Reduced immunocompetence of nestlings in replacement clutches of the European magpie (Pica pica)

Laying date is one of the most important determinants of reproductive success and recruitment probability in birds. Late breeders usually fledge fewer chicks than individuals with earlier breeding dates, and fledglings produced late in the season have high mortality rates. Food availability and nestling mass have been evoked as the principal mechanistic links between laying date and offspring survival. Here we suggest that another factor may actually account for the difference in survival rate between early and late offspring: immunocompetence. We predicted that nestlings produced later in the season or in replacement clutches should have lower immune responses when challenged with an antigen, than early nestlings or nestlings produced in first clutches. This hypothesis was tested in a population of magpies (Pica pica), in which we experimentally induced breeding failure in a group of nests and compared the immune response of nestlings in replacement clutches with the immune response of first clutch nestlings. Cellular immune response, as measured by wing web swelling (a correlate of T-lymphocyte production after injection of phytohaemagglutinin-P), significantly decreased with hatching date and was significantly lower in nestlings of replacement clutches. Furthermore, coefficients of intraclutch variation in immune response were higher in nestlings of replacement clutches. This experiment demonstrates an inverse relationship between immune responsiveness and breeding date, and reduced recruitment probability of late nestlings may be a direct consequence of their inability to cope with parasites.     

Floaters as intraspecific brood parasites in the grey starling Sturnus cineraceus

The breeding season of the grey starling Sturnus cineraceus is divided into two periods: the early and late breeding season. The birds that breed in each season are referred to as early and late breeders, respectively. In this study, the late breeders mainly consisted of new immigrants that did not breed in the early season. This suggested that these new immigrants were probably floaters in the early breeding season. Because intraspecific brood parasitism occurred frequently, it is possible that the parasites were floaters without nesting boxes. To check for the presence of floaters, two field experiments were conducted and floaters were captured with traps. With additional nesting boxes provided during the breeding season, all new boxes were quickly occupied by floaters from the period of incubation to hatching in the early breeding season, but were not occupied by floaters during the egg laying period of the late breeding season. The addition of boxes before the start of the breeding season significantly decreased the parasitic rate and number of parasitic eggs per nest. There was a positive correlation between the relative occupancy of nesting boxes and the parasitic rate. The removal of boxes again increased the parasitic rate. As for the capture of floaters with traps, the number of trapped birds per day was also related to the relative occupancy of nesting boxes. The floaters trapped between incubation and nesting periods of early breeders became the late breeders. Judging from these results, many floaters were present from the incubation to hatching periods of early breeders, and were probably intraspecific brood parasites.     

Reduced reproductive performance associated with warmer ambient temperatures during incubation in a winter‐breeding,food‐storing passerine

Timing of reproduction can influence individual fitness whereby early breeders tend to have higher reproductive success than late breeders. However, the fitness consequences of timing of breeding may also be influenced by environmental conditions after the commencement of breeding. We tested whether ambient temperatures during the incubation and early nestling periods modulated the effect of laying date on brood size and dominant juvenile survival in gray jays (Perisoreus canadensis), a sedentary boreal species whose late winter nesting depends, in part, on caches of perishable food. Previous evidence has suggested that warmer temperatures degrade the quality of these food hoards, and we asked whether warmer ambient temperatures during the incubation and early nestling periods would be associated with smaller brood sizes and lower summer survival of dominant juveniles. We used 38 years of data from a range‐edge population of gray jays in Algonquin Provincial Park, Ontario, where the population has declined over 50% since the study began. Consistent with the “hoard‐rot” hypothesis, we found that cold temperatures during incubation were associated with larger brood sizes in later breeding attempts, but temperatures had little effect on brood size for females breeding early in the season. This is the first evidence that laying date and temperature during incubation interactively influence brood size in any bird species. We did not find evidence that ambient temperatures during the incubation period or early part of the nestling period influenced summer survival of dominant juveniles. Our findings provide evidence that warming temperatures are associated with some aspects of reduced reproductive performance in a species that is reliant on cold temperatures to store perishable food caches, some of which are later consumed during the reproductive period.     

