Informed renesting decisions: the effect of nest predation risk

Animals should cue on information that predicts reproductive success. After failure of an initial reproductive attempt, decisions on whether or not to initiate a second reproductive attempt may be affected by individual experience and social information. If the prospects of breeding success are poor, long-lived animals in particular should not invest in current reproductive success (CRS) in case it generates costs to future reproductive success (FRS). In birds, predation risk experienced during breeding may provide a cue for renesting success. Species having a high FRS potential should be flexible and take predation risk into account in their renesting decisions. We tested this prediction using breeding data of a long-lived wader, the southern dunlin Calidris alpina schinzii. As predicted, dunlin cued on predation risk information acquired from direct experience of nest failure due to predation and ambient nest predation risk. While the overall renesting rate was low (34.5 %), the early season renesting rate was high but declined with season, indicating probable temporal changes in the costs and benefits of renesting. We develop a conceptual cost-benefit model to describe the effects of the phase and the length of breeding season on predation risk responses in renesting. We suggest that species investing in FRS should not continue breeding in short breeding seasons in response to predation risk but without time constraints, their response should be similar to species investing in CRS, e.g. within-season dispersal and increased nest concealment.     

Nest defence in the meadow pipit <Emphasis Type="Italic">Anthus pratensis</Emphasis>: the influence of renesting potential

The reproductive value hypothesis predicts that the level of nest defence is determined by the expected chance of offspring to survive until reproduction, and by the reproductive potential of the parents. Rates of survival from one breeding season to the next are low in small passerines, and their residual reproductive potential strongly declines as the current breeding season terminates. Therefore, we can expect that parents which have only one breeding attempt per season should defend their nests more intensively than parents with a possibility to renest. We studied nest defence in populations of meadow pipit (Anthus pratensis) breeding in Norway and the Czech Republic, differing in renesting potential. To simulate the threat from a predator, we placed a stuffed stoat (Mustela erminea) first 5 m and then 1 m away from a nest with nestlings. Parents increased or kept nest defence constant when the stoat approached their nests in Norway and, during a breeding season shortened by severe weather, in the Czech Republic (when renesting potential was limited). Parents decreased nest defence when the stoat approached the nest during “normal” breeding seasons in the Czech Republic (when renesting was common). These findings give support to the reproductive value hypothesis.     

Night conditions affect morning incubation behaviour differently across a latitudinal gradient

Intermittently incubating birds alternate between sessions of egg warming and recesses for foraging during the day, but stay on the nest continuously at night. Hence, energy costs of nocturnal incubation (which increase during longer and colder nights) cannot be replenished until the next day. Night conditions might therefore be expected to affect morning incubation behaviour the day after. We tested this prediction by exploring latitudinal and seasonal trends in characteristics of the first recess in Eastern Bluebirds Sialia sialis over a 1400-km latitudinal gradient in the continental USA. The time from civil dawn to leaving the nest (latency) increased with latitude early in the breeding season but decreased with latitude late in the season. Birds breeding at higher latitudes also took longer first recesses throughout the season, which led to a larger drop in nest temperature. At the local scale, birds rose earlier after longer nights if the night was also cold, but night length did not predict latency following warm nights. The first recess was longer if the night was warmer, probably because birds could replenish reserves at lower risk of low egg temperature. Our study shows that characteristics of the night led to behavioural changes in features of early morning incubation that were evident at both continental and local scales. These responses also affected nest temperature. Hence, night conditions carry over to incubation behaviour the following morning, which in turn may impose thermal constraints on embryonic development.     

