ENVIRONMENTAL INFLUENCES ON GROWTH AND SURVIVAL OF NESTLING HOUSE MARTINS DELICHON URBICA

The Breeding of the Kestrel, Falco Tinnunculus L., in the Reclaimed Area Oostelijk Flevoland

Data from several field experiments support the existence of a trade-off between number and quality of offspring. However, long term effects of brood size on fitness related traits of offspring have been a relatively neglected area of research. In a laboratory experiment the effect of manipulated brood size on subsequent competitive ability of adult offspring was investigated. Zebra finches, Taeniopygia guttata, were reared in small or large broods and young were exchanged in such a way that natural siblings from different rearing conditions could be compared. Competitive behaviour was assessed in two different contexts: competition for food (both sexes tested) and competition for mates (only males tested). There was no significant difference between males from small and large broods in number of succesfull attacks (after which the other male moved away) during male-male aggressive interactions provoked by the presentation of a female in an adjacent cage. Nor did brood size affect latency to eat, time spent eating or success at displacing the other bird from the feeder during food competition tests. The results thus suggest consistently that later competitive ability of offspring is not affected by brood size in this species.

For many bird species, brood size is a highly variable trait. Several aspects of the development and survival of nestlings are affected by brood size. Most scientific evidence comes from brood manipulation experiments, and the complementary information offered by studies of natural variability has received little attention in tropical and subtropical species. Here, we provide data on the effect of brood size on the development and survival of two furnariid species under natural conditions. Weekly during two breeding seasons, two forest patches (40 ha. each) from central Argentina, were exhaustively examined for active nests. Nestlings were monitored repeatedly from hatching to fledging. Factors that were considered potential confounders or effect modifiers were also recorded and included in the statistical analysis. We found a positive effect of brood size on growth, and a negative effect on survival. The first effect was only observed when preceding weeks were not rainy, and the latter was present in only one breeding season, so both effects appeared to depend on environmental conditions. Our results suggest that the benefit of a large or small brood size is dependent on context. In the studied populations, plasticity in or within‐population genetic diversity in brood size may be advantageous in the light of the unpredictable circumstances that will prevail during the breeding season.

1. An increase of competition among adults or nestlings usually negatively affects breeding output. Yet little is known about the differential effects that competition has on the offspring sexes. This could be important because it may influence parental reproductive decisions. 2. In sexual size dimorphic species, two main contradictory mechanisms are proposed regarding sex-specific effects of competition on nestling performance assuming that parents do not feed their chicks differentially: (i) the larger sex requires more resources to grow and is more sensitive to a deterioration of the rearing conditions ('costly sex hypothesis'); (ii) the larger sex has a competitive advantage in intra-brood competition and performs better under adverse conditions ('competitive advantage hypothesis'). 3. In the present study, we manipulated the level of sex-specific sibling competition in a great tit population (Parus major) by altering simultaneously the brood size and the brood sex ratio on two levels: the nest (competition for food among nestlings) and the woodlot where the parents breed (competition for food among adults). We investigated whether altered competition during the nestling phase affected nestling growth traits and survival in the nest and whether the effects differed between males, the larger sex, and females. 4. We found a strong negative and sex-specific effect of experimental brood size on all the nestling traits. In enlarged broods, sexual size dimorphism was smaller which may have resulted from biased mortality towards the less competitive individuals i.e. females of low condition. No effect of brood sex ratio on nestling growth traits was found. 5. Negative brood size effects on nestling traits were stronger in natural high-density areas but we could not confirm this experimentally. 6. Our results did not support the 'costly sex hypothesis' because males did not suffer from higher mortality under harsh conditions. The 'competitive advantage hypothesis' was also not fully supported because females did not suffer more in male-biased broods. 7. We conclude that male nestlings are not likely to be more expensive to raise, yet they have a size-related competitive advantage in large broods, leading to higher mortality of their on average lighter female nest mates.

Optimal clutch size theory predicts a trade-off between egg number and fledgling body mass, potentially affecting their survival rate. Yet, only a few studies have directly linked clutch or brood size to energy (food), nestling growth rate, and timing of breeding. We studied the relationship among all these variables in the Eurasian eagle owl. Of 150 eggs laid in 48 nests, 119 hatched and 117 nestlings fledged, indicating low nestling mortality. The number of eggs, hatchlings and fledglings (possibly indicating territory quality) decreased with ordinal date. The total amount of food brought to the nest increased with brood size, but the average amount per nestling decreased. Nestling growth rate (and thus probably fledgling body mass) was negatively correlated with brood size. A trade-off between brood size and fledgling survival may lessen the benefit of occupying food-rich territories. However, we suggest that fledglings from large broods, despite their relatively low body mass, may have higher survival rates owing to their relatively early fledging. This may be due to: (1) higher prey abundance in late spring compared with late summer and (2) a longer period before entering the cold season. Variation in brood size may therefore represent a set of strategies linked to prey abundance.

