Weight change and the body reserves of female blue tits, Parus caeruleus, during the breeding season

A female blue tit lays a large clutch that is commonly heavier than her own body weight. Body reserves cannot provide for the entire clutch and little is known of the importance of these reserves during the breeding season. This study reports on weight changes in adult female blue tits during the breeding season, observed by the use of electronic balances. In addition, data from dissecting 67 adult female blue tits, at different stages of the breeding cycle, were used to investigate the changes in body weight. There was a rapid increase in body weight during a period of 4–5 days before the first egg was laid, which was explained mostly by the development of the oviduct and the production of an egg on the final day. Body weight remained constant during the laying period, before decreasing by approximately one gram after clutch completion, owing to the absence of an egg and the absorption of the oviduct. Body reserves increased during the day after clutch completion, were maintained throughout the incubation period and were mobilized during the first five days of the nestling period. Breeding female blue tits appear to deposit small body reserves to act as a short-term buffer against adverse conditions or to support the brooding phase.     

Seasonal clutch size variation of multi-brooded bird species: comparisons between breeding season and latitudes

Single- and multi-brooded species of birds differ in their seasonal patterns of clutch size. Single-brooded species start with a maximum clutch size that declines continuously as the season progresses, whereas the clutch sizes of multi-brooded species usually increase to a mid-season maximum peak and then decrease progressively until the end of the breeding season. Previous studies have shown that multi-brooded migrant species present seasonal patterns that are similar to single-brooded species at high latitudes but similar to multi-brooded non-migratory species at lower latitudes. We studied the Greenfinch Carduelis chloris and Goldfinch C. carduelis populations in eastern Spain (Sagunto, Valencia) between 1975 and 2002 to compare seasonal variations in clutch sizes between years with early and late starts to the breeding season. The period over which clutch sizes increase was longer when the breeding season started earlier. The Goldfinch population showed no pattern of initial increase in clutch size when there was a late start to the breeding season: a late start shortens the season giving them less time to breed, and may also coincide with maximum food availability. Thus, the pattern of single-brooded species was observed. In the Greenfinch population, a trend toward the seasonal pattern of single-brooded species was also observed when the following indices were compared: clutch size increase, modal timing, initial slope and timing of maximum clutch size. We have also compared the seasonal patterns of clutch size of both species in eastern Spain with the patterns observed in Britain. Our results show that for both Goldfinches and Greenfinches, the non-migrant southern populations of Sagunto in eastern Spain do not tend towards a more multi-brooded seasonal pattern of clutch size than the migrant Goldfinches of Britain.     

Is the additional effort of renesting linked to a hormonal change in the common tern?

The production of and care for a replacement clutch can bear costs in terms of future reproduction or survival. However, renesting is quite common among seabirds and can contribute considerably to individual fitness. Prolactin and corticosterone are two hormones involved in the mediation of breeding behavior and, as they are linked to body condition or effort, it is of interest if these hormone values change during a second demanding breeding phase within a year. We compared baseline prolactin and corticosterone between the first and the renesting attempt in common terns (Sterna hirundo) on individual level. Therefore, in addition to control birds, 37 breeders were sampled during incubation of their first and their replacement clutch in 2008 and 2009. Blood samples were taken non-invasively by blood-sucking bugs. Prolactin level was lower during the renesting period, especially in birds which abandoned their clutch afterwards, whereas corticosterone did not change. Excluding the deserting birds, the reduced prolactin level was not linked to minor success, but could be related to seasonal processes. The control group of late laying common terns showed comparably low prolactin values, but increased corticosterone concentrations. Renesting individuals exhibited higher prolactin during incubation of their first clutch than non-renesting birds, probably indicating their higher quality. The fact that terns still have relatively high prolactin and low corticosterone values during renesting might confirm their higher quality and suggests that they are able to meet the costs of a second demanding breeding period without being considerably stressed.     

Evolutionary response to selection on clutch size in a long-term study of the mute swan

Life-history traits in wild populations are often regarded as being subject to directional selection, and the existence of substantial variation and microevolutionary stasis of these characters is therefore a problem in need of explanation. Avian clutch size is an archetypal life-history trait in this context, and many studies have sought to test explanations for stasis in clutch size. Surprisingly, there are many fewer studies that used long-term data to ask how selection acts on clutch size, particularly in a multivariate framework. In this article, we report selection, inheritance, and evolution of clutch size over 25 years in a colony of mute swans using a multivariate quantitative genetic framework to control for correlations with breeding time. We show that clutch size is influenced by both additive genetic and permanent environmental effects and that selection acts on clutch size in combination with breeding time. Natural selection on clutch size is strongly directional, favoring larger clutches, and we observe an increase in clutch size of 0.35 standard deviations, consistent with the expected response based on selection and inheritance of clutch size. We hypothesize that these changes result from recent relaxation of food constraints and predation risks experienced by this colony.     