Senescence and carryover effects of reproductive performance influence migration,condition, and breeding propensity in a small shorebird

Breeding propensity, the probability that an animal will attempt to breed each year, is perhaps the least understood demographic process influencing annual fecundity. Breeding propensity is ecologically complex, as associations among a variety of intrinsic and extrinsic factors may interact to affect an animal's breeding decisions. Individuals that opt not to breed can be more difficult to detect than breeders, which can (1) lead to difficulty in estimation of breeding propensity, and (2) bias other demographic parameters. We studied the effects of sex, age, and population reproductive success on the survival and breeding propensity of a migratory shorebird, the piping plover (Charadrius melodus ), nesting on the Missouri River. We used a robust design Barker model to estimate true survival and breeding propensity and found survival decreased as birds aged and did so more quickly for males than females. Monthly survival during the breeding season was lower than during migration or the nonbreeding season. Males were less likely to skip breeding (range: 1–17%) than females (range: 3–26%; β_sex = ?0.21, 95% CI: ?0.38 to ?0.21), and both sexes were less likely to return to the breeding grounds following a year of high reproductive success. Birds that returned in a year following relatively high population‐wide reproductive output were in poorer condition than following a year with lower reproductive output. Younger adult birds and females were more likely to migrate from the breeding area earlier than older birds and males; however, all birds stayed on the breeding grounds longer when nest survival was low, presumably because of renesting attempts. Piping plovers used a variety of environmental and demographic cues to inform their reproduction, employing strategies that could maximize fitness on average. Our results support the “disposable soma” theory of aging and follow with predictions from life history theory, exhibiting the intimate connections among the core ecological concepts of senescence, carryover effects, and life history.     

Aggressiveness in king penguins in relation to reproductive status and territory location

King penguins, Aptenodytes patagonicus, vigorously defend small territories in very dense colonies. The egg-laying season lasts approximately 4 months, but only pairs that reproduce during the first half of the period succeed in fledging a chick. I examined various factors affecting aggressiveness of king penguins during the breeding season and focused on the differences between central and peripheral territories. Pairs on peripheral territories experienced twice as many encounters with avian predators as did central birds. The vast majority of peripheral birds were late breeders, indicating that reproductive success was very low among penguins defending a territory on the edge of the colony. Time invested in territory defence and rate of agonistic encounters between breeding neighbours increased from the incubation to the brooding period. Parents gave most threat displays to territorial neighbours when the chick was very young and just before crèche formation. Distance to colony edge was not related to aggressiveness in incubating birds, however, the rate of pecking and flipper blows increased from the edge to the centre during brooding. In addition, aggressiveness of breeding penguins towards travelling birds trespassing into their territory increased with distance to edge. Early breeders were not more aggressive than late breeders but the proportion of time spent in territory defence increased with the number of days a bird had spent incubating. As expected, I did not detect any sex difference in aggressive behaviour. Birds occupying territories on the beach were generally more aggressive during the incubation period than those located on the valley sides. Reproductive status (incubating versus brooding) and territory location were the main factors explaining the various levels of aggressiveness observed in breeding king penguins. Copyright 2000 The Association for the Study of Animal Behaviour.     

Avoid nest predation when predation rates are low,and other lessons: testing the tropical–temperate nest predation paradigm

Nest predation is the most important cause of nest failure in most birds and latitudinal differences in nest predation rates and life histories suggest that nest predation has been influential in life history evolution. All else equal, natural selection should favor reduction of nest predation, yet evidence is equivocal. We used Monte Carlo simulations to examine the combined effects of variation in nest predation rates, breeding season length and renesting intervals on the annual number of young fledged. Simulations suggest that selection most strongly favors a reduction in nest predation when breeding seasons are short and predation rates are low (temperate characteristics). Conversely, selection favors shorter renesting intervals when breeding seasons are long and nest predation rates are high (tropical characteristics). Reducing already low rates provides a proportionately greater increase in annual nesting success than does the same reduction when nest predation rates are higher. In some tropical species, individuals increase reproductive success not by avoiding predation in subsequent nesting attempts, which is largely beyond their control, but rather by reducing renesting intervals. We suggest that the emphasis on nest predation avoidance has biased our perspectives for alternative hypotheses of how birds should respond to nest predation and the consequences of those alternatives for life history theory. Similarly to the need to control for phylogenetics in examining life history strategies, future studies must also control for differences in breeding season lengths and renesting intervals to better understand the influence of nest predation on avian life histories.     

