Testis size increases with colony size in cliff swallows

By using a sample of over 800 male cliff swallows (Petrochelidonpyrrhonota) that died during a rare climatic event in our Nebraskastudy area in 1996, we investigated how testis size was relatedto body size, age, parasite load, a bird's past colony-sizehistory, and spleen size. Testis volume increased with bodysize. After correcting for body size, testis volume was lowestfor birds age 1 and 2 years but did not vary with age for males3 years old or more. Birds occupying parasite-free (fumigated)colonies had significantly larger testes than did birds at nonfumigatedsites. Testis volume increased significantly with the size ofthe breeding colonies a bird had used in the past. Testis volumehad no relationship with spleen volume after correcting forbody size. The results show within a species that larger testesare favored in more social environments, probably reflectinga response to increased rates of extrapair copulation (and thussperm competition) among cliff swallows in large colonies. Thepresence of ectoparasites, by inflating levels of plasma corticosterone,may in turn reduce testis mass. These data provide no supportfor the hypothesis that large testes, perhaps by producing moretestosterone, are immunosuppressive and thus costly for thatreason.     

Supplementary feeding of wild birds indirectly affects the local abundance of arthropod prey

Providing supplementary food for wild birds is a globally popular pastime; almost half of the households in many developed countries participate and billions of US dollars are spent annually. Although the direct influence of this additional resource on bird survivorship and fecundity has been studied, there is little understanding of the wider ecological consequences of this massive perturbation to (what are usually) urban ecosystems. We investigated the possible effects of wild bird feeding on the size and survivorship of colonies of a widespread arthropod prey species of many small passerine birds, the pea aphid [Acyrthosiphon pisum (Harris); Hemiptera: Aphididae], in suburban gardens in a large town in southern England. We found significantly fewer aphids and shorter colony survival times in colonies exposed to avian predation compared to protected controls in gardens with a bird feeder but no such differences between exposed and protected colonies in gardens that did not feed birds. Our work therefore suggests that supplementary feeding of wild birds in gardens may indirectly influence population sizes and survivorship of their arthropod prey and highlights the need for further research into the potential effects on other species.     

INTENSE NATURAL SELECTION ON BODY SIZE AND WING AND TAIL ASYMMETRY IN CLIFF SWALLOWS DURING SEVERE WEATHER

Extreme climatic disturbances provide excellent opportunities to study natural selection in wild populations because they may cause measurable directional shifts in character traits. Insectivorous cliff swallows (Petrochelidon pyrrhonota) in the northern Great Plains must often endure periods of cold weather in late spring that reduce food availability, and if cold spells last four or more days, mortality due to starvation may result. We analyzed morphological shifts associated with viability selection, and how patterns of bilateral symmetry were affected by survival selection, during a four-day period of cold weather in 1992 and a six-day period in 1996 in southwestern Nebraska. Birds that died during the cold were compared to those still alive when the severe weather ended. The event in 1992 killed relatively few birds, but the cold spell in 1996 killed thousands of cliff swallows and reduced their population by about 53%. Climatological records suggest that mortality events comparable to that of 1996 have occurred in only one other year since 1875. Larger birds were favored in the 1996 event. Selection was more intense in 1996 than in 1992 because of more stressful conditions in 1996. Directional selection gradient analysis showed that measures of skeletal body size (tarsus length, culmen width and length) and wing length were targets of selection in 1996. Survivors had lower wing and outer tail asymmetry, and wing and tail asymmetry were targets of selection in both events. Mortality patterns did not differ by sex, but older birds suffered heavier mortality; morphological traits generally did not vary with age. Nonsurvivors were not in poorer apparent condition prior to the weather event than survivors, suggesting that selection acted directly on morphology independent of condition. Selection on body size in cliff swallows was more intense than in studies of body size evolution in other bird species. Larger swallows were probably favored in cold weather due to the thermal advantages of large size and the ability to store more fat. Swallows with low asymmetry were favored probably because low asymmetry in wing and tail made foraging more efficient and less costly, conferring survival advantages during cold weather. This population of cliff swallows may have undergone relatively recent body size evolution.     

