Increased survival and breeding performance of double breeders in little penguins Eudyptula minor,New Zealand: evidence for individual bird quality?

The little penguin Eudyptula minor is unique among penguin species in being able to fledge chicks from two clutches in one breeding season. Pairs laying two clutches in a given season make a higher reproductive investment, and may be rewarded by a higher reproductive success as they may raise twice as many chicks as pairs laying one clutch. The higher effort made by pairs laying two clutches could correlate negatively with survival, future reproductive performance or offspring survival, indicating a cost of reproduction. Conversely, a positive relationship between the number of clutches produced in a given breeding season and survival, future reproductive performance or offspring survival would indicate that birds laying two clutches belonged to a category of birds with higher fitness, compared to birds laying only one clutch in the season. In this study we used a long‐term data set taken from an increasing population of little penguins in Otago, SE New Zealand. We modelled the relationship between the number of clutches laid in a breeding season and survival probability, reproductive performance in the next breeding season and first year survival of offspring using capture‐recapture modelling.
Birds laying two clutches produced 1.7 times more fledglings during a breeding season than pairs laying one clutch. We found that birds laying two clutches had a higher probability of breeding in the following breeding season, a higher probability of laying two clutches in the following breeding season and a higher survival probability. There was no overall difference in post‐fledging survival between the young of birds producing one clutch and the young of birds producing two clutches. However, the survival of young of single clutch breeders declined with laying date, whereas the young of double clutch breeders had the same survival rate irrespective of laying date. For a subset of data with birds of known age, we found evidence that the probability of laying two clutches increased with age. However, there were also indications for differences among birds in the tendency to lay two clutches that could not be attributed to age. We tentatively interpret our results as evidence of quality difference among little penguin breeders.     

Annual and seasonal reproductive trends in the Lesser Spotted Woodpecker Dendrocopos minor

Annual and seasonal variation in reproductive timing and performance were studied in a population of the Lesser Spotted Woodpecker Dendrocopos minor over 10 years in southern Sweden. The median laying date of the first egg varied by up to 17 days between years, being generally larger than the variation of laying dates within years. Neither clutch size, brood size in successful nests, fledging success in successful nests nor mean nestling weight differed significantly between years. There was no trend for mean clutch size to vary between early and late years. In spite of a more than threefold variation in population size, no reproductive variable demonstrated an apparent density-dependence. Within the season, clutch size declined steeply with increasing clutch initiation date, whereas fledging success and nesting success did not, leading to a trend in brood size almost identical to the trend in clutch size. The survival prospects of fledged young declined with increasing clutch initiation date, and it is argued that the clutch size laid is a strategic adjustment to laying date. Out of 124 breeding attempts, 34% did not produce fledged young. In 9% of the breeding attempts, pairs laid no eggs. At least 20% of the breeding attempts failed after egg-laying. The most common cause of breeding failure was loss of the breeding partner followed by nest abandonment (40% of the failures). Only 16–28% of the failures were due to predation on the nest. Most complete failures, and also partial losses from nests, occurred at the early breeding stages. It is argued that the early nestling phase may be a critical stage, which the woodpeckers adjust to coincide with the seasonal food peak, explaining the strikingly late breeding season compared with other non-migrant species.     

Reproductive tactics of the ringed plover Charadrius hiaticula

Reproductive tactics of ringed plovers Charadrius hiaticula were studied at three localities in SW Sweden during five seasons. The usual clutch size is four, but removal experiments showed that females can produce five eggs in succession, with similar intervals between all eggs. High predation made mean breeding success per clutch low, 6.3% of eggs resulting in fledged young. Replacement clutches were common, and many pairs laid again after rearing their first brood to fledging. Egg laying spanned three months, much longer than for other waders in this region. Between years, reproductive success varied unpredictably with time of the season, but averaged over several years, the expected success was low and similar for the different parts of the breeding season. Chicks from late clutches had similar survival and recruitment as others. Because of the long breeding season and high rate of nest failure a female may produce up to five clutches of four eggs per season, containing in total about 3.7 times her own mass. Yearly local survival of adult females and males was estimated to 84.6 and 88.6%, respectively. Ability to produce many clutches with similar expected success throughout the season favours a long reproductive period, sometimes leading to double-brooding. Possible life-history trade-offs are discussed.     