Some effects of inclement weather conditions on the survival and condition of bull-headed shrike nestlings

Inclement weather struck Japan in 1993, permitting a natural experiment for examining the effect of weather conditions on nesting success and nestling growth of the bull-headed shrike, Lanius bucephalus. Aspects of the breeding biology of bull-headed shrikes in relation to weather conditions and timing of breeding in 1992 and 1993 were examined. The two breeding seasons were divided into two periods, early and late, in each year. While the probability of nest survival in nestling stages during both periods was almost equal, the probability of nest survival in the egg stage during the early period was significantly lower than that during the late period. In 1993, the probability of nestling survival during the late period was significantly lower than during the early period; the late period had larger fluctuations of precipitation and was colder than the same period in 1992. The number of disappeared nestlings positively correlated with mean precipitation per day. The greater part of the disappeared nestlings was the lightest in each brood. Late breeders fledged lighter young than the early breeders. Although shrikes adopted hatching asynchrony, the late breeders could not surmount the unpredictable inclement weather in 1993.     

Priority effects of the early breeding fire salamander on the late breeding banded newt

Early breeding intraguild predators may have advantages over late breeding predators via priority effects; early breeding predators may reduce shared prey resources before late breeders appear and may also prey upon the late breeders. Here we show that predatory larvae of the late-breeding predatory banded newt, Triturus vittatus vittatus, occupy the same temporary pond toward the end of the developmental period of the early-breeding predatory fire salamander, Salamandra salamandra, resulting in a large size disparity between larvae of these two species while they co-occur. We conducted outdoor artificial pool experiments to assess priority effects of large larval Salamandra at the end of their larval development period, on recently hatched larval Triturus. We also assessed how artificial vegetation may influence larval Triturus performance in the presence or absence of Salamandra Salamandra, introduced into the experimental pools two weeks prior to the newt larvae, strongly reduced invertebrate prey abundance shared by these two predatory urodeles and with only a one week period of overlap, strongly reduced abundance of Triturus larvae. The artificial vegetation had only a small ameliorating effect on Triturus survival when Salamandra was present. Triturus size at metamorphosis (snout-tail length) was significantly larger in the Salamandra pools, presumably due to a combination of a strong “thinning effect” and greater vulnerability of smaller Triturus individuals to predation by Salamandra. Time to metamorphosis was not significantly affected by Salamandra. These results have conservation implications as T. v. vittatus is listed as highly endangered and may also explain the largely negative spatial association of the two species. Handling editor: K. Martens     

COMPARISON OF THE DIET OF BREEDING AND NONBREEDING CAPE GANNETS MORUS CAPENSIS

Berruti, A. 1991. Comparison of the diet of breeding and nonbreeding Cape Gannets Morus capensis. Ostrich 62:8-12.

The effects of breeding status on the diet composition of Cape Gannets Morus capensis were tested at two colonies in the western Cape, South Africa. At both colonies, there were no significant differences in the species composition of the diet, but the mean energy content of regurgitations of breeders was significantly greater. Although the regurgitations of breeders were heavier and contained more fish at both colonies, only the number of fish per regurgitation from breeders at one colony was significantly greater. Birds feeding chicks at Malgas Island took significantly shorter fish than nonbreeders. The small effect of breeding status on prey species composition shows that temporal and geographical variation in prey availability is more important than breeding status in affecting diet cornposition of the Cape Gannet.     

Factors influencing plasticity in the arrival‐breeding interval in a migratory species reacting to climate change

Climate change is profoundly affecting the phenology of many species. In migratory birds, there is evidence for advances in their arrival time at the breeding ground and their timing of breeding, yet empirical studies examining the interdependence between arrival and breeding time are lacking. Hence, evidence is scarce regarding how breeding time may be adjusted via the arrival‐breeding interval to help local populations adapt to local conditions or climate change. We used long‐term data from an intensively monitored population of the northern wheatear (Oenanthe oenanthe) to examine the factors related to the length of 734 separate arrival‐to‐breeding events from 549 individual females. From 1993 to 2017, the mean arrival and egg‐laying dates advanced by approximately the same amount (~5–6 days), with considerable between‐individual variation in the arrival‐breeding interval. The arrival‐breeding interval was shorter for: (a) individuals that arrived later in the season compared to early‐arriving birds, (b) for experienced females compared to first‐year breeders, (c) as spring progressed, and (d) in later years compared to earlier ones. The influence of these factors was much larger for birds arriving earlier in the season compared to later arriving birds, with most effects on variation in the arrival‐breeding interval being absent in late‐arriving birds. Thus, in this population it appears that the timing of breeding is not constrained by arrival for early‐ to midarriving birds, but instead is dependent on local conditions after arrival. For late‐arriving birds, however, the timing of breeding appears to be influenced by arrival constraints. Hence, impacts of climate change on arrival dates and local conditions are expected to vary for different parts of the population, with potential negative impacts associated with these factors likely to differ for early‐ versus late‐arriving birds.     