Variation in age composition among colony sizes in Cliff Swallows

Variation in group size is characteristic of most social species. The extent to which individuals sort among group sizes based on age may yield insight into why groups vary in size and the age‐specific costs and benefits of different social environments. We investigated the age composition of Cliff Swallow (Petrochelidon pyrrhonota) colonies of different sizes over 18 yr at a long‐term study site in western Nebraska, USA. Using years elapsed since banding as a relative measure of age for over 194,000 birds, we found that the proportion of age‐class‐1 swallows (birds banded as nestlings or juveniles or adults in the year of banding) of both sexes increased in larger colonies and at colony sites becoming active later in the summer. Age composition was unrelated to how often a particular colony site was used. The effect of colony size most likely reflected the fact that older birds return to the same colony site in successive years even when the colony size there decreases, and that yearlings and immigrants benefit more from larger colonies than do older, more experienced individuals. The date effect probably resulted in part from later spring arrival by younger and/or immigrant swallows. At fumigated sites where ectoparasitic swallow bugs (Oeciacus vicarius) had been removed, age composition did not vary with either colony size or colony initiation date. The patterns reported here appear to be driven partially by the presence of ectoparasites and suggest that the hematophagous bugs influence variation in Cliff Swallow group composition. Our results are consistent with the hypothesis that variation in colony size reflects, in part, age‐based sorting of individuals among groups.     

An energetic correlate between colony size and foraging effort in seabirds, an example of the Adélie penguin Pygoscelis adeliae

Central-place foraging seabirds alter the availability of their prey around colonies, forming a "halo" of reduced prey access that ultimately constrains population size. This has been indicated indirectly by an inverse correlation between colony size and reproductive success, numbers of conspecifics at other colonies within foraging range, foraging effort (i.e. trip duration), diet quality and colony growth rate. Although ultimately mediated by density dependence relative to food through intraspecific exploitative or interference competition, the proximate mechanism involved has yet to be elucidated. Herein, we show that Adélie penguin Pygoscelis adeliae colony size positively correlates to foraging trip duration and metabolic rate, that the metabolic rate while foraging may be approaching an energetic ceiling for birds at the largest colonies, and that total energy expended increases with trip duration although uncompensated by increased mass gain. We propose that a competition-induced reduction in prey availability results in higher energy expenditure for birds foraging in the halo around large colonies, and that to escape the halo a bird must increase its foraging distance. Ultimately, the total energetic cost of a trip determines the maximum successful trip distance, as on longer trips food acquired is used more for self maintenance than for chick provisioning. When the net cost of foraging trips becomes too high, with chicks receiving insufficient food, chick survival suffers and subsequent colony growth is limited. Though the existence of energetic studies of the same species at multiple colonies is rare, because foraging metabolic rate increases with colony size in at least two other seabird species, we suggest that an energetic constraint to colony size may generally apply to other seabirds.     

Covariation between brain size and immunity in birds: implications for brain size evolution

Parasitism can negatively affect learning and cognition, setting the scene for coevolution between brain and immunity. Greater susceptibility to parasitism by males may impair their cognitive ability, and relatively greater male investment in immunity could compensate for greater susceptibility to parasites, in particular when males have a relatively large brain. We analysed covariation between relative size of immune defence organs and brain in juvenile and adult birds. The relative size of the bursa of Fabricius and the spleen in adults covaried positively with relative brain size across bird species. The relative size of these two immune defence organs covaried with sex differences in relative size of the brain, indicating that the relationship between immune defence and brain size was stronger for males. In contrast, liver and heart size or sexual size dimorphism in size did not covary with immune defence. Thus, species in which males have relatively large brains also have relatively large immune defence organs.     

Intraspecific variation in size and density of Avocet colonies: effects of nest-distances on hatching and breeding success

