The Effect of Parental Food Supply on Parental Feeding and Squab Growth in Pigeons,Columba livia

The growth and survival of nidicolous birds is determined by variables affecting the frequency and allocation of parental feedings. We manipulated adult pigeons' food supply and measured the effect of food availability and hatching order on the frequency and allocation of parental feeding, squab growth, and parental weight loss. Food availability did not affect any of the dependent variables during the first week, when squabs are fed crop milk. Throughout the remainder of the nestling period, when squabs are fed foraged grain, parents in the food-poor condition delivered fewer regurgitations and lost more weight, and then squabs gained less weight. Within broods, neither the allocation of parental feedings nor squab growth was differential, even in the food-poor environment. Apparently, production of crop milk is buffered from the parents' food supply, thus allowing both squabs to grow rapidly during the first few post-hatch days. The result is nondifferential feeding and growth within broods and delayed effects of food availability on the overall frequency of parental feeding, parental weight loss, and squab growth.     

Forest fragmentation is associated with primary brood sex ratio in the treecreeper (Certhia familiaris)

We studied the primary brood sex ratio of an old-growth forest passerine, the Eurasian treecreeper (Certhia familiaris), along a gradient of forest fragmentation. We found evidence that male nestlings were more costly to produce, since they suffered twofold higher nestling mortality and were larger in body size than females. Furthermore, the proportion of males in the brood was positively associated with the provisioning rate and the amount of food delivered to the nestlings. During the first broods, a high edge density and a high proportion of pine forests around the nests were related to a decreased production of males. The densities of spiders, the main food of the treecreeper, were 38% higher on spruce trunks than on pine trunks. This suggests that pine-dominated territories with female-biased broods may have contained less food during the first broods. The observation was further supported by the fact that the feeding frequencies were lower in territories with high proportions of pines. In the second broods, territories with a high forest patch density produced female-biased broods, whereas high-quality territories with a large amount of deciduous trees and mixed forests produced male-biased broods. Our results suggest that habitat quality as measured by habitat characteristics is associated with sex allocation in free-living birds.     

Effect of food availability on nestling growth and fledging success in manipulated pallid swift broods

Growth rate and fledging success were assessed in natural and manipulated broods of the pallid swift Apus pallidus. Daily measurements of chick mass, wing length, and insect abundance allowed us to examine the short-term variation of chick growth in relation to food availability.
The number of fledged nestlings increased with brood size. Wing length and body mass were slightly but significantly smaller in larger broods, and the nestlings of enlarged broods needed longer to fledge. We discuss how these differences could influence survival after fledging.
Hatching asynchrony caused a significant difference in growth among siblings, and the difference between the oldest and youngest chick was greater in larger broods.
Chick growth was independent of daily food availability. We suggest that this was due to an increased effort of the parents at their expense, when food availability was poor.
The ability of this species to raise an additional chick is in line with most findings on birds, but partially in contrast with results for the common swift in which, at least during poor seasons, the additional nestling caused an increased mortality and lowered the reproductive success.     

Harsh nutritional environment has positive and negative consequences for family living in a burying beetle

Harsh environmental conditions in form of low food availability for both offspring and parents alike can affect breeding behavior and success. There has been evidence that food scarce environments can induce competition between family members, and this might be intensified when parents are caring as a pair and not alone. On the other hand, it is possible that a harsh, food-poor environment could also promote cooperative behaviors within a family, leading, for example, to a higher breeding success of pairs than of single parents. We studied the influence of a harsh nutritional environment on the fitness outcome of family living in the burying beetle Nicrophorus vespilloides. These beetles use vertebrate carcasses for reproduction. We manipulated food availability on two levels: before and during breeding. We then compared the effect of these manipulations in broods with either single females or biparentally breeding males and females. We show that pairs of beetles that experienced a food-poor environment before breeding consumed a higher quantity of the carcass than well-fed pairs or single females. Nevertheless, they were more successful in raising a brood with higher larval survival compared to pairs that did not experience a food shortage before breeding. We also show that food availability during breeding and social condition had independent effects on the mass of the broods raised, with lighter broods in biparental families than in uniparental ones and on smaller carcasses. Our study thus indicates that a harsh nutritional environment can increase both cooperative as well as competitive interactions between family members. Moreover, our results suggest that it can either hamper or drive the formation of a family because parents choose to restrain reproductive investment in a current brood or are encouraged to breed in a food-poor environment, depending on former experiences and their own nutritional status.     