Rates and consequences of relaying in Cassin's auklets Ptychoramphus aleuticus and rhinoceros auklets Cerorhinca monocerata breeding in a seasonal environment

We removed first eggs from early‐laying females to measure rates and consequences of relaying in Cassin's auklets Ptychoramphus aleuticus and rhinoceros auklets Cerorhinca monocerata at Triangle Island, British Columbia, Canada. Based on egg size and composition, the investment that Cassin's auklets made in first eggs was very close to that predicted from adult body mass, whereas rhinoceros auklets invested more. In both species, a high percentage of females relaid (90% of Cassin's and 87% of rhinoceros auklets). Breeding success declined weakly with later laying among control Cassin's auklet pairs, but pairs that we induced to relay bred more successfully than naturally late pairs, and similar to values predicted from laying dates of their first eggs. Their chicks also fledged heavier and younger than late control chicks, and similar to values in early control chicks, but followed the population‐wide seasonal decline in wing length at fledging. Nestling diets were dominated by Neocalanus copepods until late in the season, a sign that feeding conditions remained favourable until late. In contrast, rhinoceros auklet pairs induced to relay followed the population‐wide seasonal decline in breeding success, which was driven by a decline in hatching success. Pacific sandlance Ammodytes hexapterus, thought to be a preferred prey species, virtually disappeared from nestling diets in mid‐to‐late season, yet there was no seasonal decline in fledging mass. However, chicks from replacement eggs followed the declines among control chicks in both age and wing length at fledging. Despite the female having produced a replacement egg, and despite delayed breeding, there appeared to be little immediate consequence associated with relaying for Cassin's auklets, except for a tendency for their chicks to fledge with short wings. Consequences were more marked in rhinoceros auklets (greatly reduced hatching success, and having their chicks fledge with short wings), and this may have been due to the large investment made in eggs, and/or to delayed breeding. Results of this study show that attributes of Cassin's and rhinoceros auklets that lay at different times in the season can be important in driving seasonal declines in breeding performance, as found in studies on other Alcidae. They also show how decisions taken during the egg stage can have variable yet potentially important implications for fitness, even in relatively long‐lived species that lay single‐egg clutches.     

Breeding success and chronology of Wood Storks Mycteria americana in northern and central Florida, U.S.A.

The breeding success and chronology of Wood Storks Mycteria americana were studied at eight colonies in northern and central Florida during 1981–1985. Mean ± s.d. clutch size for all colony-years was 3.07 ± 0.56 (n = 2694 nests), with three-egg clutches (72%) most frequent. Mean clutch size among all colonies and years ranged from 2.73 ± 0.55 to 3.41 ± 0.61. Many colonies exhibited significant negative trends in clutch size with, hatching date because of a proportional decrease in four-egg clutches later in the season. Mean colony clutch size was not correlated with nest numbers, nesting density or mean hatching date within most years. Mean ± s.d. number of fledglings for all colonies and years was 1.29 ± 1.16 fledglings per nest (n = 2812 nests). Mean annual fledging rates in colonies ranged from 0 (colony failed) to 2.66 fledglings per nest. Most breeding failure occurred prior to egg hatching, and the second highest mortality occurred between hatching and 2 weeks of age. Four-egg clutches fledged more storks than three-egg clutches, which in turn were more successful than two-egg clutches. However, all clutch sizes showed similar fledgling per egg rates. The seasonal decline in productivity was associated proportionally with smaller clutch sizes later in the breeding season. An increase in mean hatching date was correlated with an increase in latitude. There was greater within-year breeding synchrony among colonies than interyear breeding synchrony within each colony. Breeding synchrony was not correlated with mean hatching date, latitude, longitude, nest numbers or nesting density.     

Seasonal variation in reproductive measures of tropical Roseate Terns Sterna dougallih previously undescribed breeding patterns in a seabird