Is hissing behaviour of incubating great tits related to reproductive investment in the wild?

Anti-predator behaviour of breeding animals is a complex trait that depends on current reproductive investment as well as individual differences in risk-taking propensities. In response to nest predation, many bird species produce specific sounds, such as the hissing calls in incubating great tits (Parus major), that may provoke an acoustic startle response in the predator. However, it is still unclear whether the propensity of incubating females to produce hissing calls towards nest predators depends on the reproductive investment. With our 3-year study, we show that response type (females that do not hiss versus females giving hissing calls) to a potential nest predator, the woodpecker, is a repeatable trait. We found no differences in the studied reproductive traits between hissing and non-hissing birds. Interestingly, among hissing birds, fast-responding females started egg-laying earlier than slow-responding ones. Among non-hissing birds, heavier birds initiated clutches earlier. We also revealed that hissing birds breed in areas with decreased nest-box occupancy, suggesting either that they potentially select different areas to breed or that territory size is larger as a result of hissing birds being more aggressive. These findings demonstrate that response type is not related to the early reproductive value of the brood across distinct behavioural groups. However, our results do suggest that non-hissing and hissing females differ in terms of individual quality or dominance or personality related aspects.     

Breeding limits foraging time: evidence of interrupted foraging response from body mass variation in a tropical environment

Birds should store body reserves if starvation risk is anticipated; this is known as an ‘interrupted foraging response’. If foraging remains unrestricted, however, body mass should remain low to limit the predation risk that gaining and carrying body reserves entails. In temperate environments mass gain in female birds during breeding is often attributed to egg formation and mass loss after incubation to flight adaptation or the effect of reproductive workload, rather than as a result of an adaptive interrupted foraging response to the limited foraging time or unpredictable foraging conditions that breeding demands. In tropical environments, foraging conditions vary more within the breeding season than in temperate environments, and so studies in tropical environments are more suited to decouple the potentially confounded effects of increase in body reserves versus egg formation on the body mass of breeding birds. In this study, we test whether breeding results in an interrupted foraging response in a tropical savannah system using body mass data collected over a 15‐year period from female common bulbuls Pycnonotus barbatus. This species breeds both in the wet and dry season, despite fewer resources being available in the dry season. Breeding stage predicted female body mass: body mass peaked abruptly during incubation, but was not closely associated with the egg‐laying stage, and declined during brood rearing. Breeding females were heavier in the dry season than in the wet season. In the dry season, heavier birds were more likely to incubate eggs or brood chicks. These observations suggest that increased body reserves are required to buffer the consequence of limited foraging time or impoverished foraging conditions, which may be most pronounced during incubation and in the dry season, respectively. Such mass increases are consistent with an interrupted foraging response, which may apply to temperate zone birds experiencing foraging restrictions during breeding.     

Stage-dependent foraging in breeding black-legged kittiwakes Rissa tridactyla: distinguishing behavioural responses to intrinsic and extrinsic factors