Climate change and micro-geographic variation in laying date

Climate change has been shown to affect the timing of reproduction, with earlier reproduction being associated with an increase in temperature. Changes in the timing of reproduction arise from changes in food availability as well as other factors, and differences in the timing of reproduction among sites may cause sites with early reproduction to contribute disproportionately to local recruitment. In this study, spatial variation in the laying date of barn swallows Hirundo rustica at 39 sites in a 45-km²study area during the period 1971–2004 was used to investigate micro-geographic patterns in the timing of breeding. I found that individuals breeding at sites with early reproduction had a disproportionately large reproductive success. Early sites were characterized by early plant phenology, as determined by the date of leafing of the broad-leaved elm Ulmus glabra and date of flowering of the snowdrop Galanthus nivalis during a single year. Such early sites showed greater advancement in laying date between 1971 and 2004 than the average site. Early sites were also generally occupied during more years by a larger number of breeders than were late sites. Breeders at early sites produced more fledglings, and breeders at such sites were characterized by a smaller adult body size and larger secondary sexual characters than individuals at the average site. These observations are consistent with the hypothesis that temporal changes in the timing of reproduction occur as a consequence of differential recruitment at phenologically early sites that produce disproportionately large numbers of offspring. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Function of courtship feeding in the Bull-headed Shrike,Lanius bucephalus

We investigated the frequency of courtship feeding (CF) in the Bull-headed Shrike (Lanius bucephalus) during the 1981–1982 breeding seasons. Because no CF was observed just prior to, during or just after copulation, CF is not associated directly with copulation and does not function as a releaser for copulation. The absence of CF during the pair-formation period of the early breeder and the high frequency of CF behavior in the cold season and in the critical breeding stages (egg-laying and incubation stages) for females strongly suggests that the significance of CF in this species is not symbolic but substantial at least in early breeders. We assign one of the causes of this CF to the early onset of breeding.     

Breeding performance,mate fidelity,and nest site fidelity in a long‐lived seabird: behaving against the current?

There is evidence that breeding failure is associated with divorce and dispersal in many bird species. However, deviations from the general pattern “success‐stay/failure‐leave” seem to be common, suggesting that factors other than breeding performance may importantly influence mate and habitat selection. Moreover, variability in response to performance suggests coexistence of different evolutionary strategies of mate and site selection within a population. In this study, we assessed how individuals conform to the success‐stay/failure‐leave pattern in kittiwakes (Rissa tridactyla), and aimed to identify categories of individuals presenting different behavioural patterns. We considered individual attributes (experience, prior residence at the nest site, performance in multiple breeding attempts), pair attributes (arrival asynchrony, timing of failure, pair duration), and productivity in habitat patches. Timing of failure was an important factor. Pair reunion probability was close to 0.5 in failed pairs, but it was consistently higher in early failed than in late failed pairs. Prior residence better explained variability in probability of reunion in failed pairs than pair duration. However, the positive influence of prior residence on the probability of reunion was perceptible only in early failed pairs. Divorce probability in successful pairs increased with arrival asynchrony, and was higher in first‐time than in experienced breeders. Local productivity positively influenced site fidelity probability in early failed birds, but not in late failed ones. Using memory models, we found that dispersal decisions integrate information on individual breeding performance in a temporal scale longer than one year. This study contributed to the identification of relevant states to be considered when addressing mate and nest site choice. Natural selection may operate on slight fitness differences that cannot be detected without high levels of stratification according to the appropriate individual and habitat attributes.     

Male territorial aggression and androgen modulation in high latitude populations of the Sooty, Passerella iliaca sinuosa, and Red Fox Sparrow, Passerella iliaca zaboria

The Fox Sparrows, Passerella iliaca, include multiple groups and subspecies distributed at several latitudes from the Alaskan arctic to the southwestern United States. As such, this species represents a potential model for investigating latitudinal variation in androgen secretion and aggressive territoriality in male passerines. Breeding male Fox Sparrows from two subspecies within two groups, the Sooty Fox Sparrow, P. i. sinuosa, and the Red Fox Sparrow, P. i. zaboria, were assessed for aggressive territoriality and androgen responsiveness at multiple latitudes in arctic and subarctic Alaska. Subarctic Sooty Fox Sparrows had higher circulating androgen levels in the early (8.54 ng/ml) versus mid–late breeding season (2.44 ng/ml). Males in the mid–late breeding season did not up-regulate androgen secretion in response to social challenge, but were aggressive and spent more time within 5 m of a decoy during a simulated territorial intrusion (STI) than early breeding males. Male subarctic Red Fox Sparrows had slightly higher circulating androgen levels (2.29 ng/ml) than arctic males (1.10 ng/ml) in the mid–late breeding season. However, androgen levels were not correlated with blood collection time after a social challenge in either group, suggesting that neither arctic nor subarctic males up-regulate androgen secretion during the mid–late breeding period. Arctic males spent more time within 5 m of a decoy and sang less than subarctic males during an STI in the mid–late breeding season. These findings demonstrate that the Fox Sparrow is a tractable model for investigating the latitudinal regulation of aggressive territoriality and androgen responsiveness in passerines.     