Seasonal variation in egg-laying, egg size, hatching success, hatchling mass, fledging success and chick growth of Roseate Terms Sterna dougallii breeding on Aride Island (Seychelles), Indian Ocean, were studied in 1997 and 1998. I investigated to what extent two patterns, common in a range of species, were followed by tropical Roseate Terns: (a) seasonal decrease in clutch size, egg size and breeding success and (b) an increase in breeding success with increasing egg weight. In 1997 (a poor year), the earliest nesting birds laid significantly smaller eggs, and chicks were lighter at hatching than those of peak nesting birds. The mean clutch size, of 1.04 eggs, showed no seasonal variation and no 'b'-eggs hatched. In 1998 (a good year) the earliest nesting birds laid eggs of similar size and their chicks were of similar weight to those of peak nesting birds. Mean clutch size, of 1.25 eggs, increased significantly through the season and about 60% of the 'b'-eggs hatched. In 1997, hatching success was 57% whereas in 1998 it was 80%. In both years, breeding success declined significantly through the season. The fact that the earliest breeding birds laid smaller eggs in a poor year and smaller clutches in a good year is in marked contrast to a range of other species, and to temperate-nesting Roseate Terns. Egg volume explained about half of the variance in hatchling mass in both years, but only 15% of the variation in linear growth rate. Hatching date was the only variable with a significant effect on fledging success. Roseate Terns on Aride seemed to sacrifice egg size and clutch size for earliness of laying. Presumably it is a strategy of older birds to lay as early as possible and may be regarded as a response of tropical Roseate Terns to breeding under relatively poor, and seasonally declining, food conditions.     

Fitness cost of incubation in great tits (Parus major) is related to clutch size

Life-history theory predicts that parents produce the number of offspring that maximizes their fitness. In birds, natural selection on parental decisions regarding clutch size may act during egg laying, incubation or nestling phase. To study the fitness consequences of clutch size during the incubation phase, we manipulated the clutch sizes during this phase only in three breeding seasons and measured the fitness consequences on the short and the long term. Clutch enlargement did not affect the offspring fitness of the manipulated first clutches, but fledging probability of the subsequent clutch in the same season was reduced. Parents incubating enlarged first clutches provided adequate care for the offspring of their first clutches during the nestling phase, but paid the price when caring for the offspring of their second clutch. Parents that incubated enlarged first clutches had lower local survival in the 2 years when the population had a relatively high production of second clutches, but not in the third year when there was a very low production of second clutches. During these 2 years, the costs of incubation were strong enough to change positive selection, as established by brood size manipulations in this study population, into stabilizing selection through the negative effect of incubation on parental fitness.     

Differences in size between first and replacement clutches match the seasonal decline in single clutches in Tree Swallows Tachycineta bicolor

The seasonal decline in clutch size in birds can be a response to the environmentally conditioned decrease in prospects for offspring or a consequence of a lower physical ability of late‐breeding females. To find out which of the explanations apply in Tree Swallows Tachycineta bicolor, we assessed whether replacement clutch size in this species is affected by an individual female's ability to lay a certain number of eggs. To do this, we measured the decline in clutch size as a function of laying date between first and replacement clutches in individuals that re‐nested following natural failure, and compared this with the rate of decline in clutch size with laying date for Tree Swallows that laid only a single clutch in that season. Additionally, we assessed whether the clutch size and the rate of its seasonal decline varied across years. We accounted for the truncated and under‐dispersed nature of clutch size data by using a Bayesian approach in the analysis. We found little variation in the rate of clutch size decline across years at our breeding site. Accounting for this seasonal decline in clutch size, mean clutch size was similar between single‐time breeding females and those that laid replacement clutches, implying that the number of eggs laid on the second attempt by female Tree Swallows is determined by laying date, rather than by the female's physical ability to produce a clutch of a certain size.     

A more informative method for analyzing reproductive success

ABSTRACT.   Few investigators have examined how a female's prior history (i.e., number of clutches laid previously and whether those nesting attempts were successful or not) might influence the success of a particular clutch. Our objective was to determine the seasonal change in the mean number of fledglings successfully leaving nests in a population of Prairie Warblers ( Dendroica discolor ). To do so, we calculated the success of each clutch in the laying sequence (i.e., first, second, third, …, n ^th) and sorted them further by whether they were laid before (first-brood clutches) or after (second-brood clutches) the first brood fledged. In our sample population of 70 females we knew the fate of all clutches laid during a breeding season. Although the number of fledglings per successful clutch varied slightly, the probability that a clutch would be successful (i.e., produce at least one fledgling) and the number of fledglings per nest attempt increased during the breeding season. Thus, later clutches were more productive than earlier clutches. The low probability of producing a successful first clutch early in the breeding season would seem to favor selection for females that wait until later in the season to begin breeding. However, the seasonal reproductive success of females may be more dependent on breeding early (increasing the number of clutches laid in a season) than on the higher probability of success with later clutches. Our results indicate that further study of seasonal changes in reproductive success and their causes is needed.     