Different phases of the annual cycle in birds and mammals are often associated with characteristic and recurrent foraging behaviours. The extent to which stage‐dependent changes in foraging behaviour are caused by intrinsic or extrinsic factors is unclear. We controlled for the effects of extrinsic factors by synchronising groups of incubating and chick‐rearing black‐legged kittiwakes Rissa tridactyla. Synchrony amongst incubators and rearers was achieved experimentally by switching eggs between nests. Behavioural responses to the treatment varied between the sexes. Male kittiwakes with prolonged‐incubation made fewer foraging trips but of greater duration compared to those rearing chicks resulting in no change in the time spent on trips between the two groups. Females with prolonged‐incubation carried out fewer trips than those rearing chicks but trip duration did not differ between the two stages which resulted in prolonged‐incubating birds spending a lower percentage time on trips. In contrast, foraging ranges did not differ between prolonged‐incubation and chick‐rearing birds for either sex. This suggests that extrinsic factors, such as food availability and distribution determine kittiwake foraging locations and ranges, whereas intrinsic factors, reflected in parental duties, constrain nest attendance. Female prolonged‐incubators invested lower levels of parental effort, in terms of daily energy expenditure, compared to chick‐rearers whereas males did not show stage‐related differences in energy expenditure. This provides evidence that incubation could be an energetically cheaper stage although under normal conditions this difference may be masked by temporal variation in environmental factors. We conclude that while conditions differ between the incubation and chick rearing stages for kittiwakes at this colony, they are not the main factors prompting changes in stage‐related foraging patterns. Intrinsic factors such as sex differences, or behaviours required for each stage of the annual cycle, rather than extrinsic factors related to seasonal environments, are likely to be the main proximate cause of recurring changes in behaviour between breeding stages.     

Renesting intervals and duration of the incubation and nestling periods of Sprague's Pipits

ABSTRACT.   Sprague's Pipits ( Anthus spragueii ) are one of the least studied birds in North America, with most information about their breeding biology based on anecdotal observations at relatively few nests. I determined the renesting intervals of female Sprague's Pipits in south-central Saskatchewan and also summarized data about incubation and nestling periods of Sprague's Pipits in Saskatchewan and Manitoba to provide more robust estimates of these life-history parameters. Two of eight radio-marked females initiated replacement clutches after failed nesting attempts, with renesting intervals of 11–13 and 15–16 d, respectively. Of 74 banded females, 3 were subsequently recaptured at later nests during the same breeding season. Nests of two of these females failed due to inclement weather and their renesting attempts were initiated 6 and 7–11 d later, respectively. The third female successfully fledged young and then initiated another nest 17 d later. The new nests of females that renested were all located less than 150 m from previous nests. Incubation and nestling periods for known-aged nests ranged from 12 to 15 d (13.4 ± 0.3 [SE], N = 9) and 11 to 14 d (12.1 ± 0.2, N = 39), respectively. Variation in the length of both incubation and nestling periods was likely a function of prolonged periods of inclement weather. More data are required to determine the extent of renesting in different habitats and the extent to which Sprague's Pipits are capable of successfully raising multiple broods in a single breeding season.     

Adverse effects of instrumentation in incubating Adélie penguins (Pygoscelis adeliae)

The use of data-loggers has permitted to explore the biology of free-ranging animals. However, this method has also been reported to reduce reproductive success while the reasons of this deleterious effect remain poorly documented. In this study, we aimed to identify critical periods of the breeding cycle of Adélie penguins (Pygoscelis adeliae) when the reproductive success may decrease because of instrumentation. For this purpose, we monitored 40 pairs, where one parent was instrumented before egg laying and 30 pairs without devices (controls). These pairs were followed at least during the incubation period but the majority was monitored during the entire breeding season. Reproductive success was affected in pairs where males were instrumented. This was not due to extra chick mortality during chick rearing but to a significantly lower hatching success. Moreover, the use of artificial eggs recording incubation temperatures and egg rotation indicated that in instrumented incubating males, eggs spent as much time at optimal incubation temperatures as control eggs but were rotated at a higher frequency. In Adélie penguins, males initiate incubation and it has been established that the early stage of incubation is one of the most critical periods for embryonic development. The low hatching rate observed in instrumented males was associated with a higher egg rotation rate, perhaps as a stress response to the presence of the instrument. Even though the causal effects remain unclear, instrumentation severely affected hatching success. For these reasons, we recommend equipping birds after the early incubation.     