In many colonial bird species there is considerable intraspecific variation in colony size and inter‐nest distance (colony density). Possible causes of this variation and its effects on hatching success (survival of eggs) and breeding success (probability of a pair raising chicks) were studied in 48 Avocet Recurvirostra avosetta colonies in Schleswig‐Holstein (Germany) between 1991 and 1996. Colony density was influenced by time of year and habitat (categories: island or mainland, close to or far from feeding grounds). Colonies on islands had the highest densities. When all available space at a colony site was used, colonies became very dense (mean nearest‐neighbour nest distance less than 1 m). Colony size (number of clutches) was influenced by time of year, but not by habitat. Hatching success was low in high density colonies and in very low density ‘colonies’ (single nests) and high over a broad range of intermediate nest densities. The low success rate of single nests was caused by a very high predation rate, whereas the low success rate in very dense colonies was caused by a high rate of nest abandonment. Nest abandonment in very dense colonies was associated with a high level of aggressiveness among Avocets during the egg‐laying period. Due to territorial behaviour, Avocets seemed to be expelled from the densest breeding sites. In very dense colonies, high frequencies of clutches of unusual size occurred due to conspecific nest parasitism. The number of Avocets taking part in attacks on potential egg predators was small and (in colonies of more than one clutch) depended neither on colony size nor on colony density. Despite a low hatching success in very dense colonies, individuals breeding in the densest colonies had significantly better chances of raising chicks than Avocets breeding in less dense colonies. Coloniality seemed to be obligatory for Avocets in order to ensure hatching success. The size and density of colonies seemed to be associated with the availability of suitable nesting habitats (islands).     

Fine‐scale impacts on avian biodiversity due to a despotic species,the bell miner (Manorina melanophrys)

Bell miners (Manorina melanophrys; Meliphagidae) are a highly social and very aggressive honeyeater. They are despotic and cooperate in the defence of their territories against other bird species, leading to the almost complete exclusion of other avifauna from miner‐occupied regions. This study aimed to resolve some of the fine‐scale effects of bell miner aggression on avian diversity both within and adjacent to colonies to determine the true impact of a colony on local avifaunal abundance. Three areas, distributed throughout the range of the bell miner, were surveyed across both non‐breeding and breeding seasons to assess the temporal and spatial impacts of bell miner aggression on other bird species. Bell miner colonies were found to occupy very clearly defined areas and had the expected negative impact on avian diversity within their colony. The effects of bell miner colony presence on abundance and richness of avian species were found to cease at the colony boundary, with both recovering to normal levels immediately outside the bell miner colony. Whether bell miners were breeding or not, and irrespective of the amount of vegetation coverage, bell miner colonies were found to have relatively marginal impacts on avian richness and abundance. No impact of colony presence/absence was found on the richness or abundance of the avian species that dwell in the undergrowth, with some evidence that these species were actually more common at the colony edge. Our results demonstrate that the influence of bell miner colony presence upon avian biodiversity is restricted to the confines of the colony and does not radiate outwards into the surrounding habitat. Colony presence influences, therefore, have implications when considering the impact of bell miner behaviour on the diversity of insectivorous birds and processes, most notably the propagation of Bell Miner Associated Dieback.     

Climate,body condition and spleen size in birds

Climatic conditions may impact on the body condition of animals and thereby affect their survival prospects. However, climate may also impact directly on the survival prospects of animals by affecting the size of immune defence organs that are used for defence against parasites. We used a large long-term database on body condition and size of the spleen in birds to test for immediate and delayed relationships between climatic conditions as indexed by the North Atlantic Oscillation (NAO) and body condition and spleen mass, respectively. Across 14 species of birds, spleen mass was significantly positively correlated with the NAO index, while the delayed effect of NAO on spleen mass was not significant. Spleen mass was positively related to body condition, but body condition did not depend significantly on NAO or delayed NAO effects. Bird species with a strong positive effect of NAO on spleen mass tended to have small spleens for their body size, while species with a strong negative effect of NAO on spleen mass tended to have relatively large spleens. Since bird species with relatively large spleen have been shown to suffer more from the negative effects of parasites, we can infer that the effects of climate as indexed by NAO on the size of the spleen depends on the importance of parasite-mediated natural selection.Due to an error in the citation line, this revised PDF (published in December 2003) deviates from the printed version, and is the correct and authoritative version of the paper.     

Heritable choice of colony size in cliff swallows: does experience trump genetics in older birds?

