about how variation in incubation temperature influences the post-hatching phenotypes of avian offspring. However, among reptiles it is well known that incubation temperature affects many phenotypic traits of offspring with implications for their future survival and reproduction. Although most birds, unlike reptiles, physically incubate their eggs, and thus behaviourally control nest temperatures, variation in temperature that influences embryonic development still occurs among nests within a population. Recent research in birds has primarily been limited to populations of megapodes and waterfowl; in each group, incubation temperature has substantial effects on hatchling phenotypic traits important for future development, survival, and reproduction. Such observations suggest that incubation temperature (and incubation behaviours of parents) is an important but underappreciated parental effect in birds and may represent a selective force instrumental in shaping avian reproductive ecology and life-history traits. However, much more research is needed to understand how pervasive phenotypic effects of incubation temperature are among birds, the sources of variation in incubation temperature, and how effects on phenotype arise. Such insights will not only provide foundational information regarding avian evolution and ecology, but also contribute to avian conservation.

The prolactin response to an acute stressor in relation to parental care and corticosterone in a short-lived bird, the Eurasian hoopoe

Prolactin plays an important role in mediating parental care in birds, but little is known about changes in prolactin levels when animals disrupt their reproductive behaviour during emergency life-history stages. We investigated the variation of prolactin levels with breeding stage, sex, body condition and as a response to a standardized acute stressor in a small short-lived bird, the Eurasian hoopoe Upupa epops under natural field conditions. We found higher baseline levels of prolactin in females during the brooding phase than in their mates which feed them and their chicks at this stage. Moreover, this is the first report of a differential prolactin stress-response between sexes with contrasting parental care within a breeding phase. Capture, handling and restraint induced a clear decrease of prolactin levels which was less pronounced in females at the very early stage of brooding compared to females in later stages. In contrast, the prolactin stress response in males remained nearly constant over the breeding stages and was stronger than in females. Baseline levels of prolactin, but not handling-induced levels, were positively correlated with body condition. We found a weak relationship between the decrease in prolactin due to acute handling stress and handling-induced levels of corticosterone. Taken together, both baseline and stress response levels of prolactin were related to the amount of parental care, although we found no relationship with reproductive success. It appears that the response to an acute stressor in prolactin levels is finely tuned to parental duties and investment. Hence, prolactin appears to be involved in mediating the trade-off between current reproduction versus self-maintenance and future reproduction.

Post-fledging brood and care division in the roseate tern (Sterna dougallii)

Extended post-fledging parental care is an important aspect of parental care in birds, although little studied due to logistic difficulties. Commonly, the brood is split physically (brood division) and/or preferential care is given to a subset of the brood by one parent or the other (care division). Among gulls and tern (Laridae), males and females generally share parental activities during the pre-fledging period, but the allocation of parental care after fledging is little documented. This study examined the behaviour of male and female roseate terns (Sterna dougallii) during the late chick-rearing and early post-fledging periods, and in particular the amount of feeds and the time spent in attendance given to individual chicks/fledglings. Pre-fledging parental care was biparental in all cases. Post-fledging parental care was dependent on the number of fledglings in the brood. Males and females continued biparental care in clutches with one surviving fledgling, while in two-fledgling clutches, males fed the A-fledgling while females fed the B-fledgling. Overall, there was no difference in attendance, only in feeds. This division of care may be influenced by the male only being certain of the paternity of the A-chick but not by chick sex.

Exogenous corticosterone mimics a late fasting stage in captive Adélie penguins (Pygoscelis adeliae)

Fasting is part of penguin's breeding constraints. During prolonged fasting, three metabolic phases occur successively. Below a threshold in body reserves, birds enter phase III (PIII), which is characterized by hormonal and metabolic shifts. These changes are concomitant with egg abandonment in the wild and increased locomotor activity in captivity. Because corticosterone (CORT) enhances foraging activity, we investigated the variations of endogenous CORT, and the effects of exogenous CORT on the behavioral, hormonal, and metabolic responses of failed breeder Adélie penguins. Untreated and treated captive male birds were regularly weighed and sampled for blood while fasting, and locomotor activity was recorded daily. Treated birds were implanted with various doses of CORT during phase II. Untreated penguins entering PIII had increased CORT (3.5-fold) and uric acid (4-fold; reflecting protein catabolism) levels, concomitantly with a rise in locomotor activity (2-fold), while prolactin (involved in parental care in birds) levels declined by 33%. In CORT-treated birds, an inverted-U relationship was obtained between CORT levels and locomotor activity. The greatest increase in locomotor activity was observed in birds implanted with a high dose of CORT (C100), locomotor activity showing a 2.5-fold increase, 4 days after implantation to a level similar to that of birds in PIII. Moreover, uric acid levels increased three-fold in C100-birds, while prolactin levels declined by 30%. The experimentally induced rise in CORT levels mimicked metabolic, hormonal, and behavioral changes, characterizing late fasting, thus supporting a role for this hormone in the enhanced drive for refeeding occurring in long-term fasting birds.