Effects of presence of squabs upon plasma concentrations of prolactin and LH and length of time of incubation in ringdoves on "extended" incubatory patterns

The effects of a brief exposure to newly hatched squabs upon the concentrations of plasma prolactin and LH secretion and length of incubation time in ring doves were investigated using a novel experimental procedure. After 5 days of incubation birds were deprived of their nests for 3 days. On return to their nests, the birds immediately resumed incubation and laid a second clutch of eggs after 5 days at which time the first clutch was removed. A consequence of this procedure was that at the time of hatching, concentrations of plasma prolactin had passed their peaks and were falling to baseline values, while concentrations of plasma LH were beginning to increase. This experimental manipulation made it possible to demonstrate that brief exposure to newly hatched squabs significantly affects the concentration of plasma prolactin, an effect which was more pronounced in females than in males. This increased secretion of prolactin was associated with a reduction of plasma LH which reached significance in females but not in males. Birds exposed to squabs spent a significantly longer period of time on their nests as compared to those birds not exposed to squabs. This observation is consistent with a role for prolactin in maintaining readiness to incubate.     

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.     

Influence of Nest Deprivation on Behaviour,Hormonal Concentrations and on the Ability to Resume Incubation in Domestic Hens (Gallus domesticus)

In this study, we investigated the influence of the length of nest deprivation period (3 vs. 6 d) on the ability to renest of incubating hens. We focused on the hens' behaviour, particularly on nesting and calls, on their prolactin, luteinizing hormone (LH) and oestradiol concentrations. Nest deprivation induced a drop of prolactin and an increase of LH plasmatic concentrations in hens. These changes in circulating pituitary hormones were followed by changes in ovarian function: the persistent rise of plasmatic oestradiol gave evidence of the resumption of ovarian activity. After nest deprivation, the number of clucks increased significantly and food calls appeared; these results demonstrate that ‘maternal’ calls can be emitted outside the maternal context. Our results suggest that the onset of typical maternal calling is strongly controlled by internal state such as plasmatic hormonal concentrations, independently of social stimulation. None of the 10 hens deprived for 6 d resumed incubation when given the opportunity, whereas, after a 3 d deprivation period, three out of 10 hens renested and two hens incubated sporadically and then gradually abandoned their nests. Long periods of nest deprivation appear to disrupt the habits of sitting and nesting. Before nest reopening, all the hens presented low levels of plasmatic prolactin. Plasma prolactin concentrations of the renesting hens increased after nest boxes were reopened and returned to levels found in incubating hens. We suggest that pituitary prolactin rather than plasmatic prolactin is responsible for the maintenance of incubating potential in hens deprived of their nests.     

Effects of ovariectomy or force feeding on the plasma concentrations of prolactin and luteinizing hormone in incubating turkey hens

The effect of ovariectomy (OVX) on plasma concentrations of prolactin (PRL) and luteinizing hormone (LH) in incubating turkey hens was studied. Neither the sham-operated nor the OVX hens exhibited any change in the pattern of incubation behavior as a result of the surgery. Plasma concentrations of estradiol decreased to less than approximately 3 pg/ml by 2 days after surgery in the OVX hens. There were no significant differences in plasma levels of PRL between the sham-operated and OVX hens throughout the study. The concentration of PRL did not change in either the sham-operated or OVX hens and was maintained at high levels after surgery and during incubation of the eggs. By 2 days after hens were placed into cages, plasma levels of PRL significantly decreased and were maintained at low levels in both groups. The concentration of LH did not change in either group during the two wk after surgery when the hens were incubating eggs. After the hens were placed into cages, the concentration of LH increased in the OVX hens and was maintained at significantly higher levels than in the sham-operated hens. By contrast, the concentration of LH increased within 4 days after OVX of out-of-lay but nonincubating hens. The delay in the postcastration increase in plasma level of LH in the OVX hens was not associated with anorexia of incubating hens, since plasma levels of LH were not affected by force-feeding unless plasma levels of PRI were suppressed by nest deprivation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Reinitiation of ovulatory cycles in incubating female turkeys by an inhibitor of serotonin synthesis, P-chlorophenylalanine

