Decline In Clutch Size Research Articles

Orchestration of avian reproductive effort: an integration of the ultimate and proximate bases for flexibility in clutch size, incubation behaviour, and yolk androgen deposition

How much effort to expend in any one bout of reproduction is among the most important decisions made by an individual that breeds more than once. According to life-history theory, reproduction is costly, and individuals that invest too much in a given reproductive bout pay with reduced reproductive output in the future. Likewise, investing too little does not maximize reproductive potential. Because reproductive effort relative to output can vary with predictable and unpredictable challenges and opportunities, no single level of reproductive effort maximizes fitness. This leads to the prediction that individuals possessing behavioural mechanisms to buffer challenges and take advantage of opportunities would incur fitness benefits. Here, we review evidence in birds, primarily of altricial species, for the presence of at least two such mechanisms and evidence for and against the seasonal coordination of these mechanisms through seasonal changes in plasma concentrations of the pituitary hormone prolactin. First, the seasonal decline in clutch size of most bird species may partially offset a predictable seasonal decline in the reproductive value of offspring. Second, establishing a developmental sibling-hierarchy among offspring may hedge against unpredictable changes in resource availability and offspring viability or quality, and minimize energy expenditure in raising a brood. The hierarchy may be a product, in part, of the timing of incubation onset relative to clutch completion and the rate of yolk androgen deposition during the laying cycle. Because clutch size should influence the effects of both these traits on the developmental hierarchy, we predicted and describe evidence in some species that females adjust the timing of incubation onset and rate of yolk androgen deposition to match clutch size. Studies on domesticated precocial species reveal an inhibitory effect of the pituitary hormone prolactin on egg laying, suggesting a possible hormonal basis for the regulation of clutch size. Studies on the American kestrel (Falco sparverius) and other species suggest that the seasonal increase in plasma concentrations of prolactin may regulate both a seasonal advance in the timing of incubation onset and a seasonal increase in the rate of yolk androgen deposition. These observations, together with strong conceptual arguments published previously, raise the possibility that a single hormone, prolactin, functions as the basis of a common mechanism for the seasonal adjustment of reproductive effort. However, a role for prolactin in regulating clutch size in any species is not firmly established, and evidence from some species indicates that clutch size may not be coupled to the timing of incubation onset and rate of yolk androgen deposition. A dissociation between the regulation of clutch size and the regulation of incubation onset and yolk androgen deposition may enable an independent response to the predictable and unpredictable challenges and opportunities faced during reproduction.

Breeding biology of the Red Kite Milvus milvus in Corsica

The breeding biology of the Red Kite Milvus milvus is still little known in the southern part of its range (Mediterranean), despite recent conservation concerns and major declines in most insular populations (Sicily, Sardinia and Balearics). We report here on the breeding biology of the Red Kite in Corsica in 1996–99 and on recent population trends there. In a 42‐km2 study area located in the northwest of the island (Balagne region), breeding density was locally high (1.17–1.78 breeding pairs/km2). Breeding dispersion ranged from loosely colonial to dispersed, with average nearest‐neighbour distance of 444 ± 316 m (range 50–2000) (all data as means ± sd). Kites established breeding territories in January–February, and 92.4% of territorial pairs laid a clutch (n = 238). Laying took place between February and May (mean lay date: 27 March ± 16 days, n = 147). Clutch size averaged 2.44 ± 0.71 (1–5 eggs, n = 96), hatching success 66.9% and fledging success 78.6%. Productivity averaged 1.33 ± 0.88 young per breeding attempt (n = 221) and 1.65 ± 0.65 young per successful breeding attempt (n = 173). Overall breeding success was 51.4 ± 38.0% (n = 88). We describe the growth of young (wing, weight, tarsus and bill) and show a marked seasonal decline in clutch size and breeding performance, with pairs laying earlier producing larger clutches and being more successful than later breeding pairs. Unlike most other insular Mediterranean Red Kite populations that have recently declined, the breeding population in the northwest of Corsica, which accounts for c. 25% of the whole island population, increased from 25 to 35 pairs in 1989 to a maximum of 80–90 pairs in 1997. This increase was probably related to the lack of persecution and a local increase in abundance of Rabbits Oryctolagus cuniculus, following their introduction in the late 1970s, which provided an important feeding resource for Kites. Finally, we compare our results with those from other Red Kite populations studied in Europe. We found that there is a latitudinal gradient in laying date and productivity across Western Europe populations, but no evidence of an insular syndrome in the Corsican population.