The effects of male mating behaviour and food provisioning on breeding success in snow buntings <Emphasis Type="Italic">Plectrophenax nivalis</Emphasis> in the high Arctic

For passerine birds breeding in the Arctic, paternal effort in parental care is necessary for successful breeding. Behavioural strategies, such as mate guarding, to ensure paternity should therefore also be common in this environment. In order to investigate the relation between such behaviour and breeding success, when controlling for the effect of environmental factors, we recorded male mate-guarding behaviour, parental effort and breeding success amongst snow buntings, Plectrophenax nivalis, in the high Arctic environment of Spitsbergen, Svalbard. Mate-guarding intensity tended to positively affect male feeding frequency per nestling in one of the study years, negatively in 1 year and without effect in the 3rd year. The negative effect in 1999 was strong, but variation in estimates of the 2 other years was high and the effect of mate guarding may be negligible in these 2 years. We assume that this pattern might be related to yearly variation in the prevalence of extra-pair young. Male food provisioning was not positively related to breeding success, but late breeders had higher breeding success than early breeders. Thus, the effect of male mate guarding is highly variable and mainly affects food provisioning rates, but the benefit for total breeding success seems to be marginal, at best.     

Moult strategies of Cory’s Shearwaters Calonectris diomedea borealis: the influence of colony location, sex and individual breeding status

The replacement of old feathers is essential for birds, but it is also an energy-demanding task. As moult usually does not coincide with other stressful events in its annual cycle, such as reproduction and migration, the bird can optimise its use of time and energy allocated to different activities. There are very few studies comparing the moult strategies of populations with similar breeding calendars but occurring in areas of different habitat quality. Cory’s Shearwaters Calonectris diomedea have a partial moult–breeding overlap, an unusual phenomenon among pelagic seabirds. We have compared the moult schedules in Cory’s Shearwater colonies located in distinct environments (pelagic vs. coastal) and show that moult–breeding overlap is less extensive on Selvagem Grande, situated in deep oceanic waters, than on Berlenga, situated on the continental shelf. Colony attendance of failed breeders, most of which were moulting, was lower at Selvagem Grande than at Berlenga, which suggests that the feeding areas of birds from the former site are more distant from the colony. Failed breeders started to moult earlier than individuals still raising a chick, and breeding status had a stronger influence on determining the onset of wing-feather moult than colony location. Despite published evidence that internal circannual rhythms and external cues, such as variation in daylength, are important factors regulating moult schedules, it is clear that birds retain a considerable flexibility that allows them to respond to external factors in order to strategically manage time and energy in a way that is thought to maximise their fitness.     

Health impact of blood parasites in breeding great tits

Hypotheses of hemoparasite-mediated sexual selection and reproductive costs rely on the assumption that avian blood parasite infections are harmful to their hosts. To test the validity of this assumption, we examined the health impact of Haemoproteus blood parasites on their great tit (Parus major) host. We hypothesised that if blood parasites impose any serious health impact on their avian hosts, then infected individuals must differ from uninfected ones in respect to hemato-serological general health and immune parameters. A 3-year study of two great tit populations, breeding in contrasting (urban and rural) habitats in south-east Estonia, revealed that Haemoproteus blood parasites affected the health state of their avian hosts. Infected individuals had elevated lymphocyte hemoconcentration and plasma gamma-globulin levels, indicating that both cell-mediated and humoral immune response mechanisms are involved in host defence. The effect of parasites on cell-mediated immunity was both age- and sex-specific, as infection status affected peripheral blood lymphocyte counts only in males, and among these, the magnitude of response was greater in old individuals than yearlings. Heterophile hemoconcentration and plasma albumin levels were not affected by infection status, suggesting that blood stages of Haemoproteus infection do not cause a severe inflammatory response. Parasitism was not related to hematocrit values, indicating that Haemoproteus infection does not cause anemia. In two years, infected individuals were heavier than uninfected ones in the urban but not in the rural study area. This suggests, that under certain circumstances (possibly related to reproductive tactics), breeding great tits may avoid losing body mass in order to save resources for an anti-parasite immune response. Received: 16 February 1998 / Accepted: 22 May 1998     