Different responses to cold weather in two pied flycatcher populations

We compared how breeding parameters differ according to prevailing weather conditions between a marginal, subarctic (69°N) and temperate (61°N) population of the pied flycatcher Ficedula hypoleuca, a small migratory insectivorous passerine. We predicted that the effects of weather on breeding performance (clutch size, hatching success, nestling growth, fledging success) would be greater at northern latitudes, where the weather conditions are more extreme and unpredictable. We found that the breeding parameters, except clutch size, were not, however, inferior in the north. Northern birds, unlike the southern ones, responded to colder conditions by laying smaller clutches and maintaining a larger energy reserve (indicated by higher female body mass and higher levels of subcutaneous fat). If a cold spell occurred during the nestling period, southern flycatchers had 5–10% lower fledging success than the northern ones. Our results indicate that in the north, the breeding individuals coped with cold and variable weather better than the individuals in the southern population. This could be adaptive, because at high latitudes there is a higher probability of cold weather at the time of breeding.     

Differential maternal investment counteracts for late breeding in magpies Pica pica: an experimental study

Reproductive success in many avian populations declines throughout the breeding season. Two hypotheses have gained attention to explain such a decline: the "timing" hypothesis proposes that deteriorating food availability causes the decline in reproductive success (causal effect of breeding time), whereas the "quality" hypothesis proposes that individuals of lower phenotypic quality reproduce at the end of the breeding season, causing the correlation between breeding time and breeding success. We tested both of these hypotheses in a monogamous single breeder, the magpie Pica pica , by experimentally inducing some pairs to lay a replacement clutch, after removal of the first clutch. The first clutch was left in the nest of another magpie pair (matched by laying date and clutch size), and incubated and raised by these foster parents. In this way we obtained two clutches from the same magpie pair with full siblings raised in conditions of the first and second reproductive attempts. High quality pairs (with laying dates in the first half of the breeding season) reached similar breeding success in replacement clutches as compared to first clutches of the same female. In addition, experimental pairs reared significantly more offspring of similar quality in their replacement clutches as compared to late-season first clutches, thereby suggesting that late season first clutches were produced by pairs of lower phenotypic quality. Results indicate that high quality pairs trade-off clutch size for larger eggs in replacement clutches, which could help magpie pairs to partly compensate for poorer environmental conditions associated with a delayed breeding attempt.     

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.     

Breeding biology of the Barn Owl Tyto alba in central Mali

Data were obtained on 178 clutches of African Barn Owls in central Mali from four breeding seasons during 1979–1983. Significantly more clutches were laid in 1979–1980 and significantly fewer in 1980– 1981 than the average for the 4 years and there were significantly more clutches laid in the middle period of the annual breeding season. The egg volume was significantly smaller at the beginning of the breeding season and significantly larger in the middle than the overall mean with eggs of second clutches being larger than those of first clutches. The clutch size was 605 eggs of which 479 hatched. The number of young fledged per successful nest was 319 and was 1 83 for all nesting attempts. The month was the only variable shown to affect significantly the clutch size, eggs hatched and fledging rate, the highest success rates being associated with the middle of the breeding period. The average interval between the hatching of eggs was 2–31 days. Survival rates (47'1%) to fledging were significantly affected by year (1981–1982 being the least) and month (mid-season birds the best). The order of hatching significantly affected age at death or disappearance, the first-hatched birds surviving the longest. The year significantly affected age at fledging, the young from the year in which most clutches were laid leaving the nest at the youngest age and those associated with the year having the least number of clutches remaining in the nest the longest. The month of hatching also affected fledging age, birds at the extremes of the breeding season fledging at older ages. The discussion compares these data with those from elsewhere.     