Multitasking and the evolution of optimal clutch size in fluctuating environments

Adaptive studies of avian clutch size variation across environmental gradients have resulted in what has become known as the fecundity gradient paradox, the observation that clutch size typically decreases with increasing breeding season length along latitudinal gradients, but increases with increasing breeding season length along elevational gradients. These puzzling findings challenge the common belief that organisms should reduce their clutch size in favor of additional nesting attempts as the length of the breeding season increases, an approach typically described as a bet‐hedging strategy. Here, we propose an alternative hypothesis—the multitasking hypothesis—and show that laying smaller clutches represents a multitasking strategy of switching between breeding and recovery from breeding. Both our individual‐based and analytical models demonstrate that a small clutch size strategy is favored during shorter breeding seasons because less time and energy are wasted under the severe time constraints associated with breeding multiply within a season. Our model also shows that a within‐generation bet‐hedging strategy is not favored by natural selection, even under a high risk of predation and in long breeding seasons. Thus, saving time—wasting less time as a result of an inability to complete a breeding cycle at the end of breeding season—is likely to be the primary benefit favoring the evolution of small avian clutch sizes during short breeding seasons. We also synthesize the seasonality hypothesis (pronounced seasonality leads to larger clutch size) and clutch size‐dependent predation hypothesis (larger clutch size causes higher predation risks) within our multitasking hypothesis to develop an integrative model to help resolve the paradox of contrasting patterns of clutch size along elevational and latitudinal gradients. Ultimately, our models provide a new perspective for understanding life‐history evolution under fluctuating environments.     

Density effect on great tit (Parus major) clutch size intensifies in a polluted environment

Long-term data on a great tit (Parus major) population breeding in a metal-polluted zone around a copper–nickel smelter indicate that, against expectations, the clutch size of this species is decreasing even though metal emissions in the area have decreased considerably over the past two decades. Here, we document long-term population-level changes in the clutch size of P. major and explore if changes in population density, population numbers of competing species, timing of breeding, breeding habitat, or female age distribution can explain decreasing clutch sizes. Clutch size of P. major decreased by one egg in the polluted zone during the past 21 years, while there was no significant change in clutch size in the unpolluted reference zone over this time period. Density of P. major nests was similar in both environments but increased threefold during the study period in both areas (from 0.8 to 2.4 nest/ha). In the polluted zone, clutch size has decreased as a response to a considerable increase in population density, while a corresponding density change in the unpolluted zone did not have such an effect. The other factors studied did not explain the clutch size trend. Fledgling numbers in the polluted environment have been relatively low since the beginning of the study period, and they do not show a corresponding decrease to that noted for the clutch size over the same time period. Our study shows that responses of commonly measured life-history parameters to anthropogenic pollution depend on the structure of the breeding population. Interactions between pollution and intrinsic population characters should therefore be taken into account in environmental studies.     

Behavioural and body mass changes before egg laying in the Barn Owl: cues for clutch size determination?

To investigate laying decision and clutch size determination in indeterminate layers, we analysed in-nest activity (nest presence, and copulation, prey deliveries, and entrance frequencies) and female body mass change, as well as their relation to clutch size variation in five Barn Owl pairs (Tyto alba) nesting in eastern France. Body mass of the female and behaviour [copulation frequency, entrance frequency, and prey delivery to the nest by the male (in number and mass)] were monitored using an automated weighing system and a video camera. There was a consistent change of behaviour and foraging activity among pairs ca. 18 days before laying indicating that the females may be tied to the nest at this time. Barn Owls being indeterminate layers have their clutch size determined at the oviposition of the first egg of the clutch. Window correlation analyses between the clutch size and the female body mass gain indicate that the clutch size might be determined no later than a few days before the laying of the first egg. Our results suggest that female Barn Owls may use the pre-laying period to determine the clutch size using cues such as the male food deliveries (a proxy for male quality).     