Nest deprivation of incubating turkeys caused a decrease in serum prolactin (Prl) levels from 1184.5 +/- 116.4 ng/ml to 896.8 +/- 83.0 ng/ml 1 day after initiation of deprivation, with a further decline to 156.5 +/- 111.7 ng/ml at the end of the 22-day experimental period. Serum luteinizing hormone (LH), progesterone and estradiol levels following nest deprivation were similar to those in birds allowed to incubate (controls). Oral administration of p-chlorophenylalanine (PCPA, 50 mg/kg) to incubating turkeys for 3 consecutive days reduced nesting frequency (P less than 0.05) on the 4th day after initiation of treatment and the nesting virtually ceased by the 9th day. Pretreatment Prl was 1655 +/- 210 ng/ml and declined (P less than 0.05) after PCPA administration to a low of 28.6 +/- 2.8 ng/ml. In addition, PCPA caused a sustained rise in serum LH peaking (5.59 +/- 1.09 ng/ml) 3 days after treatment initiation. Contrary to nest deprivation, serum levels of progesterone and estradiol increased (P less than 0.05) as a consequence of PCPA treatment. Seven of 8 PCPA-treated birds later came into lay when their Prl levels and nesting frequency increased again. The results suggest a role for serotonin (5-HT) in incubation behavior, and Prl and LH secretion in turkeys.     

Plasma prolactin levels in incubating turkey hens during pipping of the eggs and after introduction of poults into the nest

Plasma levels of prolactin (Prl) associated with incubation and maternal behavior were compared in turkey hens allowed to incubate 10 fertile eggs (Group I, n = 9) or 10 infertile eggs (Group II, n = 7) in open nest boxes. At the end of the day that the first egg hatched, all unhatched eggs were removed from Group I hens and each hen was given 10 poults. At the end of the following day, infertile eggs were removed from Group II hens and each hen was given 10 poults. Although pipping of the eggs changed the incubation behavior of Group I hens, it had no effect on plasma Prl. Subsequent hatch of the eggs and/or presence of poults resulted, within 24 h, in a sharp fall in Prl levels, abandonment of the nests, and a shift to maternal behavior. Visual and auditory exposure to Group I poults had no effect on plasma Prl or incubation behavior of Group II hens incubating infertile eggs in adjacent pens. However, within 24 h after the infertile eggs were exchanged for newly hatched poults, Prl levels in Group II hens declined sharply and the hens abandoned the nests and showed maternal behavior similar to that observed in Group I hens. No significant relationships were found in either group between plasma Prl levels and quality of incubation or maternal behavior.     

Modulation of gonadotrophin and prolacrin secretion by daylength and breeding behaviour in free-living starlings, Sturnus vulgaris

Plasma levels of luteinizing hormone (LH), follicle-stimulating hormone (FSH) and prolactin were measured by radioimmunoassay in plasma samples collected from free-living starlings, Sturnus vulgaris , trapped in nest-boxes. By leaving some nest-boxes undisturbed, and repeatedly destroying nests in others, birds from a single-brooded population were trapped whilst nest-building, incubating or feeding nestlings, at different times throughout the normal breeding season. In both males and females trapped whilst nest-building, plasma LH and prolactin levels increased progressively from mid March until late May. In females sampled during incubation, LH and FSH levels were high throughout May but decreased in early June. Prolactin levels were highest in late May. In both males and females trapped during mid May, LH levels were highest in these birds which were nest-building at this time and lowest in those feeding nestlings, FSH did not change significantly, and prolactin was low in those birds which were nest-building and high in those incubating or feeding nestlings. In female starlings from a double-brooded population, levels of LH and FSH were much lower whilst feeding the second brood than when feeding the first brood, whereas prolactin levels were similar. The results suggest that incubation and feeding behaviour inhibit the increase in LH secretion caused by increasing daylength, but stimulate prolactin secretion in excess of that caused by increasing daylength.     

Don'T put all your eggs in one nest: spread them and cut time at risk

Abstract In many egg-laying animals, some females spread their clutch among several nests. The fitness effects of this reproductive tactic are obscure. Using mathematical modeling and field observations, we analyze an unexplored benefit of egg spreading in brood parasitic and other breeding systems: reduced time at risk for offspring. If a clutch takes many days to lay until incubation and embryo development starts after the last egg, by spreading her eggs a parasitic female can reduce offspring time in the vulnerable nest at risk of predation or other destruction. The model suggests that she can achieve much of this benefit by spreading her eggs among a few nests, even if her total clutch is large. Field data from goldeneye ducks Bucephala clangula show that egg spreading enables a fecund female to lay a clutch that is much larger than average without increasing offspring time at risk in a nest. This advantage increases with female condition (fecundity) and can markedly raise female reproductive success. These results help explain the puzzle of nesting parasites in some precocial birds, which lay eggs in the nests of other females before laying eggs in their own nest. Risk reduction by egg spreading may also play a role in the evolution of other breeding systems and taxa-for instance, polyandry with male parental care in some birds and fishes.     

