Pre-laying Period Research Articles

Nest Guarding in Chesapeake Bay Bald Eagles

Abstract As Bald Eagle (Haliaeetus leucocephalus) populations approach carrying capacity in the Chesapeake Bay, competition for breeding territories appears to be intensifying. Frequent territorial interactions may force breeders to adjust nest-guarding behavior. We examined nest-guarding behaviors at active Bald Eagle nests in the lower Chesapeake Bay during the nesting season (2012 and 2013). Guarding coverage was 13.7 ± 4.2% of total observation time during the pre-laying period, 6.8 ± 2.2% of observation time in the incubation period, and 26.3 ± 3.2% of observation time in the nestling period. Females were present in the nest area for 80.0 ± 2.7% of the nestling period. Although males were present only 51.2 ± 2.8% of the nestling period, male breeders guarded nests twice as often as females. Adults guarded most often from perches in adjacent trees and within 25 m of the nest. If increasing rates of conspecific interactions force males to allocate more time to nest guarding, a tradeoff may occur, with ...

Transcriptome profiling of the hypothalamus during prelaying and laying periods in Sichuan white geese (Anser cygnoides).

The Sichuan White goose is a Chinese breed well known for the quality of its meat. However, reproductive performance in this goose is not ideal, and little information is available regarding the abundance of transcripts. To better understand the molecular mechanism(s) underpinning prelaying and laying periods in the Sichuan white goose, high-throughput RNA sequencing was performed to analyze the transcriptome in the hypothalamus. After sequencing and annotation, 26,921 unigenes were obtained, with 48 transcripts up-regulated in the prelaying period and 180 transcripts up-regulated during the laying period. These transcripts were primarily related to diseases, cancers, signaling molecules and interactions, the nervous system and the immune system. Eight transcripts were selected for further analyses with quantitative reverse-transcription polymerase chain reaction (qRT-PCR). The qRT-PCR results were mostly consistent with those from the high-throughput RNA sequencing. Among these transcripts, serine/threonine-protein kinase (AMPK), heat shock protein 70 (HSP70) and NADH dehydrogenase 1 (ND1) were differentially expressed during the prelaying and laying periods. The results of this study provide a useful resource for future studies examining the hypothalamus in geese.

Effects of Long-term Graded L-arginine Supply on Growth Development, Egg Laying and Egg Quality in Four Genetically Diverse Purebred Layer Lines.

The present study was conducted to examine effects of long-term graded L-arginine supply on growth development, egg laying and egg quality in four genetically diverse purebred layer lines. The study comprised a rearing trial from hatch to week 16 and a following laying performance trial from week 17 to 41. After hatch 150 one-day-old female chicks of each genotype were distributed to three diets. The experimental diets were equivalent to 70, 100 and 200% L-arginine of age-specific recommended level (National Research Council, 1994) and were offered ad libitum to chicks (hatch to week 7), pullets (week 8 to 16) and hens (week 17 to 41). However, hens’ diets were quite low in crude protein. After a pre-laying period from week 17 to 21 thirty-six pullets of each group were used further in the laying performance trial. Independent of chicken's genetic background, insufficient L-arginine supply caused lower body weight, daily weight gain and daily feed intake during the rearing (p<0.001) and induced lower laying intensity and daily egg mass production in the laying period (p<0.05). Parameters fitted to Gompertz function suggested higher adult body weight in L-arginine supplemented birds compared to insufficient supplied ones (p<0.01). Groups fed with insufficient L-arginine reached age of maximum daily weight gain later and showed lowest maximum daily weight gain (p<0.001). As a consequence of limitations in dietary L-arginine and crude protein, high performing genotypes decreased strongly in body weight, daily feed intake and performance compared to the low performing genotypes. In conclusion, L-arginine modified the amount of weight gain and feed intake, especially in growing chicks and pullets independent of genetic background. The high performing hens were more nutritionally stressed than the low performing ones, because concentrations of dietary crude protein were relatively low.

Transitional pullet feed and its significance at sexual maturity

Two biological experiments were carried out to find out the optimum energy, protein and calcium requirements in prelay pullet diets. Experiments involved different protein, energy and calcium levels during pre-lay period, two to three weeks before sexual maturity. It was revealed that a pre-lay diet with 2700 kcal/kg of energy, 18 per cent protein with 1.5 or 2 per cent calcium produced pullet at lower cost, had better body protein and fat and influenced bone status in terms of higher defatted dry tibial bone weight and longer and wider tibial bone. It could be concluded that pre-lay diet containing 2700 kcal/kg of dietary energy, 18 per cent C.P. and 2 per cent calcium is advisable for pullets before sexual maturity.