The effect of migration strategy and food availability on White StorkCiconia ciconiabreeding success

In the mid 1970s, the breeding populations of the migrant White StorkCiconia ciconiawere close to extinction in the northeastern region of France (Alsace). A re‐introduction project was implemented, resulting in the year‐round settlement of some individuals in the region, which rely on additional food supplied by humans during the winter. Today, both resident and migrant birds breed in the same areas and take food from rubbish dumps and humans (farmers). The effects of these anthropogenic influences, altering Stork behaviour, on Stork reproductive success are not known. The aim of this study was to test the influence of bird status (resident vs. migrant) and food availability (control nests vs. nests that benefit from high food supply) on reproductive success. In control nests, the mean laying date was earlier in resident than in migrant White Storks. There was also a clear seasonal decline in clutch size. For all nests, the numbers of eggs and hatchlings were higher in resident birds than in migrants, which can be attributed to the earlier breeding of resident Storks. The large broods of resident birds showed a high mortality rate, leading to the same fledgling success (fledglings/hatchlings) and number of fledglings as in migrants. Fledgling success and the number of fledglings were higher for nests close to a reliable food supply. In summary, although resident birds can breed earlier and produce more eggs than migrants, we found no advantage in terms of number of fledglings. The higher mortality rate of chicks found in pairs with a large brood could be caused by the deterioration of their habitat. Thus, the year‐round settlement of Storks may not present a biological advantage if the quality of their habitat is not guaranteed by the conservation of their grasslands.

Selection for synchronous breeding in the European starling

Colonial birds often demonstrate considerable breeding synchrony. In southern Sweden the semi‐colonial European starling initiated the vast majority of clutches within one week. Laying dates were positively skewed so that many birds initiated clutches at similar dates early in the season. Breeding was further synchronised by a particularly strong clutch‐size reduction equivalent to one third of an egg per day during the first part of the breeding season. The decline in clutch size with season also held true for separate age‐classes of females, for individual females laying at different times at different years and for individual females laying at different times the same year. Trends in breeding success during nestling rearing were unlikely to explain the high degree of breeding synchrony or the seasonal decline in clutch size; nestling survival and growth were weakly related or unrelated to reproductive timing. In contrast recruitment success of fledged offspring declined sharply with season. Even within the synchronous laying period, defined as clutches initiated during the first week each year, local recruitment success declined. It is suggested that the early seasonal decline is caused by selection for synchronous fledging permitting the immediate formation of flocks after fledging, whereas the late seasonal trends may be caused by either population differences in female quality or deteriorating conditions for raising young.

Effect of food abundance on laying date and clutch size in the White Stork Ciconia ciconia

CapsuleFood independently affects both laying date and clutch size, suggesting that seasonal decline in clutch size is related to a decrease in food availability. Aim To test the effect of food abundance on laying date and clutch size of the White Stork and identify the cause of seasonal decline in the number of eggs laid. Methods During 1991 and 1996 we recorded clutch size and laying date of pairs breeding next to rubbish dumps (food abundant and constant throughout the breeding season) and birds breeding far from rubbish dumps (using natural food sources). Results In 1991 there was no difference in mean laying date between pairs nesting at rubbish dumps and control pairs. Clutch size was significantly larger at rubbish dump nests. In contrast, mean laying date was earlier in control pairs in 1996 and there was no significant differences in clutch sizes, even when controlling for laying date effect. Conclusion The results support the hypothesis that food availability independently affects both laying date and clutch size. The seasonal decline in clutch size close to rubbish dumps was negligible (1991) or much smaller than in the control zone (1996) suggesting that a progressive deterioration of natural food sources is the most probable reason for a decline in clutch size as the season advances.