Rainfall preceding egg-laying — a factor of breeding success in Common Terns (Sterna hirundo)

Summary The temporary reduction of clutch size, egg size and breeding success of the Common Tern during the breeding period of 1983 at the Lower Saxonian Wadden Sea (West Germany) was correlated with heavy rainfalls on the days preceding egg-laying (Figs. 2–4). Possible effective mechanisms and the result that the reduction of clutch size was not observed for colonies at the mainland coast are being discussed (Fig. 4).Supported by the Deutsche Forschungsgemeinschaft     

Playback of colony sound alters the breeding schedule and clutch size in zebra finch (Taeniopygia guttata) colonies

The hypothesis that social stimulation, derived from the presence and activities of conspecifics, can hasten and synchronize breeding in colonies of birds was tested. A modified playback/recorder system was used to continuously exaggerate the amount of colony sound available to zebra finches throughout their courtship period. Males that heard 'sound supplements' generated from their own colony sang more than males in control colonies that did not receive playback; males that heard samples from a different colony, sang at an intermediate level. Females that were exposed to the vocalizations of their mate and playback from a colony other than their own, laid eggs earlier and more synchronously than females in control colonies. Females that heard the vocalizations of their mate along with playback samples generated from their own colony, laid eggs more synchronously but not earlier than control females. Both acoustic treatments caused females to lay larger clutches. Social stimulation influences the breeding schedule and clutch size in zebra finch colonies. If there are advantages associated with these effects, social stimulation may contribute to the maintenance of colonial breeding systems.     

THE SOCIABLE WEAVER,PART 3: BREEDING BIOLOGY AND MOULT

Maclean, G. L. 1973. The Sociable Weaver, Part 3: Breeding biology and moult. Ostrich 44: 219–240.

Rain or some associated phenomenon is the principal Zeitgeber releasing breeding in the Sociable Weaver. The species does not breed in the absence of rain. The same nest chambers are used for breeding as are used for roosting throughout the year. The Sociable Weaver is monogamous. The clutch size varies from two to six eggs, larger clutches being more common after good rains than in relatively poorer breeding periods. Food supply may therefore be the proximate factor regulating clutch size. Replacement clutches are not necessarily smaller than first clutches. The mean clutch size within a breeding period decreases with an apparent decrease in food supply. The parents share parental duties about equally. Up to four successive broods may be raised in a single breeding period; a breeding period may last up to nine months and may occur at any time of the year according to the somewhat erratic rainfall which averages about 226 mm per year in the study area.

First broods help their parents to feed later broods; fourth brood chicks may therefore be fed by as many as 11 birds (nine young and two parents). This has survival value especially toward the end of a breeding period when food is scarce. Of similar value is the habit of starting incubation with the first or second egg of the clutch; in a relatively poor season older chicks will survive while younger ones will starve, thereby effectively and quickly reducing brood size. Young birds moult into adult plumage at four months, but do not normally leave the home colony. The sexes are indistinguishable at all ages, but there is an approximate ratio of eight males to five females in the study area.

Wing moult is slow: each remex takes about a month for replacement. Body moult occurs within the space of a month, usually after rain while the birds are breeding. Primary remiges are moulted proximo-distally from 1 to 9; secondaries are moulted disto-proximally from 1 to 6. Body moult is antero-posterior with the dorsal surface slightly in advance of the ventral surface.     

Seasonal patterns of sperm storage in the Yellow-headed Blackbird Xanthocephalus xanthocephalus

The seasonal pattern of sperm storage was examined in a wild population of Yellow-headed Blackbirds Xanthocephalus xanthocephalus by collecting females at eight different stages of their breeding cycle. Sperm storage tubules (SSTs) were present in simiktr numbers throughout all stages of the breeding season, from arrival on the breeding grounds to departure for winter. In contrast, the size of SSTs varied enormously over the season, reaching maximum size just prior to clutch initiation, when SSTs were more than three times the length of those found in post-breeding birds. The amount of sperm stored also varied over the season, in parallel with the growth and regression of SSTs. Sperm were first observed in SSTs approximately 2–5 days before laying and increased rapidly in abundance until clutch initiation. The amount of sperm stored then decreased gradually during the egg-laying period so that by clutch completion approximately 70% of stored sperm had disappeared from the SSTs. The rapid seasonal development and subsequent regression of SSTs in the Yellow-headed Blackbird contrast sharply with the pattern found in domestic poultry, where the imposition of continuous laying regimes has; led to the maintenance of SSTs at maximum size for extended periods of time.     