The variation in breeding-colony size seen in populations of most colonial birds may reflect heritable choices made by individuals who are phenotypically specialized for particular social environments. Although a few studies have reported evidence for genetically based choice of group sizes in birds, we know relatively little about the extent to which animals potentially rely on experience versus innate preferences in deciding with how many conspecifics to settle at different times of their lives. We conducted a cross-fostering experiment in 1997-1998 on cliff swallows (Petrochelidon pyrrhonota) in southwestern Nebraska, USA, in which some individuals were reared in colonies different in size from those in which they were born. Breeding-colony sizes chosen by this cohort of birds were monitored by mark-recapture throughout their lives. A multistate mark-recapture analysis revealed that birds in their first breeding year chose colony sizes similar to those of their birth, regardless of their rearing environment, confirming a previous analysis. Beyond the first breeding year, however, cliff swallows' colony choice was less dependent on where they were born. Birds born in small colonies and reared in large colonies showed evidence of a delayed rearing effect, with these birds overwhelmingly choosing large colonies in later years. Heritabilities suggested strong genetic effects on first-year colony choice but not in later years. Cliff swallows' genetically based colony-size preferences their first year could be a way to ensure matching of their phenotype to an appropriate social environment as yearlings. In later years, familiarity with particular colony sites and available information on site quality may override innate group-size preferences when birds choose colonies.     

Immune competence and spleen size scale with colony status in the naked mole-rat

Naked mole-rats (NM-R; Heterocephalus glaber) live in multi-generational colonies with a social hierarchy, and show low cancer incidence and long life-spans. Here we asked if an immune component might underlie such extreme physiology. The largest lymphoid organ is the spleen, which plays an essential role in responding to immunological insults and may participate in combating cancer and slowing ageing. We investigated the anatomy, molecular composition and function of the NM-R spleen using RNA-sequencing and histological analysis in healthy NM-Rs. Spleen size in healthy NM-Rs showed considerable inter-individual variability, with some animals displaying enlarged spleens. In all healthy NM-Rs, the spleen is a major site of adult haematopoiesis under normal physiological conditions. However, myeloid-to-lymphoid cell ratio is increased and splenic marginal zone showed markedly altered morphology when compared to other rodents. Healthy NM-Rs with enlarged spleens showed potentially better anti-microbial profiles and were much more likely to have a high rank within the colony. We propose that the anatomical plasticity of the spleen might be regulated by social interaction and gives immunological advantage to increase the lifespan of higher-ranked animals.     

Movements of radio-tagged Grey Herons Ardea cinerea during the breeding season in a large pond area

This paper describes the strategies of resource utilization in the course of the breeding season by five radio-tagged Grey Herons Ardea cinerea. The seasonal changes in exploitation of the environment by two breeding adults, one non-breeding adult and two non-breeding first-year birds were studied from March to August 1982, near Zonhoven in Belgium. Two adult breeding birds could be followed continuously from the end of March until the middle of June. During the first month they explored an extended area all around the colony, but each concentrated its search in a specific direction. From the end of April until the beginning of June, most probably from egg-hatching until the end of breeding activities, each bird spent a very large proportion of its time at a particular feeding site, from which other herons were actively excluded. In the first part of June they again visited different sites, each maintaining its preferred direction. From the middle of June onwards they seemed to have left the fish-pond area. The pattern of movements of the first-year birds differed markedly from that of the breeding adults. In April, although both non-breeding and breeding birds explored large areas, only the areas used by non-breeders were centred on the colony. From the end of April onwards, probably after general egg-hatching in the colony, the non-breeders very rarely revisited the colony, and from May till August their ranges became more and more restricted to very small areas at an increasing distance from the colony. They were never observed defending particular sites. The results are discussed with regard to recent speculations about the evolution of colonies as an adaptation for the exploitation of food resources. Breeding herons seem to explore a large part of the environment during incubation and defend a particular site while feeding their young. Choice of feeding site by non-breeding birds may be influenced by the site defence of the breeding birds. Non-breeding birds exploit a large area when breeding birds occupy feeding territories. Perhaps they are forced to forage in less suitable places at this time. Colonies might have evolved as a strategy to minimize effort in resource esploitation as, especially at the beginning of the breeding season, the colony could act as an assembly point in the exploration of the environment. However, its importance as an assembly point diminishes in the course of the season, as non-breeding birds no longer visit the colony and adults defend territories.     

Do sleeping king penguins influence the movement of conspecifics through a colony?