Altricial species rely on parental provisioning for early-life sustenance, and a larger brood or litter size leads to higher levels of competition between siblings for parental resources. Early-life stress can have severe and lifelong effects on Darwinian fitness. Indeed, it is well established that being reared in a larger brood impairs growth and fitness prospects of birds, but the mechanistic underpinnings of this effect are still largely unknown. Specifically, it is not well known to what extent the reduced growth and fitness prospects of nestlings reared in large broods is due to increased resource allocation to competition versus a per capita reduction in parental provisioning rate, or a combination of the two. We cross-fostered zebra finch ( Taeniopygia castanotis ) chicks into small and large broods, and recorded their growth as well as the behaviour of parents and offspring throughout the nestling period. As in previous experiments, growth rate was higher in small broods. In large broods, chicks begged more and parents fed more frequently, fully compensating for brood size differences to yield comparable per capita feeding rates. We therefore conclude that the lower growth rate for nestlings raised in large broods is likely attributable, at least in part, to increased energy expenditure on begging and limits to parental allocation efficiency, rather than solely a reduction in their feeding frequency. These patterns are consistent with significant energetic costs associated with begging and raise the interesting possibility that brood size would not have negatively affected growth in large broods if chicks had not increased their begging effort due to increased levels of competition in the nest.

We investigated the relationship between parental behavior and brood size, and the consequences of this relationship in terms of parental fitness (timing of molt and body mass at onset of molt in same year as breeding, and probability of return, timing of breeding, and clutch size in following year) in the precocial Lesser Snow Goose (Chen caerulescens caerulescens) at La Perouse Bay, Manitoba. The percentage of time parent birds spent feeding decreased with increasing brood size, from greater than 90% for pairs without off-spring to less than 80% for broods of seven and eight. The number of vigilant (head-up) postures per minute by parental birds increased up to brood size five and then decreased. Parental females also spent significantly less total time feeding and more time in alert behavior as brood size increased from one to five goslings. The relationship between parental behavior and brood size remained significant for small brood sizes even if pairs without goslings were excluded (range one to five goslings), and this relationship was independent of female age. Males (but not females) rearing larger broods molted later than those with smaller broods, although only by one to two days. This was directly related to rearing of offspring; in both sexes, birds that hatched four or more goslings and subsequently lost one or more goslings during brood-rearing molted significantly earlier than birds rearing all of their hatched goslings. There was no relationship, in either sex, between number of goslings reared and the adult mass five to six weeks posthatch (molt) in the same year, or probability of return or timing of breeding (laying date or hatch date) in the following year. Partners of males that reared the largest number of goslings laid significantly larger clutch sizes the following year, suggesting that these were ''better-quality'' pairs. Over the range of naturally observed brood sizes, the effect of increasing brood size on parental behavior does not appear to be associated with any negative effects on residual parental reproductive effort or fitness in this species.

Allometric relationships of reproductive output were compared in 20 species from 7 families of brachyuran crabs from the east and west coasts of North America, using regression analysis of log reproductive parameters versus log body weight. Comparisons of crabs spanning 4 orders of magnitude in body weight indicated that female body size was the principal determinant of reproductive output: 95% of the variance in brood weight, 79% of the variance in the number of eggs per brood, 63% of the variance in annual brood weight, and 74% of the variance in annual fecundity were explained by body weight. Brood weight exhibited an isometric constraint to about 10% of body weight. Allometric limitations on space available for yolk accumulation in the body cavity appeared to be the main constraint on brood size. Ovum size increased significantly, but only slightly, with increasing body size. There was a significant trade-off between ovum size and the number of eggs per brood. There was no significant relationship between the number of broods per year and body size. The number of eggs per brood was significantly better than brood weight as a predictor of the number of broods produced per year by a species, indicating that demographic pressure on fecundity rather than energetic considerations is the primary selective mechanism influencing annual reproductive effort. Each of the 7 families of crabs exhibited trends toward distinct patterns for the suite of co-adapted reproductive traits. However, no interspecific reproductive patterns were apparent with respect to the variables of feeding type, salinity tolerance, habitat, and geographic range represented by the 20 species.

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

Resource limitations, either due to environmental conditions or constraints on parental provisioning effort, can drive intense competition among offspring. In communal groups, resource availability may increase if parents receive assistance from other group members; however, if those caregivers also produce young, offspring demand may increase at the same time. It is possible, therefore, that the costs of intrabrood competition in large broods may outweigh the benefits of provisioning from additional caregivers. We tested the relationships between group size, brood size, and provisioning rates in the greater ani (Crotophaga major), a communally nesting cuckoo in which multiple breeding pairs and nonreproductive helpers cooperatively raise a shared brood. Crucially, brood and group size can vary independently in this species, allowing us to test changes in each variable separately. Using video footage of 2255 prey deliveries across 10 nests, we found that an increase in the number of adult caregivers within a group did not sufficiently offset a corresponding increase in the number of dependent young within a brood: prey availability per average nestling decreased with brood size, regardless of group size. In larger broods, last-hatched nestlings received significantly less prey than their broodmates, in part due to greater hatching asynchrony that exacerbated competitive asymmetries and facilitated inequality in food allocation. Our results indicate that last-hatched ani nestlings suffer a “double cost” in large broods: they must compete with more nestmates, and suffer disproportionately from asynchronous hatching. These costs may contribute to increased parent–offspring conflict and may constrain group size in communal breeders.