Intersexual niche differentiation in the blue tit (Parus caeruleus)

The intersexual niche differentiation hypothesis, according to which sexual size dimorphism (SSD) has evolved to facilitate niche separation between the sexes, has received equivocal support in studies of birds. One explanation for the inconsistency of the results is that other factors crucial for reproductive success, such as variation in food abundance among territories, may obscure food niche differentiation between parents. We studied the effects of SSD, food availability and food demands of broods on niche differentiation in the blue tit ( Parus caeruleus ). Parents attending experimentally enlarged broods delivered the same amount, but more diverse food items, to their offspring than parents attending reduced broods, although this effect was significant in only one of the two study years. The time-lag (deviation from the peak food abundance in habitat in days) and squared time-lag were the most important determinants of the diversity of food items delivered to young and their condition (fitness surrogate). SSD, as measured by principal component scores of morphological traits, did not explain any variation neither in the diversity or amount of food provided, nor in offspring condition. These results suggest that synchronization of breeding with peak food availability is the most important determinant of diversity of food delivered to young and, more importantly, Their condition. No evidence for intersexual food niche differentiation was found; at best, it is likely to constitute only a weak selection pressure on the maintenance or enhancement of SSD in this system.     

Unpredictable food supply modifies costs of reproduction and hampers individual optimization

Investment into the current reproductive attempt is thought to be at the expense of survival and/or future reproduction. Individuals are therefore expected to adjust their decisions to their physiological state and predictable aspects of environmental quality. The main predictions of the individual optimization hypothesis for bird clutch sizes are: (1) an increase in the number of recruits with an increasing number of eggs in natural broods, with no corresponding impairment of parental survival or future reproduction, and (2) a decrease in the fitness of parents in response to both negative and positive brood size manipulation, as a result of a low number of recruits, poor future reproduction of parents, or both. We analysed environmental influences on costs and optimization of reproduction on 6 years of natural and experimentally manipulated broods in a Central European population of the collared flycatcher. Based on dramatic differences in caterpillar availability, we classified breeding seasons as average and rich food years. The categorization was substantiated by the majority of present and future fitness components of adults and offspring. Neither observational nor experimental data supported the individual optimization hypothesis, in contrast to a Scandinavian population of the species. The quality of fledglings deteriorated, and the number of recruits did not increase with natural clutch size. Manipulation revealed significant costs of reproduction to female parents in terms of future reproductive potential. However, the influence of manipulation on recruitment was linear, with no significant polynomial effect. The number of recruits increased with manipulation in rich food years and tended to decrease in average years, so control broods did not recruit more young than manipulated broods in any of the year types. This indicates that females did not optimize their clutch size, and that they generally laid fewer eggs than optimal in rich food years. Mean yearly clutch size did not follow food availability, which suggests that females cannot predict food supply of the brood-rearing period at the beginning of the season. This lack of information on future food conditions seems to prevent them from accurately estimating their optimal clutch size for each season. Our results suggest that individual optimization may not be a general pattern even within a species, and alternative mechanisms are needed to explain clutch size variation.     

Effects of experimental food supplementation on movements of juvenile northern goshawks (<Emphasis Type="Italic">Accipiter gentilis atricapillus</Emphasis>)

Food availability is thought to strongly influence the leaving phase of natal dispersal in animals with condition-dependent dispersal. We conducted a food supplementation experiment to determine the influence of extra food on the onset of dispersal and early dispersal movement patterns of 42 radio-tagged northern goshawks (Accipiter gentilis atricapillus) from 28 broods in north-central New Mexico during 1992 and 1993. We randomly assigned half of the broods each year as treatments and the other half as controls. Treatment broods were given supplemental food from hatching (late April) until mid-October. Control broods received the same visitation rate but no food. Birds were located approximately every 2 days from fledging until mid-October in 1992 and 1993. Timing of fledging and independence were not affected by the treatment. However, extra food significantly influenced post-fledging movements of juvenile goshawks. During the late fledgling-dependency period (>65 days of age until independence) control birds were located in the natal area (<2 km from nest tree) more frequently than supplemented birds. This pattern reversed after independence (approximately 82 days of age) when supplemented birds were located more frequently in the natal area than controls. After independence the control birds were never located in the natal area and by the end of September in both years the controls had all left the study area (study area boundaries were approximately 25 km from nest tree). Supplemented birds were never located outside of the study area for the duration of the experiment. We conclude that the control birds dispersed out of the study area and the supplemented birds remained. Since the experimentally fed juveniles remained near a known food source and the controls did not, our study demonstrates that food availability influences at least the first 4 months of post-fledging movement patterns in this population. These results also suggest individuals base dispersal decisions on knowledge of their environment at a local scale, which can influence juvenile recruitment.     