Increased ambient temperature alters the parental care behaviour and reproductive success of the three-spined stickleback (<Emphasis Type="Italic">Gasterosteus aculeatus</Emphasis>)

The effects of environmental warming on aquatic poikilotherms, such as fish, are likely to be manifest during periods of high metabolic demand. For species that invest heavily in parental-care, such as the three-spined stickleback, Gasterosteus aculeatus L., their reproductive success may be adversely affected. In two separate experiments with temperatures raised by 2°C to 6°C above 16–17°C ambient over a whole breeding season, we quantified changes to parental-care behaviour and the resultant reproductive success of G. aculeatus. As temperature increased, male parental-care behaviour was altered, particularly the fanning of the fertilised eggs. Fanning behaviour was highly variable among individual fish however it increased over the course of incubation. Furthermore, all egg incubating fish consistently fanned at a faster rate in higher temperatures. The male fish responded to the increased temperature by putting more effort into fanning. The consequence was that these fish had a higher rate of incubation failure and an increased likelihood of mortality. The pattern of alteration to parental care behaviour and decreased reproductive success with higher temperature was remarkably consistent across the individual fish, which suggests consequences at the population level of increased ambient temperatures.     

Time-dependent reproductive decisions in the blue tit

Many breeding attempts in birds do not result in any fledged young due to predation on eggs or young. Consequently, the influence of time constraints on reproductive decisions are integrated parts of the reproductive behaviour of birds breeding within short, seasonal climate zones. In this study, I mimicked nest predation by removing blue tit (Parus caeruleus) clutches shortly after completion. Around 75% of the removed clutches were followed by a repeat clutch. Females producing their first clutch early in the season and females with an early onset of incubation in the laying sequence (an indication of high parental or territory quality) were most likely to initiate a repeat clutch. A trade‐off between the benefits of a repeat clutch and survival likely stopped late females in bad condition from investing more in the current reproductive season. Females producing a repeat clutch laid fewer eggs, had an earlier onset of incubation in the laying sequence and produced larger eggs than they did when producing their original first clutch. Eggs produced after the onset of incubation were especially large in the repeat clutches. Since food availability was presumably higher when the female produced her repeat clutch compared with her first clutch, females made a strategical decision when reducing clutch size, whereas onset of incubation and egg size may have been energetically constrained when producing the first clutch. Females that produced a relatively large clutch, had a relatively early onset of incubation, and laid relatively large eggs in their first clutch also did so when producing a repeat clutch, indicating that some of the variation in breeding parameters are due to differences in parental or territory quality. Differences between years in the temperature‐dependent development rate of caterpillars seem to affect the time constraints on breeding. A year with a predicted early seasonal decline in caterpillars resulted in short intervals between removal and relaying, small clutches and an early onset of incubation.     

Dynamics of an immune response in house sparrows Passer domesticus in relation to time of day, body condition and blood parasite infection

European starlings ( Sturnus vulgaris ) introduced to North America compete with native cavity-nesting birds for nest sites. I examined whether natural selection could favour a strategy of delayed nesting in a population of northern flickers ( Colaptes auratus ), a native woodpecker, to reduce overlap in breeding phenology with starling competitors. I developed a mathematical model based on reproductive parameters for a population of flickers from central British Columbia, Canada. On average, 7% of flicker nests each year were usurped by starlings; daily probabilities of takeover declined through the season but were relatively low (1–2%). Flickers laid between 3 and 13 eggs and there was a negative correlation between clutch size and date. The probability of renesting after nest failure also declined during the season, but renesting intervals (2–19 days) were not associated with female age, or stage of nest loss. The model suggested that costs of delaying reproduction would outweigh benefits of an early start except when the probability of nest usurpation is very high (>75%) early in the season and declines rapidly through the summer. Thus, early laying dates should be favoured especially in northern latitudes where breeding seasons are short.     