Egg concealment is an antipredatory strategy in a cavity‐nesting bird

Birds have developed different behavioural strategies to reduce the risk of predation during the breeding period. Bird species that nest in the open often cover their eggs to decrease the risk of predators detecting the clutches. However, in cavity nesters, the potential functions of egg covering have not been explored despite some bird species that nest in cavities also covering their eggs as open nesters do. We analysed whether egg covering is an antipredatory behaviour in the blue tit (Cyanistes caeruleus). We simulated an increase in the perceived risk of predation at experimental nests by adding predator scent inside the nest boxes during the egg‐laying period, whilst adding lemon essence or water to control nest boxes. Birds exposed to predator chemical cues in the nest of experimental pairs more frequently covered their eggs than birds exposed to an odorous control. These results suggest that egg covering may have evolved as an antipredatory behaviour also in cavity nesters to reduce the risk of egg predation and thus increase reproductive success in birds.     

The breeding biology of the endemic Berthelot's Pipit Anthus berthelotii in a harsh oceanic island environment (Tenerife,Canary Islands)

We describe the density, breeding and foraging ecology of the Berthelot's Pipit (Anthus berthelotii), an endemic passerine to the Atlantic islands, in the semi-arid coastal scrub of Tenerife. Furthermore, we analyse the effects of temperature and rainfall in the onset of reproduction and the behaviour (nest site and foraging site selection). The average density of the Pipit (using DISTANCE methodology) in the semi-arid coastal scrub was 7.1 birds 10ha^?1, and the overall density estimate in grassland was 3.3 birds 10ha^?1. The Pipit bred early (mean average first-egg laying date 12 February) and in response to rainfall (but not temperature). By the Mayfield method, the probability of the nests surviving the egg stage was 0.77 and of and the nests surviving the chick stage 0.67 — assuming an incubation period of 13 days and a chick stage of 12.5 days — based on the European average for the similar-sized Meadow Pipit Anthus pratensis. Overall nest survival pooling stages were 0.51. The Pipits had a low clutch size (3.3 eggs, range of 2–4). Nest damage by heavy rainstorms was the major cause of nest failures (three out of five failures). The Pipits nested preferentially in grass with relatively low densities of Euphorbia balsamifera bushes. Birds selected grassy areas as their main foraging site but also preferred to forage on large rocks during nesting: caterpillars were the main food items (14 out of 17 prey captures).     

The functional significance of multiple nest-building in the Australian Reed Warbler Acrocephalus australis

The vast majority of bird species build a nest in which to breed. Some species build more than one nest, but the function of most multiple nest-building remains unclear. Here we describe the unusual nest-building behaviour of the Australian Reed Warbler Acrocephalus australis , and test experimentally the hypotheses that multiple nest-building is related to individual condition or territory quality, and plays a role in mate assessment. Australian Reed Warblers built two types of nest structures: 'type I' nests, which were used for eggs and nestlings, and 'type II' nests, which were structurally distinct from type I nests, did not support eggs, nestlings or adults and were not essential for successful breeding. The number of type II nests built in each territory varied. Type II nests were only built before breeding had commenced in a territory and females were not observed participating in their construction, supporting a role in female mate choice. Birds provided with supplementary food built significantly more type II nests than control birds. However, supplementary-fed birds did not have greater pairing success, and the addition of further type II nests to territories did not increase the pairing rate or type II nest construction in those territories. There was no relationship between the presence of type II nests and either reproductive success or likelihood of nest predation. We discuss the implications of these results in light of previous suggestions regarding the function of multiple nest-building in birds.     