Restoration of body mass in King Penguins after egg abandonment at a critical energy depletion stage: early vs late breeders

In fasting‐incubating seabirds, it has been proposed that egg abandonment and refeeding should be induced when a low body mass (BM) threshold is attained, thus ensuring adult survival at the expense of immediate breeding. In the context of life‐history trade‐offs in long‐lived birds, we have tested this hypothesis by comparing short‐term survival and restoration of BM in King Penguins Aptenodytes patagonicus that abandoned their egg to those that were relieved normally by their mate at the end of the first incubation shift. Since King Penguins have an extended laying period, the possible influence of seasonal factors was also examined by comparing early and late breeders. Forty incubating males were experimentally forced to fast until egg abandonment by preventing relief by the female. At egg abandonment of both early and late breeding males, BM was below the BM threshold, fasting duration was eight days (about 30%) longer than for relieved birds, and plasma uric acid level was elevated (signature of increased body protein catabolism, phase III of fasting). All abandoning birds survived and came back from sea at a BM similar to that of relieved penguins. The duration of the foraging trip of abandoning early breeders was the same as that of relieved birds, and some abandoning birds engaged in a new breeding attempt. Abandoning late breeders, however, made foraging trips twice as long as those of relieved males. This difference can be explained by time constraints rather than nutritional constraints, abandoning early breeders having enough time left in the breeding season to engage in a new breeding attempt in contrast to abandoning late breeders. These observations lend support to the suggestion that not only BM but also an internal clock intervene in the decision to engage in breeding or not. By preventing a lethal energy depletion ashore and by acting at a fasting stage where the capacity to restore BM at sea is unaffected, abandonment at a low body condition threshold plays a major role in the trade‐off between adult penguin survival and reproduction.     

Pre-breeding energy requirements: thyroid hormone, metabolism and the timing of reproduction in house sparrows Passer domesticus

We measured thyroid hormone (T3) levels and energy expenditure of pre-breeding house sparrows Passer domesticus in relation to the timing of breeding and reproductive success. The onset of reproduction was synchronised in two waves, separated by a three-week interval. On an annual basis, early breeders (birds that bred for the first time during the first wave) made significantly more breeding attempts, laid significantly more egg and raised 2.3 times more chicks to fledging than late breeders (birds that bred for the first time during the second wave). By the end of March, about one month before the first egg was laid in the population, plasma titres of testosterone in males and estradiol in females were still low and did not differ between early and late breeders. However, birds that subsequently bred early had higher titres of plasma triiodothyronine (T3) than birds that started to breed late. We show for the first time in a free-living bird population that Basal Metabolic Rate (BMR) is positively correlated with the plasma titre of T3. Differences in plasma T3 accounted for 48% of the inter-individual variation in BMR. Elevated T3 levels indicate that energy requirements increase prior to breeding. Although early breeding appears to be advantageous in terms of the number of offspring raised on an annual basis, the increased energy requirements prior to breeding are thought to delay the onset of reproduction in those birds that cannot afford the additional energy expenditure early in the season.     

Population change in breeding boreal waterbirds in a 25-year perspective: What characterises winners and losers?

1. Understanding drivers of variation and trends in biodiversity change is a general scientific challenge, but also crucial for conservation and management. Previous research shows that patterns of increase and decrease are not always consistent at different spatial scales, calling for approaches combining the latter. We here explore the idea that functional traits of species may help explaining divergent population trends.
2. Complementing a previous community level study, we here analyse data about breeding waterbirds on 58 wetlands in boreal Fennoscandia, covering gradients in latitude as well as trophic status. We used linear mixed models to address how change in local abundance over 25 years in 25 waterbird species are associated with life history traits, diet, distribution, breeding phenology, and habitat affinity.
3. Mean abundance increased in 10 species from 1990/1991 to 2016, whereas it decreased in 15 species. Local population increases were associated with species that are early breeders and have small clutches, an affinity for luxurious wetlands, an herbivorous diet, and a wide breeding range rather than a southern distribution. Local decreases, by contrast, were associated with species having large clutches and invertivorous diet, as well as being late breeders and less confined to luxurious wetlands. The three species occurring on the highest number of wetlands all decreased in mean abundance.
4. The fact that early breeders have done better than late fits well with previous research about adaptability to climate change, that is, response to earlier springs. We found only limited support for the idea that life history traits are good predictors of wetland level population change. Instead, diet turned out to be a strong candidate for an important driver of population change, as supported by a general decrease of invertivores and a concomitant increase of large herbivores.
5. In a wider perspective, future research needs to address whether population growth of large-bodied aquatic herbivores affects abundance of co-occurring invertivorous species, and if so, if this is due to habitat alteration, or to interference or exploitative competition.