Within a breeding colony, movements of penguins to and from territories can be associated with high energetic costs and risks of injury induced by the high aggressiveness of territorial breeders. Here, we tested whether the presence of sleeping king penguins (namely non-aggressive birds) could influence the trajectory of walking conspecifics. Also, we determined how an aggressive response from the sleeping individual's neighbours could affect the trajectory adopted by a travelling bird. We observed 150 sleeping king penguins on Possession Island (Crozet Archipelago) during the breeding season and determined whether or not travelling birds walk preferentially beside sleeping individuals. Moving penguins not attacked by territorial birds directly surrounding the sleeping individual walked preferentially beside it in 80.3% of the observations. However, birds attacked by the sleeping individual's neighbours walked beside the sleeping bird in only 27.9% of the cases. Whether the sleeping individual's neighbours were directly facing the intruder or not did not affect the probability of the intruder being attacked. Hence, travelling penguins actively chose to walk beside a sleeping individual when not attacked by the latter's neighbours. This striking tropism could represent an adapted motivated behaviour, reducing energetic costs and risks of injury associated with fighting. Accepted: 21 December 1998     

Effects of experimental food provisioning on reproduction in the Jackdaw Corvus monedula, a semi-colonial species

Two Jackdaw Corvus monedula colonies were given supplementary food before and during breeding in 1983. Breeding density and cavity use were compared with those of the same colonies in previous years, when no food was provided. Predation rate and reproductive parameters were compared with those in the same colonies in previous years and with those of two control colonies, without experimental food. Jackdaws preferred safe cavities with small minimum nest-entrance dimensions and avoided those with a high risk of nest predation. In experimental (fed) colonies, however, there was a tendency to use all cavities, which resulted in an increased breeding density. No nests were preyed upon by Ravens Corvus corax in the experimental colonies because supplemental food favoured group defence by increasing colony size and by increasing the time the Jackdaws spent in the colony. Additional food advanced laying date, increased clutch size independently of laying date and increased fledging success. Supplementary food significantly increased fledging success in less than half of all experimental studies on birds. We suggest that the key to this problem is the species' breeding strategy, and we show that supplementary food significantly increased fledging success in brood-reduction strategist species but not in species which directly adjusted their clutch size.     

Balance between site fidelity and habitat preferences in colony site selection by herons and egrets

Habitat selection in avian species is a hierarchical process driven by different factors acting at multiple scales. Habitat preferences and site fidelity are two main factors affecting how colonial birds choose their breeding locations. Although these two factors affect how colonial species choose their habitats, previous studies have only focused on one factor at a time to explain the distribution of species at regional scales. Here we used 28 yr of colony location data of herons and egrets around Ibaraki prefecture in Japan in order to analyze the relative importance of habitat preferences and colony site fidelity for selecting breeding locations. We used Landsat satellite images together with a ground survey‐based map to create land‐use maps for past years and determine the habitats surrounding the herons and egrets colonies. Combining the estimated colony site fidelity with the habitat data, we used a random forest algorithm to create habitat selection models, which allowed us to analyze the changes in the importance of those factors over the years. We found high levels of colony site fidelity for each year of study, with its relative importance as a predictor for explaining colony distribution increasing drastically in the most recent five years. The increase in collective site fidelity could have been caused by recent changes in the population size of grey herons Ardea cinerea, a key species for colony establishment. We observed a balance between habitat preferences and colony site fidelity: habitat preferences were a more powerful predictor of colony distribution until 2008, when colony site fidelity levels were lower. Considering changes in the relative importance of these factors can lead to a better understanding of the habitat selection process and help to analyze bird species’ responses to environmental changes.     

Hidden patterns of colony size variation in seabirds: a logarithmic point of view

Explaining the huge variability present in bird colony sizes within and between species is intimately related to the understanding of the proximate and ultimate reasons for bird coloniality. However, natural patterns of colony size frequency distributions (CSFDs) remain poorly known. It is widely believed that colonial birds have similar long‐tailed (highly right‐skewed) CSFDs and that species mainly differ in their maximum colony sizes (in the length of the ‘tail’ of their CSFDs). We used data from the Seabird 2000 project (20 species; 19 978 colonies; 3 779 919 nests), the largest and most detailed dataset currently available, to analyse the CSFDs of seabird breeding in Britain and Ireland. Log‐transformations of colony sizes revealed that the often reported long‐tailed CSFDs in common histograms were hiding contrasting patterns, mainly log‐normal but also power law CSFDs. The different statistical characteristics of CSFDs did not co‐occur at random within species and were in fact highly correlated (e.g. a large geometric mean correlated with a large coefficient of variation). A PCA with these characteristics revealed a smoothed transition between species’ CSFD. Therefore, (a) a logarithmic analysis will allow different aspects of what is currently only referred to as ‘colony size variation’ to be quantified; (b) we challenge the current idea that all species show similar long‐tailed CSFDs; (c) we offer a new (unified) view of colony size variation and discuss how these new patterns confirm, challenge and may advance theoretical and applied research into bird coloniality.     