Incubation routine, body mass regulation and egg neglect in the Blue Petrel Halobaena caerulea

The incubation routine and mass changes of male and female Blue Petrel Halobaena caerulea were studied at the Kerguelen Islands to investigate factors influencing the durations of incubation stints and foraging trips at sea and the factors determining nest desertion and return to the nest.
The body mass at the start of an incubation shift and also when the bird was relieved varied throughout the incubation period, whereas the mass when birds deserted the nest was stable. Birds deserted the nest when their mass decreased to threshold, independent of the duration of the fast. Temporary egg neglect was observed in successful as well as in unsuccessful breeding attempts, but it increased the risk of breeding failure. The net and daily massgained at sea during the second part of the incubation period were higher than during the first part, suggesting an increase in food availability. During the first part, the mass gained at sea and time spent foraging were inversely related to the mass of the bird before it left the burrow, whereas a similar relationship did not occur thereafter.
The results suggest the occurrence of a fixed mass threshold when birds decide to leave the nest if not relieved by their partner. The mass when a bird left its nest inffuenced the time spent foraging or mass gained when food was scarce. Although decision rules to leave the nest or return from the sea are related to body condition. the possibility of neglecting the eggs temporarily enables Blue Petrels to regulate the trade-off between risks of breeding failure and risks of an increase in adult mortality. A model for behavioural decision to stop incubating or stop feeding, based on a variable set point, is proposed.     

Endocrine correlates of parental care in an Antarctic winter breeding seabird, the emperor penguin, Aptenodytes forsteri

Plasma levels of luteinizing hormone (LH) and prolactin associated with parental behavior were measured in the Antarctic winter breeding emperor penguin, Aptenodytes forsteri. Males exclusively incubate the egg while females exclusively brood the nonhomeothermic young; both sexes alternate in rearing the homeothermic young. Birds were sampled on arrival from the sea through egg laying, incubation, and brooding. All parent birds lost their chicks at the end of the brooding period due to harsh weather but sampling continued. In females, LH titers dropped after egg laying but levels were restored when the birds returned from the sea to brood the chicks and were not depressed by high prolactin levels. Plasma prolactin remained low in males captured on arrival and kept until the free-living males finished incubation. In breeders, prolactin secretion increased during the prelaying period when day length decreased. Prolactin levels stayed elevated in males during incubation and in brooding females returning after a 2-month absence at sea. Prolactin values were higher in brooding females than in males ending incubation or returning in late brooding. These levels did not drop after chick loss, and the sexual difference in prolactin values was maintained after breeding failure. In emperor penguins, increased prolactin secretion appears to be triggered around the time of egg laying and continues, driven by an endogenous mechanism, through incubation and brooding until rearing is completed. Prolactin secretion independent of external stimuli may have evolved in pelagic seabirds to maintain parental care despite long absences at sea from the breeding colony.     

The role of partial incubation and egg repositioning within the clutch in hatching asynchrony and subsequent effects on breeding success

The main mechanism to achieve hatching asynchrony (HA) for incubating birds is to start heating the eggs before clutch completion. This might be achieved through partial incubation and/or early incubation. Even in the absence of incubation behaviour during the laying phase, clutches still experience a certain degree of asynchrony. Recent studies have shown that eggs located in the centre of the nest receive more heat than peripheral ones during incubation. As eggs receiving more heat would develop faster, we hypothesized that HA should be shorter in nests where eggs were moved homogeneously along the centre–periphery space during incubation than in those nests where eggs repeatedly remained in the same locations, either centrally or peripherally. We explored the relative roles of egg repositioning and partial incubation in determining HA in wild birds by (1) removing eggs from 20 Great Tit Parus major nests on the day of laying and replacing them with fake eggs to avoid partial incubation, and returning them when full incubation began; (2) monitoring twice a day the position of each individually marked egg relative to the clutch centre during incubation, and estimating the coefficient of variation of the distances; and (3) determining HA in each nest. Preventing partial incubation reduced HA by 51% days in experimental nests. It also caused negative effects for the incubating females (lengthening the full incubation period) and positive effects for the brood (increasing fledging success). However, our hypothesis about the role of egg repositioning on HA was not supported: all the females moved the eggs with remarkable consistency, generally attaining a coefficient of variation of the distances around 33%, and it was not related to the HA experienced. We therefore conclude that partial incubation is an important factor regulating HA, and females compensate for the potential effects of differential heating by moving the eggs homogeneously within the clutch.     