Gene expression profiles in the pituitary glands of Sichuan White geese during prelaying and laying periods.

To better understand the molecular mechanism(s) underlying egg-laying in Sichuan white geese, the profiles of genes in the pituitary gland were investigated during the prelaying and laying periods. Total RNA was extracted from the pituitary glands of geese during prelaying or laying periods and cDNA was generated. After sequencing and annotation, 54 upregulated and 84 downregulated genes were obtained from gene libraries. These genes were related primarily to biosynthetic processes, cellular nitrogen metabolic processes, transport, cell differentiation, cellular protein modification processes, signal transduction, and small molecule metabolic processes. Eleven genes were selected for further analyses using quantitative real-time PCR, and the results were generally consistent with the profiling results. Among these genes, levels of gonadotropin-releasing hormone, gonadotropin-inhibitory hormone, vasoactive intestinal peptide and its receptor, follistatin, estrogen receptor beta, and the progesterone receptor were differentially overexpressed during the prelaying period compared with the laying period. These results provide a solid foundation for elucidating the molecular mechanism of egg-laying performance in Sichuan white geese.

On the “real estate market”: Individual quality and the foraging ecology of male Cory's Shearwaters

ABSTRACT An extended reproductive period and high variability in food resource availability at sea make good quality nest sites particularly important for the survival of pelagic seabird chicks. Despite high philopatry during the early pre-laying period, males compete strongly for nests, making this period a unique opportunity to independently assess the influence of nest-site characteristics and individual quality on the foraging behavior of Cory's Shearwater (Calonectris diomedea borealis) individuals. We found significant differences in the at-sea foraging behavior of males and females at temporal (trip duration) and spatial (foraging areas, trip distance, and trip sinuosity) scales, both of which are greater in females. Furthermore, we suggest that nests of higher quality are deeper and closer to the nest of a conspecific neighbor because both variables were associated with males foraging closer to the colony. Finally, we showed that during the early pre-laying period the influence exerted by nests on...

No selection on immunological markers in response to a highly virulent pathogen in an Arctic breeding bird.

In natural populations, epidemics provide opportunities to look for intense natural selection on genes coding for life history and immune or other physiological traits. If the populations being considered are of management or conservation concern, then identifying the traits under selection (or ‘markers’) might provide insights into possible intervention strategies during epidemics. We assessed potential for selection on multiple immune and life history traits of Arctic breeding common eiders (Somateria mollissima) during annual avian cholera outbreaks (summers of 2006, 2007 & 2008). We measured prelaying body condition, immune traits, and subsequent reproductive investment (i.e., clutch size) and survival of female common eiders and whether they were infected with Pasteurella multocida, the causative agent of avian cholera. We found no clear and consistent evidence of directional selection on immune traits; however, infected birds had higher levels of haptoglobin than uninfected birds. Also, females that laid larger clutches had slightly lower immune responses during the prelaying period reflecting possible downregulation of the immune system to support higher costs of reproduction. This supports a recent study indicating that birds investing in larger clutches were more likely to die from avian cholera and points to a possible management option to maximize female survival during outbreaks.

Trophic ecology of the Grey Wagtail Motacilla cinerea before and during the breeding season in the region of Bejaia (Algeria)

The diet composition of the Grey Wagtail Motacilla cinerea of the Babor Range is followed by analysis of faecal samples (90 faeces) before and during the breeding season 2010. The Grey Wagtail's diet varies depending on the stage of the breeding cycle at the southern edge of their breeding area in North Africa (Bejaia). The diet consists predominantly of aquatic preys (51.79%), with Coleoptera being the most frequent constituent (n=331, 45.5%). During the pre-laying period (February–March), the diet was variable (91 prey-taxa and H’=3,36 bits) and preys of medium size (5 to 8mm) were most common. During the incubation period (April–May), preys were mainly aquatic (60%) and larger (20 to 32mm). At the end of the breeding season (June–July), there was a greater occurrence of terrestrial preys (31 aquatic versus 30 terrestrial taxa).