     

Comparison of Color and Body Condition Between Early and Late Breeding King Penguins

Early breeding is associated with greater reproductive success in many species. In king penguins, Aptenodytes patagonicus , laying extends for 6 mo. Early breeders may fledge a single chick at best, but late breeders virtually never fledge a chick. For early and late breeders, we compared colored ornaments known to be important in mate choice: yellow–orange feathers of the breast and auricular areas, and an ultraviolet and yellow–orange beak spot. Our purpose was to discern differences between males and females in this highly sexually monomorphic species, as well as to discern whether colored ornaments are more important for the more successful early breeders (aspects of color were hue, chroma, and brightness). For this, we weighed and measured 130 penguins. Early males had greater reflectance of ultraviolet color from the beak spot than did early females and late breeders of both sexes, and the early males were heavier and in better condition than late breeding males or females. Late breeding females were the yellowest in breast hue, a trait that has been linked to immunocompetence. Within pairs, males and females were significantly correlated in body mass, but only early in the breeding season. We concluded that early in the breeding season when reproductive success was greatest, potential mates were not only more similar in body mass, but also that females may have chosen males that had brighter beak spots and were in better body condition.     

Dispersal and recruitment during population growth in a colonial bird, the great cormorant Phalacrocorax carbo sinensis

While the factors influencing reproduction and survival in colonial populations are relatively well studied, factors involved in dispersal and settlement decisions are not well understood. The present study investigated exchanges of great cormorants Phalacrocorax carbo sinensis among six breeding colonies over a 13‐year period when the breeding population in Denmark increased from 2800 to 36 400 nests. We used a multistate capture‐recapture model that combined multisite resightings and recoveries to examine simultaneously recruitment, natal dispersal, breeding dispersal and annual survival of first‐year, immature and breeding great cormorants. Mean survival of first‐year birds (0.50±0.09, range=0.42–0.66 among colonies) was lower than survival of breeders (0.90±0.06, range=0.81–0.97). Mean survival of immature birds over the study period was 0.87±0.08. Dispersal from a colony increased with decreasing mean brood size in the colony in both first‐time and experienced breeders. The choice of the settlement colony in first‐time breeders was affected by conditions in the natal colony and in the colonies prospected during the pre‐breeding years. In particular, first‐time breeders recruited to colonies where they could expect better breeding success. Experienced breeders relied mainly on cues present early in the season and on their own breeding experience to choose a new breeding colony. Newly established colonies resulted mainly from the immigration of first‐time breeders originating from denser colonies. Dispersal was distance‐dependent and first‐time breeders dispersed longer distances than breeders. We suggest that the prospecting behaviour allows first‐time breeders to recruit in nearby as well as more distant potential breeding colonies. Dispersing breeders preferred to settle in neighbouring colonies likely to benefit from their experience with foraging areas. We discuss the importance of these movements for growth and expansion of the breeding population.     

Adaptive variation in senescence: reproductive lifespan in a wild salmon population

The antagonistic pleiotropy theory of senescence postulates genes or traits that have opposite effects on early-life and late-life performances. Because selection is generally weaker late in life, genes or traits that improve early-life performance but impair late-life performance should come to predominate. Variation in the strength of age-specific selection should then generate adaptive variation in senescence. We demonstrate this mechanism by comparing early and late breeders within a population of semelparous capital-breeding sockeye salmon (Oncorhynchus nerka). We show that early breeders (but not late breeders) are under strong selection for a long reproductive lifespan (RLS), which facilitates defence of their nests against disturbance by later females. Accordingly, early females invest less energy in egg production while reserving more for nest defence. Variation along this reproductive trade-off causes delayed or slower senescence in early females (average RLS of 26 days) than in late females (reproductive lifespan of 12 days). We use microsatellites to confirm that gene flow is sufficiently limited between early and late breeders to allow adaptive divergence in response to selection. Because reproductive trade-offs should be almost universal and selection acting on them should typically vary in time and space, the mechanism described herein may explain much of the natural variation in senescence.