Modulation of the hypothalamic-pituitary-adrenal axis of an Arctic-breeding polygynandrous songbird,the Smith's longspur,Calcarius pictus

To successfully reproduce in the Arctic, birds must modulate their neuroendocrine and behavioural systems. These adjustments include an attenuation of the stress responsiveness of the hypothalamic-pituitary-adrenal (HPA) axis to external stimuli and a behavioural insensitivity to high corticosterone (B) levels. The HPA axis was examined in free-living territorial polygynandrous Smith's longspurs (Calcarius pictus) that migrate to breed on the Arctic tundra. Basal and stress-induced B levels were measured through the breeding season and were found to be significantly lower in females compared with males. This was not a consequence of adrenal insensitivity, because intrajugular injections of adrenocorticotrophin hormone (ACTH) enhanced B release in incubating females. In males the adrenocortical response to stress was significantly attenuated during the parental phase compared with arrival at the breeding ground. In contrast to temperate passerines, there was no significant decrease in male territorial aggressive behaviour when B was experimentally elevated, suggesting a behavioural insensitivity to glucocorticoids. This mechanism is hypothesized to increase reproductive success by preventing interruptions to parental care during transient deleterious environmental perturbations, which are often experienced in the short Arctic breeding season. Modulation of the HPA axis in this species in relation to life-history stage, lifetime reproductive success and the polygynandrous mating system is discussed.     

The mating system of the White-throated Magpie-jay Calocitta formosa and Greenwood's hypothesis for sex-biased dispersal

Greenwood explained the different sex bias in dispersal of birds (usually female biased) and mammals (usually male biased) by a difference in mating systems: male birds primarily defend resources while male mammals primarily defend females. The White-throated Magpie-jay Calocitta formosa is unusual among birds in that females are philopatric and jointly defend permanent resource territories while males disperse before they are 2 years of age. One female in a group is the primary breeder. One male joins the group permanently as her mate. Males that do not have a permanent breeding position circulate among groups and attempt to mate with both the primary breeding female and other group females. Other females feed the primary breeder and her offspring and also pursue other reproductive behaviour, including secondary nesting in the territory and egg dumping into the primary breeder's nest. I argue that the unusual dispersal pattern in this species is a result of the alternative reproductive strategies that can be pursued by males and females excluded from being primary breeders. The White-throated Magpie-jay conforms to Greenwood's predictions: males pursue a mate defence rather than resource defence mating system and they are the dispersing sex. The primary factor influencing alternative reproductive tactics may be asynchronous reproduction among groups during the long breeding season arising from frequent renesting in an area of high nest predation.     

Plasma levels of prolactin and gonadal steroids in relation to multiple-brooding and renesting in free-living populations of the song sparrow, Melospiza melodia

Plasma levels of testosterone (T), estradiol (E2), and prolactin were measured in samples collected from free-living song sparrows, Melospiza melodia. In males, plasma levels of T were elevated early in the season when territories were established and when females laid the first clutch of eggs. Thereafter, T levels declined and remained low throughout the remainder of the breeding cycle. However, if the first brood was lost to a predator, or by experimental removal of the nest, plasma levels of T increased as renesting, to replace the clutch, occurred. Circulating levels of prolactin in males began to rise during the egg-laying stage of the first brood, reached a maximum toward the end of the incubation stage, remained elevated until breeding was terminated, and then declined throughout the moulting stage to basal values in October. Prolactin levels remained high throughout the breeding season irrespective of whether a brood was raised successfully or whether the nest was lost and renesting occurred. In females, plasma levels of E2 were elevated prior to the egg-laying stage for each brood as is typical of multiple-brooded species. However, prolactin titers rose dramatically during egg-laying for the first clutch (slightly higher than in males) and were maximal by onset of incubation. Only females of this species incubate, although males do feed young. As in males, plasma prolactin in females remained high between broods and during experimentally induced renesting, and then declined to basal by the end of the moult stage in October. These data suggest that there are no differences in the temporal patterns of prolactin concentrations in blood between multiple-brooding and renesting. In a separate experiment, captive male song sparrows were transferred from a short day to artificial long days (18L 6D) and a control group was maintained on 9L 15D. In the long-day group, prolactin levels rose abruptly over the first 20 days, as the testes developed, and remained high well into postnuptial moult after the gonads had regressed. Prolactin remained basal in the control group. These data suggest that the temporal pattern of circulating prolactin levels throughout the breeding season is regulated at least partly by changing photoperiod. However, nonphotoperiodic factors are also important since photoperiodically induced increases in prolactin are significantly less than those seen in free-living individuals. These differences may be related to parental behavior.     