Colony attendance and at‐sea distribution of thin‐billed prions during the early breeding season

Procellariiform seabirds have extreme life histories; they are very long‐lived, first breed when relatively old, lay single egg clutches, both incubation and chick‐rearing are prolonged and chicks exhibit slow growth. The early part of the breeding season is crucial, when pair bonds are re‐established and partners coordinate their breeding duties, but is a difficult period to study in burrow‐nesting species. Miniature geolocators (Global Location Sensors or GLS loggers) now offer a way to collect data on burrow attendance, as well as determine at‐sea movements. We studied the early breeding season in thin‐billed prions Pachyptila belcheri breeding at New Island, Falkland Islands. Males and females arrived back at the colony at similar times, with peak arrival in the last days of September. However, males spent more time on land during the pre‐laying period, presumably defending and maintaining the burrow and maximising mating opportunities. Males departed later than females, and carried out a significantly shorter pre‐laying exodus. Males took on the first long incubation shift, whereas females returned to sea soon after egg laying. During the pre‐laying exodus and incubation, males and females travelled at similar speeds (&gt; 250 km d−1) and were widely distributed over large areas of the Patagonian Shelf. Inter‐annual differences in oceanographic conditions were stronger during the incubation than during the pre‐laying exodus and were matched by stronger differences in distribution. The study thus suggests that extended trips and flexible distribution enable thin‐billed prions to meet the high energy demands of egg production and incubation despite low productivity in waters around the colony during the early summer.

Annual variation in the timing of breeding, pre‑breeding foraging areas and corticosterone levels in an Arctic population of black-legged kittiwakes

Late breeding usually occurs during years of poor environmental conditions, but the proximate mechanisms underlying this phenological pattern have been poorly documented. Here, we combined the deployment of GPS devices (from 2008 to 2010) and the monitoring of breeding parameters and baseline corticosterone levels (from 2007 to 2011) during the pre-laying period to investigate the proximate regulation of breeding date in an Arctic population of black-legged kit- tiwake Rissa tridactyla. The timing of breeding varied considerably during the course of the study and late breeding was associated with reduced clutch size and low breeding success at the indi- vidual level. Foraging strategies differed considerably between males and females and among years. All but one of the females tracked using GPS during the pre-laying period foraged inside the fjord, whereas tracked males foraged both inside and outside the fjord, using the deep waters of the Greenland-Svalbard ridge. Trips lasted longer and were to greater distances in 2009, the year of late breeding, compared to 2008 and 2010, highlighting a food scarcity in 2009. Cortico- sterone levels differed among years, and were the lowest in 2010, the year of earliest breeding. Moreover, kittiwakes exhibiting higher corticosterone levels tended to undertake longer trips when foraging outside the fjord. Breeding decision and laying date were not related to corticoster- one levels at the individual level, but were positively influenced by body condition, suggesting that complex proximate mechanisms may affect timing of breeding in kittiwakes.

Seasonal Sexual Segregation by Monomorphic Sooty Shearwaters Puffinus griseus Reflects Different Reproductive Roles during the Pre-Laying Period

Tracking technology has revolutionized knowledge of seabird movements; yet, few studies have examined sex differences in distribution and behavior of small to medium-sized, sexually-monomorphic seabirds. Application of bird-borne geolocation-immersion loggers revealed seasonal segregation in the sexually-monomorphic Sooty Shearwater Puffinus griseus, mainly in the pre-laying period, when there were clear differences in reproductive roles. Shearwaters first returned to the Falkland Islands on 27 Sept±8 d; males, on average, 8 d earlier than females. Prior to egg-laying, distribution at sea, colony attendance and behaviour depended on sex. Males foraged locally over the southern Patagonian Shelf and Burdwood Bank, spending mainly single days at sea and intervening nights in the burrow. Females, who flew for more of the day during this time, foraged in more distant areas of the northern Patagonian Shelf and Argentine Basin that were deeper, warmer and relatively more productive. Attendance of females at the colony was also more variable than that of males and, overall, males were present for significantly more of the pre-laying period (38 vs. 19% of time). Sex differences were reduced following egg-laying, with males and females using similar foraging areas and making trips of similar mean duration in incubation (7.6±2.7 d) and chick-rearing (1.4±1.3 d). Congruence continued into the non-breeding period, with both sexes showing similar patterns of activity and areas of occupancy in the NW Atlantic. Thus, seasonal changes in reproductive roles influenced patterns of sexual segregation; this occurred only early in the season, when male Sooty Shearwaters foraged locally, returning regularly to the colony to defend (or maintain) the burrow or the mate, while females concentrated on building resources for egg development in distant and relatively more productive waters.