Passive auditory stimulation improves vision in hemianopia

Techniques employed in rehabilitation of visual field disorders such as hemianopia are usually based on either visual or audio-visual stimulation and patients have to perform a training task. Here we present results from a completely different, novel approach that was based on passive unimodal auditory stimulation. Ten patients with either left or right-sided pure hemianopia (without neglect) received one hour of unilateral passive auditory stimulation on either their anopic or their intact side by application of repetitive trains of sound pulses emitted simultaneously via two loudspeakers. Immediately before and after passive auditory stimulation as well as after a period of recovery, patients completed a simple visual task requiring detection of light flashes presented along the horizontal plane in total darkness. The results showed that one-time passive auditory stimulation on the side of the blind, but not of the intact, hemifield of patients with hemianopia induced an improvement in visual detections by almost 100% within 30 min after passive auditory stimulation. This enhancement in performance was reversible and was reduced to baseline 1.5 h later. A non-significant trend of a shift of the visual field border toward the blind hemifield was obtained after passive auditory stimulation. These results are compatible with the view that passive auditory stimulation elicited some activation of the residual visual pathways, which are known to be multisensory and may also be sensitive to unimodal auditory stimuli as were used here. TRIAL REGISTRATION: DRKS00003577.     

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

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

Multiple breeding in the Great Tit. A trade-off between successive reproductive attempts?

1. Multiple breeding (raising more than one batch of young per breeding season) is a common life-history tactic that has received very little attention. A simple static optimization model was developed, applicable to iteroparous animals with parental care, that predicts: (1) when an animal should be a multiple breeder, (2) the optimal interval between successive clutches, and (3) when a clutch should be deserted.
2. More specifically, it was predicted that (I) animals should invest less in a clutch when it is followed by another clutch in the same season, and (II) as a consequence the contribution of the first clutch to the parents' fitness will be reduced.
3. These predictions were tested experimentally in a population of Great Tits ( ParusM. major L.). The experiment consisted of the removal of second clutches. When second clutches were removed, parents tended their first brood fledglings for a longer period than undisturbed control pairs, in agreement with the first prediction. Removal of second clutches did not affect fledgling survival, but recruits enjoyed higher reproductive success in their first year of breeding when the second clutch of their parents had been removed, in agreement with the second prediction.
4. It is concluded that a trade-off exists between successive reproductive attempts, which will affect the optimal rate of reproductive attempts.     

Pre-incubation feeding activities and energy budgets of Snow Geese: can food on the breeding grounds influence fecundity?

The potential contribution of early spring feeding conditions in the Arctic to clutch size variation was examined in a population of Lesser Snow Geese Anser caerulescens caerulescens. Behavioural observations were combined with energetic analyses of food material to derive an estimate of the energy budgets of pre-laying and laying females. Food intake of females between arrival on the breeding grounds and incubation was considerable; estimated energy gains in this period were in the same magnitude as the cost of one or several eggs. The pre-laying period spent on the breeding grounds can thus be energetically beneficial rather than costly. Accumulation of resources for reproduction in Snow Geese is a continual process including the breeding grounds, and nutrient limitation after arrival in the Arctic cannot sufficiently explain the environmental component of clutch size variation. The timing of migration and follicle development is such that clutch size decisions are sometimes made during the late stages of migration and some-times after arrival. In the latter case food conditions on the breeding grounds may greatly influence clutch size; in the former case they may still influence readjustments of clutch size after the initial decision. The universal negative correlation between clutch size and laying date in Snow Geese can be explained by negative fitness consequences of late hatching, which outweigh the benefits of delayed laying and further nutrient accumulation. Food shortage on the breeding grounds may sometimes be a secondary factor contributing to seasonal clutch size decline.     

The association between the emergence of cooperative breeding and clutch size

Previous theoretical work has suggested that smaller brood sizes helped facilitate the emergence of cooperative breeding in birds. However, recent empirical evidence has found no statistically significant difference between the clutch sizes of cooperative breeders and that of noncooperative breeders. One explanation for this finding is that while small clutch sizes may predispose species to cooperative breeding, the emergence of cooperative breeding itself may influence the evolution of clutch size. Here, we develop a set of models using population dynamics to describe how the emergence of cooperative breeding influences clutch size. We find, in contrast to previous theoretical work, that the emergence of cooperative breeding does not necessarily decrease (and under certain conditions may actually increase) clutch size. In particular, clutch size may increase after the emergence of cooperative breeding if helpers – philopatric individuals that assist their breeding relatives – are able to substantially improve breeder fecundity at low costs to their own survival, and if the association between breeder and helper is brief. In many cases, clutch size increases following the emergence of cooperative breeding not because it is optimal for the breeder, but as the result of breeder–helper conflict over resource allocation.     