Spatial and temporal variation in breeding parameters of two south-temperate populations of House Wrens

Studies of variation in breeding parameters are often based on temporal analyses of a single population. However, to differentiate between the effects of regional and local factors, neighboring populations with limited interpopulational dispersal need to be compared. We studied two nearby (< 5 km apart) populations of House Wrens (Troglodytes aedon bonariae) at two ranches (Los Zorzales, 10 years; La Esperanza, 13 years) in south-temperate Argentina to assess the possible effects of regional and local factors on breeding phenology. For each breeding season, we recorded laying dates, clutch sizes, and length of the breeding season, and estimated the reproductive synchrony of first and second breeding attempts. We examined how these breeding parameters were affected by weather, population density, and rates of nest failure. With favorable temperatures during the pre-reproductive period (September–October), wrens in both populations initiated first breeding attempts earlier. However, ordinal laying dates were also affected by local factors, with wrens at Los Zorzales initiating breeding attempts earlier than those at La Esperanza. We found a spatial correlation in clutch sizes between populations for the 2007–2012 breeding seasons, but clutch sizes of first and second nesting attempts showed low variability. Reproductive synchrony of first nesting attempts varied among years, suggesting an effect of regional factors. However, we detected no synchronization between populations and were unable to identify environmental variables that explained the temporal variation. Ordinal laying dates of second clutches were strongly correlated with the ordinal laying dates of first clutches. We also found that the length of breeding seasons was longer when daily nest mortality rates were lower. Although environmental factors seemed to affect the decision of when to start breeding, pairs with successful first nesting attempts were more likely to initiate second nests, thus affecting the length of the breeding season. The spatial variation and temporal variation of the breeding parameters of House Wrens in our study provide evidence of marked plasticity in their breeding decisions and allowed us to identify local and regional environmental factors related to this variation.     

FACTORS AFFECTING THE EGG SIZE OF RED-BILLED GULLS LARUS NOVAEHOLLANDIAE SCOPULINUS

The factors influencing the egg size of the Red-billed Gull Larus novaehollandiae scopulinus were studied at Kaikoura, New Zealand, between 1964 and 1972. In two- and three-egg clutches there was a trend for the eggs to become smaller in the sequence of laying. Length, breadth and volume of eggs of one-, two- and three-egg clutches declined significantly as the season progressed. The size of eggs from single-egg clutches tended to be smaller than eggs from two-egg clutches laid at the same time. There were correlations between the proportions of one-egg and of three-egg clutches being laid at a given period and the mean egg volume of two-egg clutches. When the mean egg volume of two-egg clutches increased there was a corresponding increase in the proportion of two- and three-egg clutches laid. When the mean egg volume of two-egg clutches decreased there was an increase in the proportion of single-egg clutches laid. The egg size of the Red-billed Gull showed no direct correlation with the abundance or availability of food; the largest eggs were produced early in the season when food was in short supply. In spite of an increase in the food supply in the middle of the breeding season, birds laying at this time produced smaller eggs than birds which laid earlier in the season. However, early breeders which relayed at the peak in food abundance on average produced a larger replacement clutch than originals laid early in the season. It is suggested that the birds nesting early in the season are able to produce the largest eggs because they are the most efficient foragers for food, and those which nest later in the season produce smaller eggs, even at peak food abundance, because of their inefficiency or inexperience. Early breeders laying replacement clutches tended to lay larger eggs and larger clutches than birds which are producing their first clutches at the same time. Two-year-old females laid eggs which were significantly shorter than older aged birds while the breadth and volume of the egg increased with the age of the female up to the fifth year. There was a trend for females to lay larger eggs when mated with older rather than younger males. No statistical differences in egg size were detected between females changing or retaining the partner of the previous season. Female body weight and egg volume were positively correlated in females weighing less than 275 g but not for heavier females. It is suggested that the seasonal decline in egg size and clutch size results from a decrease in the availability of food and the ability of the individual to exploit the resource.     

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.     

The effect of body condition on the timing and success of breeding in Little Penguins Eudyptula minor

We studied the effect of parental body condition on the breeding biology of Little Penguins Eudyptula minor . Daily attendance patterns and body mass were recorded using an Automated Penguin Monitoring System, which collected arrival and departure masses for approximately 200 breeding birds over the 2000 and 2001 breeding seasons. Breeding success varied between the two years; 2000 was a year of average breeding success (fledging 1.07 chicks per pair), and 2001 a year of poor breeding success (fledging 0.53 chicks per pair). In both years, adult body condition (body mass divided by flipper length) increased significantly prior to laying. The laying period began over a month later in 2000 than in 2001, and birds in 2000 exhibited significantly better body condition at laying. However, the mean laying dates in 2000 were less variable than in the 2001 breeding season. Body condition appeared to influence both the time of breeding and breeding synchrony in Little Penguins. Breeding success was correlated with adult body condition at incubation in 2000 but not in 2001, indicating that success was not solely influenced by adult body condition at incubation.     