Molecular cloning and expression analysis of the synaptotagmin-1 gene in the hypothalamus and pituitary of Huoyan goose during different stages of the egg-laying cycle.

BackgroundSynaptotagmin-1 (Syt1) is an abundant, evolutionarily conserved integral membrane protein that plays essential roles in neurotransmitter release and hormone secretion. Neurotransmitters secreted by hypothalamic neurons can alter GnRH (gonadotropin-releasing hormones) neuronal activity by binding to and activating specific membrane receptors in pituitary cells and, in turn, control the release of gonadotropin hormones from the pituitary gland. To reveal the influence of Syt1 on the process of goose egg-laying, we cloned and characterized the cDNA of goose Syt1 originating from hypothalamus and pituitary tissues of Huoyan goose and investigated the mRNA expression profiles during different stages of the egg-laying cycle.MethodsHypothalamus and pituitary tissues were obtained from 36 Huoyan geese in the pre-laying period, early laying period, peak-laying period, and ceased period. The cDNA sequences of goose Syt1 were cloned and characterized from Huoyan goose tissues using 5’-RACE and 3’-RACE methods. Multiple alignments and phylogenetic analyses of the deduced Syt1 amino acid sequence were conducted using bioinformatics tools. The expression profiles of the Syt1 mRNA in the hypothalamus and pituitary during pre-laying, early laying, peak-laying and ceased period were examined using real-time PCR (qRT-PCR).ResultsThe cDNA of Syt1 consisted of a 274 bp 5’ UTR, a 1266 bp open reading frame (ORF) encoding 421 amino acids, and a 519 bp 3’ UTR. The deduced amino acid sequence of goose Syt1 is highly conserved with the sequence from other species, especially with birds (more than 98%), and contains two protein kinase C2 conserved regions (C2 domain) from amino acids residue 157 to 259 and 288 to 402. The results of qRT-PCR demonstrated that the expression of Syt1 mRNA increased from the pre-laying period to the peak-laying period, reached its peak in the peak-laying period, and then decreased in the ceased period.ConclusionsTo the best of our knowledge, this study is the first to obtain full-length cDNA sequences of the goose Syt1 gene, and the results of Syt1 mRNA expression profiling in the hypothalamus and pituitary tissues suggested that Syt1 may play an important role in regulating the secretion of hormones relevant to the reproduction and egg-laying of female geese.

Sexual Maturity and Performance of Pullets Fed Different Preparations and Concentrations of Brown Marine Algae (Sargassum Dentifebium) in Pre-Laying and Early Laying Periods

The effect of brown marine algae (BMA; Sargassum dentifebium) as alternative feed source for pullets was studied in three processed and at three concentrations in the pullets from 14-42 weeks. The processing forms were sundried brown marine algae (SBMA), sundried and boiled brown marine algae (BBMA) and sundried and autoclaved brown marine algae (ABMA). The concentrations of BMA were 2%, 4% and 6% that composed 10 treatments along with control. Each treatment was replicated 6 times using 30 pullets per treatment. Different criteria on pullets and eggs, including feed intake, body weight, feed conversion efficiency, laying rate and egg mass and quality were studied. Results indicated that BMA could be used up to 6% in the pullets diets from 14 to 42 weeks without adverse effects (P≥0.05) on sexual maturity (139-142 days), laying rate (80.7-87.9%), egg mass (44.99-51.86 g/hen/day), feed conversion ratio (2.468-2.868 kg feed/kg egg) and Haugh unit (82.9-90.6) and shell percentage (8.61-9.87%). Furthermore, egg yolk color and calcium content in eggshell were improved (P≤0.05) by 12.31% and 9.1%, respectively.