Host immune function and sexual selection in birds

Parasites have been hypothesized to affect sexual selection of their hosts, if secondary sexual characters reliably signal absence of infectious parasites, superior parenting ability caused by the absence of parasites, or heritable resistance to parasites, for which there is some intraspecific and interspecific evidence. Measures of immune defence of hosts provide reliable information on the current infection status of individuals of the chosen sex, usually males, and correlations between immune defence and development of secondary sexual characters thus provide a novel critical test of parasite-mediated sexual selection. In a comparative study of birds, sexually dichromatic species had higher immune defences, measured in terms of leukocyte concentration and the size of spleen and bursa of Fabricius, respectively, than closely related, monochromatic species. Male plumage brightness was consistently negatively related to the size of the spleen in males of sexually dichromatic species, but not in males of monochromatic species. Hence, the brightest males, which frequently are preferred as mates by choosy females, had low levels of immune defence, suggesting that such males were healthy. This provides evidence for a general role of parasites in sexual selection among their bird hosts.     

Mechanisms of colony selection by first‐year Lesser Kestrels Falco naumanni

In colonial species, first‐time breeders may use the number of settled conspecifics in colony selection, but such a relationship is confused by the correlation between colony size and nest‐site availability. To distinguish conspecific attraction from neutral colony selection, we experimentally increased nest availability for first‐year Lesser Kestrels Falco naumanni, allowing us to dissociate the number of vacant nest‐sites from colony size at the arrival time of first‐year birds. Under natural conditions, the number of first‐year birds settling was positively correlated with both the number of philopatric and the total number of breeding pairs (colony size) already settled. However, the probability of occupation of experimentally manipulated nests by first‐year birds was independent of colony size. In experimental colonies, the number of first‐year birds settling was positively correlated with the number of manipulated nest‐sites but not with the number of conspecifics. Overall, these results support a neutral colony selection by first‐year Lesser Kestrels based on nest‐site availability.     

Phenotypic sorting in morphology and reproductive investment among sociable weaver colonies

Colony sizes in birds can vary by orders of magnitude within species, and many studies have shown that selection pressures differ dramatically among small and large colonies. Does such selection result in phenotypic sorting at the level of individuals? This study describes inter-colony differences in morphology and reproductive investment in a population of a highly colonial, communal and sedentary African passerine bird, the sociable weaver Philetairus socius. Relative colony sizes were fairly stable over a 10-year period. Adults differed among colonies in terms of bill morphology, condition, body size and degree of ectoparasite infestation, and the last two declined consistently with colony size. In larger colonies, smaller eggs were laid, and nestlings were more parasite-infested, showed weaker cell-mediated immune responses, and experienced higher levels of brood reduction and snake predation. Taken together with another study showing that adult survival is higher in larger colonies, these results suggest that patterns of age-specific mortality are consistently related to colony size in the sociable weaver. Based on these observations I suggest two hypotheses that might account for the observed phenotypic sorting, involving colony size-dependent patterns in (1) density-dependent competition for food and (2) adaptive life-history adjustment.     

Egg-size variation and reproductive success in the Herring Gull Lams argentatus: adaptive or constrained size of the last egg?

Herring Gull Larus argentatus eggs from a study colony in the Baltic showed a slight but significant variation in egg size within the laying sequence. Last-laid eggs were only about 5% smaller by volume than first eggs. There was no significant difference in dry yolk weight or dry albumen weight, although possible differences were evident. The chicks had nearly equal hatching weights and equally long tarsi. There was no differential mortality in the third chick in the study colony, and there were no indications of egg-size-mediated mortality. The birds in the colony produced an average of 1.45 fledglings per pair. Compared with several other studied colonies, the difference in egg size within a clutch was low, and a comparison of colonies from northwestern Europe suggests that variance within the clutch is negatively correlated with fledging success, so that a large difference in size between first and last eggs is associated with low fledging success. We suggest that the size of the last egg in the clutch reflects the feeding potential in the environment and is mainly a nonadaptive response to poor feeding conditions during laying.