Should I stay or should I go? Hormonal control of nest abandonment in a long-lived bird, the Adélie penguin

According to life-history theory, long-lived birds should favor their survival over the current reproductive attempt, when breeding becomes too costly. In seabirds, incubation is often associated with spontaneous long-term fasting. Below a threshold in body reserves, hormonal and metabolic shift characteristics of a switch from lipid to protein utilization (phase III, PIII) occur. These metabolic changes are paralleled by nest abandonment and stimulation of refeeding behavior. Parental behavior is then under control of two hormones with opposite effects: corticosterone (CORT) and prolactin which stimulate foraging and incubation behavior, respectively.The aim of this study was to determine the respective role of these two hormones in nest abandonment by Adélie penguins. To this end, plasma hormone levels were measured before egg-laying and at departure from the colony (i.e. when birds were relieved by their partner or abandoned their nest), and related to nutritional state and incubation success.We found that males abandoning their nest in PIII presented high CORT levels and low prolactin levels. Interestingly, males which presented high plasma levels of prolactin in PIII did not abandon. We show that although CORT is the first hormone to be affected by prolonged energy constraints, the combined effects of high CORT and low prolactin levels are necessary for parents to favor self-maintenance and abandon the nest. We provide insights into time-course changes of the endocrine profile as PIII proceeds and report that reaching proteolytic late fasting is not sufficient to induce nest abandonment in a long-lived bird.     

Should I stay or should I go? Hormonal control of nest abandonment in a long-lived bird,the Adélie penguin

According to life-history theory, long-lived birds should favor their survival over the current reproductive attempt, when breeding becomes too costly. In seabirds, incubation is often associated with spontaneous long-term fasting. Below a threshold in body reserves, hormonal and metabolic shift characteristics of a switch from lipid to protein utilization (phase III, PIII) occur. These metabolic changes are paralleled by nest abandonment and stimulation of refeeding behavior. Parental behavior is then under control of two hormones with opposite effects: corticosterone (CORT) and prolactin which stimulate foraging and incubation behavior, respectively.The aim of this study was to determine the respective role of these two hormones in nest abandonment by Adélie penguins. To this end, plasma hormone levels were measured before egg-laying and at departure from the colony (i.e. when birds were relieved by their partner or abandoned their nest), and related to nutritional state and incubation success.We found that males abandoning their nest in PIII presented high CORT levels and low prolactin levels. Interestingly, males which presented high plasma levels of prolactin in PIII did not abandon. We show that although CORT is the first hormone to be affected by prolonged energy constraints, the combined effects of high CORT and low prolactin levels are necessary for parents to favor self-maintenance and abandon the nest. We provide insights into time-course changes of the endocrine profile as PIII proceeds and report that reaching proteolytic late fasting is not sufficient to induce nest abandonment in a long-lived bird.     

Nocturnal incubation recess and flushing behavior by duck hens

Incubating birds must balance the needs of their developing embryos with their own physiological needs, and many birds accomplish this by taking periodic breaks from incubation. Mallard (Anas platyrhynchos) and gadwall (Mareca strepera) hens typically take incubation recesses in the early morning and late afternoon, but recesses can also take place at night. We examined nocturnal incubation recess behavior for mallard and gadwall hens nesting in Suisun Marsh, California, USA, using iButton temperature dataloggers and continuous video monitoring at nests. Fourteen percent of all detected incubation recesses (N = 13,708) were nocturnal and took place on 20% of nest‐days (N = 8,668). Video monitoring showed that hens covered their eggs with down feathers when they initiated a nocturnal recess themselves as they would a diurnal recess, but they left the eggs uncovered in 94% of the nocturnal recesses in which predators appeared at nests. Thus, determining whether or not eggs were left uncovered during a recess can provide strong indication whether the recess was initiated by the hen (eggs covered) or a predator (eggs uncovered). Because nest temperature decreased more rapidly when eggs were left uncovered versus covered, we were able to characterize eggs during nocturnal incubation recesses as covered or uncovered using nest temperature data. Overall, we predicted that 75% of nocturnal recesses were hen‐initiated recesses (eggs covered) whereas 25% of nocturnal recesses were predator‐initiated recesses (eggs uncovered). Of the predator‐initiated nocturnal recesses, 56% were accompanied by evidence of depredation at the nest during the subsequent nest monitoring visit. Hen‐initiated nocturnal recesses began later in the night (closer to morning) and were shorter than predator‐initiated nocturnal recesses. Our results indicate that nocturnal incubation recesses occur regularly (14% of all recesses) and, similar to diurnal recesses, most nocturnal recesses (75%) are initiated by the hen rather than an approaching predator.     