Effects of pre-laying attendance and body condition on long-term reproductive success in Wandering Albatrosses

To conserve threatened species it is important to protect the most productive individuals in the population, yet there is little information on the characteristics that distinguish such individuals. Among experienced Wandering Albatrosses (Diomedea exulans), we show that current breeding performance is correlated with previous breeding success. By comparing birds with unproductive and productive breeding histories, we test whether we can differentiate more and less successful breeders based on body condition, arrival date and attendance at the colony during the pre-laying period. Males in pairs with successful reproductive histories were present in the colony for longer than unsuccessful males. Productive females spent more days with their mates, suggesting that time spent with partners may be indicative of long-term reproductive success. Overall, birds that bred in a given season arrived earlier and spent more time at the colony than those that deferred breeding. Individuals with better body condition arrived earlier and spent more time at the colony. Body condition was independent of year and may indirectly affect the capacity to breed through its influence on arrival date and pre-laying attendance. Body condition and pre-laying attendance (and indirectly climate and food availability), may influence reproductive success, differentiating more and less successful breeders.

To breed or not to breed: endocrine response to mercury contamination by an Arctic seabird

Mercury, a ubiquitous toxic element, is known to alter expression of sex steroids and to impair reproduction across vertebrates but the mechanisms underlying these effects are not clearly identified. We examined whether contamination by mercury predicts the probability to skip reproduction in black-legged kittiwakes (Rissa tridactyla) from Svalbard. We also manipulated the endocrine system to investigate the mechanism underlying this relationship. During the pre-laying period, we injected exogenous GnRH (gonadotropin-releasing hormone) to test the ability of the pituitary to release luteinizing hormone (LH, a key hormone for the release of sex steroids and hence breeding) in relation to mercury burden. Birds that skipped reproduction had significantly higher mercury concentration in blood than breeders. Endocrine profiles of these birds also varied based on breeding status (breeders versus non-breeders), mercury contamination and sex. Specifically, in skippers (birds that did not breed), baseline LH decreased with increasing mercury concentration in males, whereas it increased in females. GnRH-induced LH levels increased with increasing mercury concentration in both sexes. These results suggest that mercury contamination may disrupt GnRH input to the pituitary. Thus, high mercury concentration could affect the ability of long-lived birds to modulate their reproductive effort (skipping or breeding) according to ongoing environmental changes in the Arctic, thereby impacting population dynamics.

Overcoming difficult times: the behavioural resilience of a marine predator when facing environmental stochasticity

Annual changes in the behaviour and distribution of top predators at sea may be linked to environmental variability. Here we report, for the first time to our knowledge, the inter- annual (2007�2011) foraging ecology during the pre-laying period of female Cory's shearwaters Calonectris diomedea from Berlenga (Portugal), based on biotelemetry data. Our aim was to exam- ine the degree of flexibility in the at-sea distribution, behaviour and habitat selection of foraging birds, and relate this with marine environmental stochasticity. Productivity proxies in the closer for- aging areas decreased noticeably between 2007 and 2011 (i.e. an increase of sea surface tempera- ture and a decrease of primary productivity). Female Cory's shearwaters perceived the oceanic changes, and shifted their distribution to an area that could have the food resources required for egg formation. In 2011, in order to exploit the productive Grand Banks and Newfoundland Shelf domains, female Cory's shearwaters embarked on one of the largest foraging excursions of the pre- laying period when compared to other seabird species (nearly 4000 km). The option to forage on such a distant area in 2011 decreased the females' body condition and reduced their hatching success, which may be interpreted as an adaptation to local productivity during this energetically highly demanding period to favour their own survival. Long-term monitoring of the foraging be - haviour of top predators such as Cory's shearwaters in years to come may serve as a 'sensitive' proxy to help understand the medium- to long-term effects of environmental stochasticity in marine ecological systems.

Characteristics and Sexual Functions of Sky-Dancing Displays in a Semi-Colonial Raptor, the Montagu'S Harrier (Circus pygargus)

Abstract Raptors perform spectacular aerial displays, but our understanding of their complexity and functions is still limited. We describe the characteristics of Montagu's Harrier (Circus pygargus) sky-dancing displays and evaluate possible sexual functions. Sky-dancing was performed by both sexes, but much more frequently by males. Although occurring throughout the breeding season and into the post-fledging period, sky-dancing displays by males were more frequent during the pre-laying period (from 3 wk before laying, when pair formation is occurring, until just after laying onset). Pre-laying display frequency by males and females increased with colony size and vole abundance. A principle component analysis on display characteristics (height, speed, number of loops, twists, undulations, and calls) identified three main axes (PC1-3). PC1 correlated with all parameters, identifying displays that started at higher altitude, descended at higher speeds, with higher frequencies of twists, undulations, loops, ...