Body condition, migration, and timing of reproduction in snow geese: a test of the condition-dependent model of optimal clutch size.

The seasonal decline of avian clutch size may result from the conflict between the advantage of early breeding (greater offspring value) and the advantage of a delay in lay date (improved body condition and hence clutch size). We tested predictions of a condition-dependent individual optimization model based on this trade-off (Rowe et al. 1994) in a long-distance migrant, the greater snow goose (Chen caerulescens atlantica), using data on condition, migration, and reproductive decisions of individuals. We closely tracked radio-marked females at their main spring staging area and on their breeding grounds. Our results were consistent with predictions of the model. Early-arriving females had a longer prelaying period and initiated their nests earlier than late arrivals. After controlling statistically for arrival date, we determined that females with high premigration condition had an earlier lay date than those in low condition. After controlling for the seasonal decline (i.e., lay date), we observed that clutch size was not related to premigration condition. Moreover, we took advantage of an unplanned manipulation of the prebreeding condition that occurred during our long-term study. We found that a reduction in condition caused a delay in lay date. However, after controlling for the seasonal decline, it did not affect clutch size. Our study indicates that geese simultaneously adjust their lay date and clutch size according to their premigration condition and migratory behavior as predicted by the condition-dependent optimization model.

Geographical differences in seasonal clutch size variation in multi‐brooded bird species

The seasonal pattern of clutch size variation in birds varies among species. In single‐brooded bird species clutch sizes decline continuously with date from an early season maximum. In resident multi‐brooded species, clutch sizes first increase to a mid‐season maximum and then decrease again. Limited data for multi‐brooded migratory birds suggest that clutches in these species also show a continuous decline throughout the season, but it remains to be resolved whether this applies generally and whether migration adds a constraint to the system. We here report on a study of the Eastern BluebirdSialia sialisconducted using data collected by volunteers across most of its range. In the southern part of its range, where Eastern Bluebirds are largely year‐round residents, clutches initially increase, reach a mid‐season maximum and gradually decline later in the season. In the northern part of the range, where the majority of Eastern Bluebirds are migratory, clutch sizes decline continuously throughout the season. To determine whether seasonal changes in the clutch size of multi‐brooded species are determined by migratory behaviour of the population we compared our results with published data on the DunnockPrunella modularisand conclude that in multi‐brooded species migratory behaviour is not a sufficient condition for a continuous seasonal decline in clutch size, but it could be a necessary one. We propose two mechanisms for the pattern observed among multi‐brooded migrants: (i) a time or energy cost of migration, and (ii) a more abrupt increase in seasonal resources in spring at more northerly latitudes.

Cyclic variation in seasonal recruitment and the evolution of the seasonal decline in Ural owl clutch size.

Plastic life-history traits can be viewed as adaptive responses to environmental conditions, described by a reaction norm. In birds, the decline in clutch size with advancing laying date has been viewed as a reaction norm in response to the parent's own (somatic or local environmental) condition and the seasonal decline in its offspring's reproductive value. Theory predicts that differences in the seasonal recruitment are mirrored in the seasonal decrease in clutch size. We tested this prediction in the Ural owl. The owl's main prey, voles, show a cycle of low, increase and peak phases. Recruitment probability had a humped distribution in both increase and peak phases. Average recruitment probability was two to three times higher in the increase phase and declined faster in the latter part of the season when compared with the peak phase. Clutch size decreased twice as steep in the peak (0.1 eggs day-1) as in the increase phase (0.05 eggs day-1). This result appears to refute theoretical predictions of seasonal clutch size declines. However, a re-examination of current theory shows that the predictions of modelling are less robust to details of seasonal condition accumulation in birds than originally thought. The observed pattern can be predicted, assuming specifically shaped seasonal increases in condition across individuals.