A theoretical investigation of the effect of latitude on avian life histories

Tropical birds lay smaller clutches than birds breeding in temperate regions and care for their young for longer. We develop a model in which birds choose when and how often to breed and their clutch size, depending on their foraging ability and the food availability. The food supply is density dependent. Seasonal environments necessarily have a high food peak in summer; in winter, food levels drop below those characteristic of constant environments. A bird that cannot balance its energy needs during a week dies of starvation. If adult predation is negligible, birds in low seasonal environments are constrained by low food during breeding seasons, whereas birds in high seasonal environments die during the winter. Low food seasonality selects for small clutch sizes, long parental care times, greater age at first breeding, and high juvenile survival. The inclusion of adult predation has no major effect on any life-history variables. However, increased nest predation reduces clutch size. The same trends with seasonality are also found in a version of the model that includes a condition variable. Our results show that seasonal changes in food supply are sufficient to explain the observed trends in clutch size, care times, and age at first breeding.     

Kittiwakes strategically reduce investment in replacement clutches

Many life-history traits are expressed interactively in life, but to a varying extent on different occasions. Changes in trait expression can be accounted for by differences in the quality of the environment ('environmental constraint' hypothesis) or by strategic adjustments, if the relative contribution of the trait to fitness varies with time ('strategic allocation' hypothesis). In birds, egg production is lower in replacement clutches than in first clutches, but it is unknown whether this reduction results from an environmental constraint (e.g. food being less available at the time when the replacement clutch is produced) or from a strategic allocation of resources between the two breeding attempts. To distinguish between these two hypotheses, we performed an experiment with black-legged kittiwakes (Rissa tridactyla). Pairs were either food-supplemented or not before the first clutch was laid onwards and we induced them to produce a replacement clutch by removing eggs once when the first clutch was complete. As predicted by the 'strategic allocation' hypothesis, egg production of food-supplemented and non-food-supplemented birds decreased between first and replacement clutches. This suggests that kittiwakes strategically reduce investment in egg production for their replacement clutches compared to first clutches.     

Increased detrimental effects of ectoparasites on their bird hosts during adverse environmental conditions

Parasites are usually assumed to negatively affect fitness components of their hosts, particularly during adverse environmental conditions. We experimentally tested whether the ectoparasitic house martin bug (Oeciacus hirundinis) had larger negative effects on its house martin (Delichon urbica) host during second clutches. Conditions for reproduction decreased during the season, being less favourable for the second clutch than the first clutch. This was demonstrated by smaller clutch sizes, lower breeding success and smaller and lighter nestlings during the second clutch, as compared to the first, in the absence of parasites. Experimental manipulation of the intensity of parasitism resulted in the reproductive success of the host being reduced with increasing numbers of house martin bugs. This effect was much stronger during the second, as compared to the first, clutch. The house martin bug therefore had a greater detrimental effect on the reproduction of its host when conditions for reproduction were poor.     

A long-term large-scale study of the breeding biology of the Spanish imperial eagle (Aquila adalberti)

We present data from a 17-year study of the population biology of a growing population of Spanish imperial eagles Aquila adalberti across most of its breeding range. The objective of this study was to analyse the effects of age, supplemental feeding and rabbit haemorrhagic disease (RHD) on several breeding parameters of this population of eagles. Average clutch size was 2.2 eggs per clutch, and the average incubation time was 41.7 days per clutch. Fledging occurred an average of 76.8 days after hatching, the length of the fledgling period was not correlated to clutch size. The annual average percentage of pairs laying eggs was 88%. A significant reduction in the percentage of pairs laying eggs in the period 1992–1994 (from 91 to 81%) coincided with most of the eagles’ territories being affected by the rabbit epizootic disease, RHD, which reduced their food supply significantly. Average productivity was 1.23 chicks per monitored territory, average breeding success was 1.40 chicks in a clutch per territory and the average fledging rate was 1.69 chicks per territory with hatching success. The main known causes of breeding failure during incubation were nest collapse and human disturbance. During chick-rearing, total or partial chick losses were mainly caused by siblicide, disease, malnutrition or nest collapse. In 26.2% of the 1372 monitored breeding attempts, at least one of the breeding birds was a subadult (the male in 56.1% of the cases, the female in 15.5% and both sexes in 28.4% of cases). In cases of mixed-aged pairs (n = 205), 70.7% were the result of a substitution, and 29.3% were the result of the forming of a new pair. Egg laying took place significantly earlier and breeding success was higher in territories occupied by adults than in those occupied by subadults. Breeding parameters were higher in high-quality (rabbit-rich) territories than in low-quality (rabbit-poor) territories, but only for those territories occupied by adults. The values obtained in the territories occupied by adults were only significantly higher than in those of the subadults in high-quality territories. Age and territory quality thus simultaneously affected reproductive output.