Effects of Food Availability on Yolk Androgen Deposition in the Black-Legged Kittiwake (Rissa tridactyla), a Seabird with Facultative Brood Reduction

In birds with facultative brood reduction, survival of the junior chick is thought to be regulated primarily by food availability. In black-legged kittiwakes (Rissa tridactyla) where parents and chicks are provided with unlimited access to supplemental food during the breeding season, brood reduction still occurs and varies interannually. Survival of the junior chick is therefore affected by factors in addition to the amount of food directly available to them. Maternally deposited yolk androgens affect competitive dynamics within a brood, and may be one of the mechanisms by which mothers mediate brood reduction in response to a suite of environmental and physiological cues. The goal of this study was to determine whether food supplementation during the pre-lay period affected patterns of yolk androgen deposition in free-living kittiwakes in two years (2003 and 2004) that varied in natural food availability. Chick survival was measured concurrently in other nests where eggs were not collected. In both years, supplemental feeding increased female investment in eggs by increasing egg mass. First-laid (“A”) eggs were heavier but contained less testosterone and androstenedione than second-laid (“B”) eggs across years and treatments. Yolk testosterone was higher in 2003 (the year with higher B chick survival) across treatments. The difference in yolk testosterone levels between eggs within a clutch varied among years and treatments such that it was relatively small when B chick experienced the lowest and the highest survival probabilities, and increased with intermediate B chick survival probabilities. The magnitude of testosterone asymmetry in a clutch may allow females to optimize fitness by either predisposing a brood for reduction or facilitating survival of younger chicks.

Intracellular glucocorticoid receptors in spleen, but not skin, vary seasonally in wild house sparrows (Passer domesticus)

Over the short-term and at physiological doses, acute increases in corticosterone (CORT) titres can enhance immune function. There are predictable seasonal patterns in both circulating CORT and immune function across many animal species, but whether CORT receptor density in immune tissues varies seasonally is currently unknown. Using radioligand binding assays, we examined changes in concentrations of glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) in spleen and skin in wild-caught house sparrows in Massachusetts during six different life-history stages: moult, early winter, late winter, pre-egg-laying, breeding and late breeding. Splenic GR and MR binding were highest during the pre-laying period. This may help animals respond to immune threats through increased lymphocyte proliferation and/or an increase in delayed-type hypersensitivity reactions, both of which CORT can stimulate and in which spleen is involved. A decrease in splenic GR and MR during the late breeding period coincides with low baseline and stress-induced CORT, suggesting immune function in spleen may be relatively CORT-independent during this period. We saw no seasonal patterns in GR or MR in skin, suggesting skin's response to CORT is modulated primarily via changes in circulating CORT titres and/or via local production of CORT in response to wounding and other noxious stimuli.

Pre-breeding ecophysiology of a long-distance migratory seabird

The energetic demands of animals change throughout their annual life cycle. In migrating birds, reproduction and migration are the two most energy demanding processes; the transition from one to the other require a number of physiological adjustments. When arriving to the breeding grounds, long-distance migratory birds need to recover from migration and prepare for reproduction. This process is crucial, since pre-breeding body condition has been established as a critical factor influencing reproductive success in several avian species. In the present study we examined the physiological condition of the Cory's shearwater (Calonectris diomedea borealis, Cory 1881) after arrival from migration to a breeding colony, as well as its changes throughout the pre-laying period. We weighed and took blood samples from 149 shearwaters newly arrived to their breeding colony. If birds were found again after 10, 20, 30 or 40days since their first capture, they were resampled. We examined individual variation of biochemical parameters, body mass and blood oxygen transport capacity in both males and females. Our results indicate that birds were improving their body condition after arrival from migration. Plasma metabolites revealed shearwaters were depositing fat and degrading muscular mass during post-migration and pre-laying, suggesting shearwaters were remodeling their body composition to have more space for lipid reserves. Females started fat deposition earlier than males, probably to cope with the lipid demands of egg formation. This study shows that biochemical blood parameters are good indicators of the ecophysiological changes experienced by post-migrating and pre-breeding birds.