Seasonal decline in clutch size in European starlings: a novel randomization test to distinguish between the timing and quality hypotheses

Summary The seasonal decline in clutch size observed in many avian populations may be due to (a) a causal effect of the timing of breeding or (b) variation in individual quality (i.e. lower quality individuals lay fewer eggs and lay later in the season). To distinguish between these two hypotheses, we manipulated the timing of breeding of European starlings (Sturnus vulgaris L.) by removing first clutches to induce the production of replacement clutches. This experiment was performed at two sites over three breeding seasons. To test whether the within‐individual trend in clutch size differed from the seasonal decline in clutch size observed in the population, we devised a novel randomization test that overcomes some of the potential limitations of previously used analyses. We observed no within‐female decline in clutch size in any year at either site. Therefore, clutch removal did not appear to manipulate female quality (due to the costs of egg production), a potentially confounding factor in such experiments; only the timing of breeding was manipulated. The within‐individual seasonal trend in clutch size differed significantly from the population decline in four of six cases (year–site combinations); there is good reason to believe that the lack of a significant difference in the two other cases was due to low statistical power. Thus, our data support the hypothesis that the season decline in clutch size in this species is due to variation in female quality. The between‐year repeatability of clutch size was 0·47 and significantly different from zero, whereas that of relative laying date was much lower (−0·23) and non‐significant. The repeatability data are thus consistent with the quality hypothesis: clutch size is characteristic of individuals regardless of their timing of breeding. Female body mass at the beginning of incubation was not related consistently to clutch size or laying date, and therefore does not appear to be an indicator of female quality in this context.

How Many Baskets? Clutch Sizes That Maximize Annual Fecundity of Multiple-Brooded Birds

We developed deterministic models on the basis of nest survival rates and renesting behavior capable of predicting annual fecundity in birds. The models calculate probabilities of fledging from one to four nests within a discrete breeding season. We used those models to address theoretical issues related to clutch size. In general, birds require at least one day to lay an egg, and many species delay incubation until their entire clutch is laid. Because it takes longer to complete a larger clutch, and fewer such clutches can fit into a limited breeding season, there exists a clutch size for which annual fecundity is maximized. We asked, for a given amount of reproductive effort (i.e. a set number of eggs), does the age-old maxim “don't put all your eggs in one basket” apply? If so, in how many “baskets” should a nesting bird place its eggs? The answer depends on both likelihood of nest predation and length of the breeding season. Those results are consistent with the observed increase in clutch size with latitude (shorter breeding season length) and larger clutch sizes characteristic of cavity-nesting species (with higher nest survival rates). The models also predict that the size of replacement clutches should decrease as the breeding season progresses, and that intraseasonal decline in clutch size should be more pronounced when the breeding season is short.

How Many Baskets? Clutch Sizes That Maximize Annual Fecundity of Multiple-Brooded Birds

Abstract We developed deterministic models on the basis of nest survival rates and renesting behavior capable of predicting annual fecundity in birds. The models calculate probabilities of fledging from one to four nests within a discrete breeding season. We used those models to address theoretical issues related to clutch size. In general, birds require at least one day to lay an egg, and many species delay incubation until their entire clutch is laid. Because it takes longer to complete a larger clutch, and fewer such clutches can fit into a limited breeding season, there exists a clutch size for which annual fecundity is maximized. We asked, for a given amount of reproductive effort (i.e. a set number of eggs), does the age-old maxim “don't put all your eggs in one basket” apply? If so, in how many “baskets” should a nesting bird place its eggs? The answer depends on both likelihood of nest predation and length of the breeding season. Those results are consistent with the observed increase in clutch size with latitude (shorter breeding season length) and larger clutch sizes characteristic of cavity-nesting species (with higher nest survival rates). The models also predict that the size of replacement clutches should decrease as the breeding season progresses, and that intraseasonal decline in clutch size should be more pronounced when the breeding season is short.