Sexual differences in moult–breeding overlap and female reproductive costs in pied flycatchers, Ficedula hypoleuca

1. In this study I show that a sexual difference in timing of the post-nuptial moult frequently occurs in a sub-arctic population of the pied flycatcher.
2. Most pairs started to moult after fledging of their young, but an overlap between moult and nestling feeding was more common among males than females. This sexual difference in moult–breeding overlap increased as the season progressed.
3. Females with moult–breeding overlap laid smaller clutches than non-moulting females. In addition to many other factors explaining the seasonal decline in clutch size that has been found for many bird species, it is possible that females adjust their clutch size according to their own risk of having to start moulting while still feeding the nestlings.
4. Nearly 24% of the females were deserted by their mate before the young fledged. Desertion imposed no fitness costs to males in terms of fledgling number or quality, suggesting that their females managed to adjust their care for the loss of male care.
5. Deserted females started moulting later than aided females, which may be a result of their increased reproductive investment.
6. Deserted females and females aided by moulting males had lower survival rate than females aided by non-moulting males.
7. These findings suggests that delayed moult may be one mechanism causing inter-annual reproductive costs in birds, and the relationship between a sexual difference in timing of moult and its fitness consequences may be widespread among passerine birds.     

Seasonal decline in provisioning effort and nestling mass of Blue Tits Cyanistes caeruleus: experimental support for the parent quality hypothesis

A common feature of many birds breeding in seasonal environments is that fitness‐related parameters such as nestling mass or survival decline as the breeding season progresses. Consequently, there is a tendency for early breeders to have better reproductive performance than individuals breeding later in the season. This variation could be caused by factors associated with the date of laying, such as changing environmental conditions (date hypothesis), or by differences in parental quality between early and late breeders (parent quality hypothesis). To evaluate the relative importance of both hypotheses, we manipulated hatch dates of Blue Tits Cyanistes caeruleus by exchanging clutches with different incubation stages and assessed the impact on nestling mass, nestling diet and provisioning rates. Mean nestling mass declined significantly over the season. This was the combined result of differences in parental quality, which dominated in the early part of the season, and the influence of hatching date (date effect per se), which prevailed later in the season. Nestling diet composition was apparently uninfluenced by the manipulation, suggesting that deteriorating food supplies are the primary reason for the seasonal variation in the nestling diet. Counter to the date hypothesis, delayed parents did not feed their young less than control pairs did, but in fact exhibited higher provisioning rates. Our results suggest that in this population, parental quality seems to constrain post‐hatching reproductive performance and such intrinsic limitations may help to explain why certain individuals breed later.     

Multiple responses to increasing spring temperatures in the breeding cycle of blue and great tits (Cyanistes caeruleus,Parus major)

Many bird species start laying their eggs earlier in response to increasing spring temperatures, but the causes of variation between and within species have not been fully explained. Moreover, synchronization of the nestling period with the food supply not only depends on first‐egg dates but also on additional reproductive parameters including laying interruptions, incubation time and nestling growth rate. We studied the breeding cycle of two sympatric and closely related species, the blue tit Cyanistes caeruleus and the great tit Parus major in a rich oak‐beech forest, and found that both advanced their mean first‐egg dates by 11–12 days over the last three decades. In addition, the time from first egg to fledging has shortened by 2–3 days, through a decrease in laying interruptions, incubation time (not statistically significant) and nestling development time. This decrease is correlated with a gradual increase of temperatures during laying, suggesting a major effect of the reduction in laying interruptions. In both species, the occurrence of second clutches has strongly decreased over time. As a consequence, the average time of fledging (all broods combined) has advanced by 15.4 and 18.6 days for blue and great tits, respectively, and variance in fledging dates has decreased by 70–75%. Indirect estimates of the food peak suggest that both species have maintained synchronization with the food supply. We found consistent selection for large clutch size, early laying and short nest time (laying to fledging), but no consistent changes in selection over time. Analyses of within‐individual variation show that most of the change can be explained by individual plasticity in laying date, fledging date and nest time. This study highlights the importance of studying all components of the reproductive cycle, including second clutches, in order to assess how natural populations respond to climate change.