桂西南褐翅鸦鹃的孵卵行为与节律

2016年3~6月,在广西西南部龙州县弄岗村(22°26′35.20′′~22°30′46.90′′N,106°57′46.35′′~107°03′32.99′′E),通过野外观察和自动温度记录仪相结合的方法对褐翅鸦鹃(Centropus sinensis)的孵卵行为与节律进行了研究。结果表明,1)褐翅鸦鹃边筑巢边产卵,每2 d产1枚卵,卵长径和短径分别为(36.11±0.42)mm和(28.46±0.38)mm,卵重(16.35±0.51)g(n=44枚)。窝卵数3~5枚,孵卵期为(16.75±1.65)d(n=4巢),孵化率为45.45%(n=44枚)。孵卵期与窝卵数之间无显著相关性(r=0.865,P0.05);2)白天双亲共同参与孵卵,夜晚则由其中1只负责。夜间亲鸟的在巢时间从19时左右持续至翌日晨6时左右;3)亲鸟采取离巢次数少和离巢时间长的孵卵策略。亲鸟日活动时间在700 min以上(n=45 d),日离巢次数为(8.82±0.34)次(n=45 d),平均每次离巢持续时间为(52.91±2.35)min(n=397次),每次离巢持续时间与环境温度呈显著负相关关系(r=﹣0.113,P0.05);4)巢内平均孵卵温度为(31.7±0.3)℃(n=4巢),随孵卵天数增加而增加,并与环境温度(最高温r=0.566,最低温r=0.537,平均温r=0.706,P0.01)和日活动时间正相关(r=0.506,P0.01);5)有延迟孵卵行为。延迟孵卵期间夜晚巢内最低温是22.1℃。在桂西南北热带气候环境中,高的环境温度是保障褐翅鸦鹃孵卵成功的主要因素之一。     

Endocrine changes in the incubating and brooding turkey hen

Turkey hens were allowed to incubate eggs and to hatch and rear young. Plasma prolactin (Prl) levels increased prior to the start of continuous incubation and rose sharply as incubation progressed to reach a peak of 1178.2 +/- 221.8 ng/ml (mean +/- SEM) just before hatching. Prl levels then fell precipitously before the hens left the nest, and returned to preincubation levels (36.8 +/- 3.4 ng/ml) by the time the poults were 2 weeks old. These results show that the high plasma concentrations of Prl found during incubation are not initiated or maintained only by the stimulus of nesting. We suggest that the decline in Prl levels at the end of incubation could be related to the pipping and hatching of eggs, and the consequent shift to maternal behavior. Plasma growth hormone (GH) levels were significantly increased in hens which were brooding poults, but not in hens incubating eggs. An elevenfold, 1-day increase in plasma GH was observed immediately after the hens left the nests. Mean plasma GH levels rose from 12.0 +/- 4.7 ng/ml on the day that the hens left the nests to 133.0 +/- 32.0 ng/ml on the following day, and then declined to 23.1 +/- 9.6 ng/ml after an additional day. There were no significant changes in plasma thyroxine levels during laying, incubation and brooding. Plasma glucose concentration was significantly depressed during incubation.     

Plasticity of brood patch development and its influence on incubation periods in the yellow-eyed penguin Megadyptes antipodes: an experimental approach

In many bird species, eggs laid late in the laying period hatch after a shorter incubation period than early-laid eggs. However, the mechanisms that explain these seasonal declines in incubation periods among clutches remain poorly understood. In this study we investigated the plasticity of brood patch development during incubation in yellow-eyed penguins Megadyptes antipodes and established whether differences exist in brood patch formation among early, mean and late-breeding penguins. We also examined whether brood patch development was influenced by sex and age of birds. We then placed an artificial egg in nests a few days prior to egg laying to investigate whether the presence of an egg influences brood patch development and whether an advanced brood patch development at the time of egg laying causes declines in incubation periods. Initial brood patch width on the day the first egg was laid was dependent on sex and age, while the development of brood patch width after first egg laying was slower in early-laying birds than in mean- and late-laying birds. Initial brood patch temperature as well as temperature throughout incubation was largely dependent upon sex, whereby males had higher brood patch surface temperatures than females. Placement of an artificial egg in nests stimulated successfully brood patch development in manipulated birds, so that by the time they laid their own first egg, their brood patches were wider and had higher temperatures than those of control birds. Moreover, incubation periods of first eggs from manipulated nests were significantly shorter (43.5 days) than were those from control nests (47.3 days). Thus, variation in brood patch development and related differences in incubation temperature during early incubation could contribute to seasonal declines in incubation periods.