COVARIATION OF CLUTCH SIZE, LAYING DATE, AND INCUBATION TENDENCY IN THE AMERICAN KESTREL

Seasonal decline in clutch size is common in birds, but the proximate mechanisms for this phenomenon have not been elucidated. The most credible model to date posits that late-laying females lay fewer eggs due to a seasonal increase in the tendency to incubate during laying, which inhibits egg production. We tested this model with free-living and laboratory American Kestrels (Falco sparverius) by quantifying changes in clutch size and incubation tendency during laying over the course of the breeding phase. Consistent with the model, clutch size in free-living kestrels decreased while incubation increased with progress of the 74-day breeding phase. Inconsistent with the model, variation in incubation tendency during laying was not associated with clutch size in either the field or the laboratory. In the laboratory, incubation increased but clutch size did not decrease over the course of the 77-day breeding phase. In the laboratory, females that nested early in one breeding phase renested more quickly and nested in a second breeding phase more quickly than females nesting late in the first breeding phase. This indicates that timing of laying is, in part, a property of individual females, independent of environmental factors. Together, our findings suggest that both clutch size and timing of laying are inherent, correlated properties of particular females. Although incubation tendency may influence clutch size, other factors appear to override its influence. Covariación del Tamaño de la Nidada, la Fecha de Postura y la Tendencia de Incubación en Falco sparverius Resumen. Aunque la disminución estacional en el tamaño de la nidada es común entre las aves, los mecanismos proximales para este fenómeno no han sido elucidados. El modelo de mayor credibilidad propuesto hasta el momento sugiere que las hembras que ovipositan tarde ponen menos huevos debido a un aumento estacional en la tendencia a incubar durante la postura que inhibe la producción de huevos. Pusimos a prueba este modelo con individuos de Falco sparverius en condiciones naturales y en cautiverio, cuantificando los cambios en el tamaño de la nidada y la tendencia a incubar durante la postura a lo largo de la época reproductiva. Tal como el modelo plantea, el tamaño de la nidada en individuos silvestres de F. sparverius disminuyó mientras que la incubación aumentó a medida que transcurrían los 74 días de la época de reproducción. Mientras tanto, en contraste con el modelo, la variación en la tendencia a incubar durante la postura no estuvo asociada con el tamaño de la nidada ni en el campo ni en el laboratorio. En el laboratorio, la incubación aumentó pero el tamaño de la nidada no disminuyó en el transcurso de la época reproductiva (77 días). En condiciones de laboratorio, las hembras que anidaron temprano en una fase reproductiva volvieron a anidar más rápidamente en una segunda fase que las hembras que anidaron tarde en la primera. Esto indica que el tiempo en que se hace la postura es en parte una propiedad de cada hembra y es independiente de factores ambientales. En conjunto, nuestros hallazgos sugieren que tanto el tamaño de la nidada como el momento de la postura son propiedades correlacionadas de cada hembra en particular. Aunque la tendencia a incubar podría influenciar el tamaño de la nidada, otros factores parecen anular su influencia.

Density dependence of avian clutch size in resident and migrant species: is there a constraint on the predictability of competitor density?

The presence of density dependence of clutch size is tested in 57 long‐term population studies of 10 passerine bird species. In about half of the studies of tit species Parus spp. density dependence of clutch size was found, while none was found in studies of two flycatcher species Ficedula spp. One hypothesis explaining this difference is that migrants are less able to predict the final competitor density, because new pairs are still settling when the first females start laying eggs. Such unpredictability is only a problem for early laying females. If this explanation is true, the commonly observed negative correlation between clutch size and laying date should be stronger in high‐density years. I tested this prediction in three populations of Pied Flycatcher Ficedula hypoleuca, and compared the results with three populations of Great Tit Parus major. In none of the six populations was there a significant correlation between the strength of the seasonal decline in clutch size and population density. Thus the lack of density dependence of clutch size in Pied Flycatchers was not consistent with the idea that this is caused by the unpredictability of final density at the time of egg‐laying of the earliest females in the population. Furthermore, density does not have any adverse effect on reproductive output of Pied Flycatchers, and therefore they do not adjust clutch size to density.