Strategies of mass change in breeding birds

Birds experience strikingly different patterns of mass change during breeding. In some species with uniparental incubation, the incubating parents (mostly females) lose mass and attain their lowest point when the chicks hatch (Incubatory mass loss strategy = IML). In species with shared incubation, or with intense incubation feeding, the incubating parents maintain or increase in body mass without reducing attentiveness levels (Incubatory mass constancy = IMC). In other species, uniparental incubation with IMC is associated with reduced levels of attentiveness. The mobilization of fat stores or its preservation are involved in the two strategies. IML leads to mass increases after hatching of the young, while the opposite is true for IMC. The non-incubating sex in uniparental species does not experience significant mass changes during breeding. Fasting endurance, predation risk and mode of development are proposed as the main selective factors determining strategy. Latitude, climate, diet and food availability interact with the main factors. From a revision of the literature we can deduce that IML is mainly found among large birds with precocial development, while IMC is typical of smaller species with altricial development. Proportional mass losses are positively correlated with body mass in IML-species, as well as in precocial species, where body mass explains more than 70% of the variation in proportional mass change. Incubation periods increase with body mass in uniparental IMC-species, but not in IML-species. IML is thus associated to fast embryonic growth in large species. Successful raising of highly-dependent young in altricial and semialtricial species apparently depends on one of the parents retaining reserves until hatching. Subsequent mass losses may be necessary to maintain the brooding parent through a period when nestlings require heat, insulation and food. Patterns of mass change are not mere consequences of reproductive stress but the outcome of adaptive compromises between different selective factors and constitute an important aspect of the breeding biology and life history of birds.     

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.     

Territorial aggression does not feed back on testosterone in a multiple-brooded songbird species with breeding and non-breeding season territoriality,the European stonechat

Testosterone mediates reproductive behaviours in male vertebrates. For example, breeding season territoriality depends on testosterone in many species of birds and in some, territorial interactions feed back on testosterone concentrations. However, the degree to which territorial behaviour and testosterone are associated differs even between species with seemingly similar life histories, especially between species that also defend territories outside the breeding season. Here, we investigate the link between territorial behaviour and testosterone in European stonechats. Previous studies found that territorial aggression in stonechats depends on testosterone in a breeding, but not in a non-breeding context. We investigated whether stonechats show a rise in testosterone during simulated territorial intrusions (STI) during the breeding season. Post-capture testosterone concentrations of males caught after an STI were not higher than those of males caught in a control situation regardless of breeding stage. However, most of the males would have been able to mount a testosterone response because the same individuals that did not increase testosterone during the STI showed a substantial increase in testosterone after injections of gonadotropin-releasing hormone (GnRH). GnRH-induced and post-capture testosterone concentrations were positively correlated and both decreased with successive breeding stages. Further, territory owners with a short latency to attack the decoy expressed higher post-capture testosterone concentrations than males with a longer latency to attack the decoy. Thus, there is no evidence for behavioural feedback on testosterone concentrations during male-male interactions in stonechats. In combination with previous studies our data suggest that testosterone functions as an on/off switch of high intensity territorial aggression during the breeding season in stonechats. The among-species variation in the androgen control of territorial behaviour may be only partly a result of environmental differences. Instead, potential differences in how territoriality evolved in different species may have influenced whether and how a reproductive hormone such as testosterone was co-opted into the mechanistic control of territorial behaviour.