We investigated the effects of ecological context (by comparing data from two consecutive years) and experimentally manipulated nestling developmental conditions (large vs. small brood size) on immune function (immunoglobulin Y [IgY]) and oxidative stress in nestling European starlings Sturnus vulgaris. On the basis of annual differences in chicks' morphological traits and body masses close to fledging, we established that 2007 was a relative low-quality year and 2008 was a relatively high-quality year. Total antioxidant capacity (TAC) was significantly lower in experimentally enlarged broods, but only in the low-quality year (2007). Total oxidant status (TOS) was independent of brood size in both years but was 45% higher in the low-quality year. Consequently, plasma oxidative status (the ratio between TOS and TAC) was higher in 2007. In contrast, plasma IgY levels were higher in the experimentally enlarged broods and in the high-quality year (2008). Thus, immune function and oxidative stress showed inverse relationships with developmental conditions and annual variation in year quality. Finally, TOS and TAC were positively correlated, but only in the low-quality year (2007), and there was no relationship observed between IgY and markers of oxidative stress. Our results demonstrate the importance of taking into account year effects or ecological context when assessing environmental effects on physiological mechanisms underlying the life-history traits of chicks, such as oxidative stress.

1. Little is known about long-term effects of brood size on fitness components of offspring. This is unfortunate because such information is needed to predict optimal brood size. Furthermore, ontogenetic circumstances are potentially important in explaining the large individual differences in lifetime reproductive success documented in many species. 2. In a laboratory study the long-term effect of brood size manipulation on mortality and morphological development of offspring was investigated. Young zebra finches Taeniopygia guttata were reared in small or large broods. Young were exchanged in such a way that natural siblings from different rearing conditions could be compared. 3. Manipulated brood size affected offspring morphology permanently (measurements were taken up to the age of 12 months). Individuals reared in small broods were heavier, had longer tarsi and wings, higher beaks, were in better condition, and males had redder beaks. The experiment did not affect beak length or female beak redness. Individuals from large broods had caught up on wing length and males from large broods on beak redness by 6 months of age, which may reflect priority of investment in traits important to fitness. 4. Mortality of offspring raised in large broods was higher both before and after independence. After independence the effect of manipulated brood size on mortality was sex specific; females were most likely to die. This suggests that the optimal sex ratio of offspring may depend on brood size. 5. The effect of brood size on mortality after independence was probably partly mediated by size and condition during the nestling phase and possibly by peer aggression among adult offspring. Sex-linked mortality could not be explained by a sex difference in morphological development. Why females are more vulnerable remains to be discovered.

Peer Reviewed

Brood weight and number of eggs per brood are primarily determined by female body size in brachyuran crabs. Brood weight exhibits an isometric relationship spanning more than four orders of magnitude in body weight among 33 non-pinnotherid species, with brood size being constrained to about 10% of female body weight by the space available for yolk accumulation within the cephalothorax. In contrast, two pinnotherids ( Pinnotheres ostreum, Fabiasubquadrata ) have relative brood sizes which are 66% and 97%, respectively, of body weight. The allometric constraint is circumvented in these extraordinarily large brood sizes by two pinnotherid features which allow more space for yolk accumulation. Unlike other brachyurans, their ovaries extend out of the cephalothorax into the abdomen, and calcification of their exoskeleton is greatly reduced, potentially allowing distension of the body. As in crabs, pinnotherid egg size is highly variable among species, which tends to increase the variance among species in fecundity per brood, making pinnotherid fecundities not extraordinary. Thenumber of broods produced per year in these two species is limited by short reproductive seasons, which tends to bring their annual reproductive output in line with other brachyurans. The trade-offs among reproductive variables in pinnotherids is adaptive for the small body size and parasitic niche of these species.

When eggs hatch asynchronously, offspring arising from last-hatched eggs often exhibit a competitive disadvantage compared with their older, larger nestmates. Strong sibling competition might result in a pattern of resource allocation favoring larger nestlings, but active food allocation towards smaller offspring may compensate for the negative effects of asynchronous hatching. We examined patterns of resource allocation by green-rumped parrotlet parents to small and large broods under control and food-supplemented conditions. There was no difference between parents and among brood sizes in visit rate or number of feeds delivered, although females spent marginally more time in the nest than males. Both male and female parents preferentially fed offspring that had a higher begging effort than the remainder of the brood. Mean begging levels did not differ between small and large broods, but smaller offspring begged more than their older nestmates in large broods. Male parents fed small offspring less often in both brood sizes. Female parents fed offspring evenly in small broods, while in large broods they fed smaller offspring more frequently, with the exception of the very last hatched individual. These data suggest male parrotlets exhibit a feeding preference for larger offspring—possibly arising from the outcome of sibling competition—but that females practice active food allocation, particularly in larger brood sizes. These differential patterns of resource allocation between the sexes are consistent with other studies of parrots and may reflect some level of female compensation for the limitations imposed on smaller offspring by hatching asynchrony.