Time-saving mechanisms in the reproduction of Marsh Tits (Parus palustris)

Producing young late in the season is very costly in Marsh Tits, therefore any means which allow late-laying females to compensate for an initial delay are highly advantageous. Any attempts to make up for the initial delay should result in shorter breeding cycles late in the season and in shorter reproductive cycles in late-laying females. A shortening of the nesting cycle should be achievable via reduction of the clutch size or by shortening the duration of the incubation and/or nestling periods. These predictions are examined using data from an extensive study of Marsh Tits carried out in remnants of primaeval European forest (Bialowieza National Park, E Poland). As anticipated, the breeding cycle late in the seasons was significantly shorter. This was due to a decline in clutch size, and shortening of the incubation period. The shortening of the cycle occurred also within a season, late-laying birds had significantly shorter cycles than early-laying ones. This was due to a very steep seasonal decline in clutch size and, to a lesser extent, by a shortened incubation period. With the five day overall difference, the late-laying birds could compensate for up to a third of the initial delay in egg-laying, a substantial saving of time. Young females commenced laying about two days later than older ones; as predicted, they laid smaller clutches but contrary to the predictions, they incubated for longer. The nestling period showed no relationship with season neither among years nor within a single year.

Nest-site selection and breeding success in the Roller (Coracias garrulus) in the Southwest of the Iberian peninsula

Nest-site selection and breeding success was studied in a breeding population of Roller (Coracias garrulus) in the Southwest of Spain. 567 nest-boxes were installed on electric pylons in seven different habitats: fallow land, cereals, holm oaks, olive groves, pasture land, irrigated crops (rice and maize) and shrubs. The more favoured habitats were the irrigated crops, the olive groves and the holm-oaks, while only the cereals were clearly avoided. Fallow land, pastures and shrublands were preferred approximately according to their availability in the area. The mean laying date of Rollers was significantly different between habitats, however, the other reproductive variables did not vary with habitat type. The seasonal trend in reproductive value of the Roller varied relative to habitat type. So, in olive groves and irrigated crops no clear seasonal trends were detected in clutch size, while a clear seasonal decline in clutch size was detected in the rest of the habitat types. The intensity of selection of each habitat was only marginally related to the hatching success of the rollers in Extremadura. These results indicate that the decrease in natural nest-sites as a result of the intensification of agriculture is the principal cause of the decline in Roller populations in contrast to previous studies which argued that this was a consequence of the decrease in available food.

Reproductive consequences of egg‐laying decisions in snow geese

Summary1. Even though feeding conditions typically improve over time during the laying period, clutch size decreases over the course of the nesting period in most bird species. We examined whether seasonal decrease in offspring value could explain the seasonal decline in clutch size in arctic‐nesting greater snow geese (Anser caerulescens atlanticusL.).2. Nesting was synchronized within a year, with more than 90% of the nests being initiated within about 8 days. Despite this high synchrony, there was a steep seasonal decline in clutch size in each of the 7 years of the study, from about five eggs in early clutches to three in late ones (−0·20 egg day−1).3. Late parents performed more poorly than early parents in most components of reproductive success. The relationship between laying date and nesting success was curvilinear, early and late nests having a higher failure rate than those initiated near the median. Prefledging survival decreased by about 50% over the season, although the earliest hatched goslings also tended to have a reduced survival. The postfledging survival showed the strongest seasonal decline, as survival probability of late‐hatched birds was about five times lower than in early‐hatched ones.4. Overall reproductive success showed a very steep seasonal decline as the number of young surviving to the first winter was about eight times lower in late‐nesting birds than in early‐nesting ones. Reproductive success declined slightly in the earliest‐nesting birds, suggesting a cost to nesting too early.5. The observed clutch size generally matched the clutch size that yielded the highest reproductive success for each laying date, except in earliest‐nesting birds which should have done better by slightly delaying nesting. Our data suggest that trading off additional eggs for earlier nesting to increase reproductive success is an option in geese. Consequently, the seasonal decline in clutch size may be an adaptive response to seasonally declining survival prospects of offspring.

Fitness Components Associated with Laying Date in the Cliff Swallow

We report how clutch size, annual reproductive success, and annual survival of breeders and offspring vary with laying date in Cliff Swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, from 1982-1993. Clutch size declined significantly across the season for both parasite-free (fumigated) and naturally infested nests. Larger clutches of 4 and 5 eggs were initiated significantly earlier in warm and dry seasons; laying dates for smaller clutches showed no relationship with weather. Incubation period declined significantly with laying date and was shorter in the absence of ectoparasites. Fledging success declined significantly with laying date in nonfumigated nests, primarily through greater loss of nestlings to parasitic swallow bugs (Oeciacus vicarius). First-year survival probabilities of offspring varied significantly with date among nonfumigated nests but not among fumigated nests; earlier-hatched young survived best in some years, and in other years intermediate- or late-hatched young had highest survival. Number of young locally recruited was highest for early nesters in 5 of 7 years with intermediate nesters doing best in the remaining years. Annual survival of female breeders increased with laying date, suggesting a benefit to late nesting and a potential life history trade-off between reproduction and adult survival. Nestling body mass did not vary with laying date, suggesting no decline in food availability over the summer. Female body mass prior to and during laying declined significantly with laying date but during brood rearing increased significantly with laying date. Female body mass had no significant effect on clutch size independent of laying date. The primary environmental factor causing the seasonal decline in reproductive success is ectoparasitism. A seasonal decline in clutch size may be a strategic adjustment to reduce ectoparasitism by lowering the parasite carrying capacity per nest. Greater apparent survival of late-nesting females may prevent directional selection for earlier breeding times in Cliff Swallows.

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.

Variation in Clutch Size of the Common Eider: A Study Based on 41 Breeding Seasons on Coquet Island, Northumberland, England

--The clutch size of the Common Eider (Somateria mollissima) nesting on Coquet Island, Northumberland, UK has been studied for 41 successive years, during which time there has been a progressive decline in the annual mean clutch size, from a peak in 1962 of 5.37 eggs per nest to 3.43 eggs in 1996. There is evidence of a density-dependent relationship between clutch size and numbers of adult ducks from 1958 to 1981, but since 1982 the relationship has changed significantly and there has been a positive correlation between clutch size and numbers of adult ducks. In the early years of the study, the mean clutch size laid by young birds was lower than those laid by older birds, but this effect has been lost in recent years, apparently associated with the overall decline in clutch size. Clutch size did not change with age after the first breeding attempt. Comparisons of annual average clutch sizes for ten years' results from the Sands of Forvie, and for twelve years from The Netherlands, with those in the same years on Coquet Island showed no significant correlations. The mean clutch size in the sample of birds captured each year was significantly higher than that in the birds which left the nest early on the observer's approach. This could be an age effect or reflect the general body condition of the birds. I suggest the decline in clutch was caused by a deterioration in the food of the eider around Coquet Island, which in turn is possibly linked with an effect of sewage discharges on mussel beds. The eider appears to be a very suitable species for monitoring environmental conditions of inshore waters near its breeding sites, through systematic recording of clutch size and numbers of adults. Received 28January 1999, accepted 22 May 1999.

Clutch Size of Kestrels: Seasonal Decline and Experimental Evidence for Food Limitation under Fluctuating Food Conditions

We studied the clutch size of Eurasian kestrels, Falco tinnunculus in western Finland during 1985-1996 (the total number of nests 546). Voles, the main food of kestrels, fluctuated in three-year population cycles. In low vole years, kestrels started to lay eggs in mid-May and on average 8 to 10 days later than in the increase and decrease years. Yearly clutch sizes were also smaller in low vole years (yearly mean 4.4 to 5.3) than in the increase and decrease years (5.4 to 5.9). Clutch size showed a similar seasonal decline (0.05 egg per day) in each phase of the vole cycle. After controlling for the effect of laying date, the phase of the vole cycle appeared to be the main determinant of clutch size; kestrels produced smaller clutches in the low than in the increase and decrease vole years. Supplementary feeding before and during the egg-laying periods in 1986-1988 did not advance the start of egg-laying and did not prevent the seasonal decline in clutch size, but resulted in larger clutches. These results show that both laying date and abundance of main food at the early stage of breeding are crucial determinants of clutch size in kestrels. Our result, that variation in food abundance can alter the relationship between clutch size and laying date, supports the quality hypothesis (that differences in quality between individual birds and/or their territories may result in the seasonal decline of clutch size). We suggest that food is limiting clutch size in kestrels both during the egg-laying and nestling periods, and not only at low but also at high levels of natural food abundance.