The effects of leaf deprivation on leaf-cutting ants and their mutualistic fungus

1. When leaf-cutting ants were deprived of leaves for 5 days, they increased their consumption rates of 'staphylae', the nutritive bodies produced by their mutualistic fungus. They also pruned their fungus garden more intensively, presumably stimulating increased staphyla production.
2. Depriving fungus gardens of leaves for 5 days led to greater rates of staphyla production compared with control gardens. The rate of substrate exhaustion also increased, possibly because of this increased staphyla production, which led to greater removal of fungal resources by the ants.
3. The availability of leaf substrate directly affected staphyla production by the fungus garden. When leaf fragments were inserted into fungus garden samples, staphyla production was depressed, suggesting that the fungus can allocate resources either to hyphal colonization of fresh substrate or to staphyla production.
4. Under conditions of leaf deprivation, food production for the ants effectively increased, due to the combined effects of increased pruning by the ants and the response of the fungus to lack of substrate. This will allow nests to survive for long periods in the field when forage availability is reduced.
5. Exposing parts of the colony to the stress of leaf deprivation showed that there is a high level of flexibility in the fungus garden's ability to produce staphylae. This may be important during the cyclic production of broods, when demands for larval food will fluctuate widely. Under normal conditions, the optimal strategy may be for the fungus garden to have a moderate rate of production with a low rate of turnover, unless demand for staphylae increases dramatically.     

The Impact of Increased Food Availability on Reproduction in a Long-Distance Migratory Songbird: Implications for Environmental Change?

Many populations of migratory songbirds are declining or shifting in distribution. This is likely due to environmental changes that alter factors such as food availability that may have an impact on survival and/or breeding success. We tested the impact of experimentally supplemented food on the breeding success over three years of northern wheatears (Oenanthe oenanthe), a species in decline over much of Europe. The number of offspring fledged over the season was higher for food-supplemented birds than for control birds. The mechanisms for this effect were that food supplementation advanced breeding date, which, together with increased resources, allowed further breeding attempts. While food supplementation did not increase the clutch size, hatching success or number of chicks fledged per breeding attempt, it did increase chick size in one year of the study. The increased breeding success was greater for males than females; males could attempt to rear simultaneous broods with multiple females as well as attempting second broods, whereas females could only increase their breeding effort via second broods. Multiple brooding is rare in the study population, but this study demonstrates the potential for changes in food availability to affect wheatear breeding productivity, primarily via phenotypic flexibility in the number of breeding attempts. Our results have implications for our understanding of how wheatears may respond to natural changes in food availability due to climate changes or changes in habitat management.     

Seasonal changes in cell‐mediated immunocompetence and mass gain in nestling barn swallows: a parasite‐mediated effect?

Reproduction in seasonal environments is usually timed so peak demand for food by offspring coincides with peak availability. Hence, late breeders will encounter a scarcity of food. Since parasite populations grow during the reproductive season of their hosts, late reproducing animals will also face an increasing challenge by parasites. We hypothesised that seasonal decrease in food availability and seasonal increase in parasite abundance will cause a trade-off between growth and immune function. This prediction was tested in nestling barn swallows ( Hirundo rustica ) from first and second broods. Nestlings from second broods mounted stronger T cell mediated immune responses to a challenge with a novel antigen, but had lower rates of mass gain, than nestlings from first broods, consistent with the prediction. Broods in which at least one nestling died had lower levels of T cell mediated immune response, but not lower rates of mass gain, than broods without mortality, suggesting that brood reduction is mediated through an inability of offspring to defend themselves against parasites rather than an inability to grow. Possible mechanisms include scarcity of specific nutrients needed for immune responses, and/or parasites being concentrated on a single or few nestlings.     

Effects of short-term hunger and competitive asymmetry on facultative aggression in nestling black guillemots Cepphus grylle

Siblings in a diversity of species are facultatively aggressive,yet the proximate control of the aggressive response and theecological conditions selecting for such systems are poorlyunderstood. In this study, we investigated the effects of foodamount (food amount hypothesis) and competitive asymmetry onsibling aggression in black guillemot broods. Parental provisioningrates were experimentally manipulated in broods comprisinga range of hatching intervals over a 12-h period. Aggressionbecame evident only after parental provisioning rates wereexperimentally reduced. When parental provisioning resumed,adults did not increase their feeding rate to compensate forthe induced food deficit, and the result of sibling rivalry was a change in the allocation of parental deliveries from oneof equality to one in favor of the dominant chick. Food-deprivedchicks from synchronous broods were more aggressive than thosefrom asynchronous broods, suggesting that one benefit of hatchingasynchrony in the black guillemot is to establish an efficientcompetitive hierarchy among siblings which minimizes the needfor costly aggressive interactions. On the following day, siblingaggression ceased, and chicks regained an equal share of parentalfeeds. Our results provide the first evidence that short-termfood shortage per se acts as an initial trigger for aggressionand also show that the aggressive response is complicated byfactors associated with hatching and laying order.     

Unusual sex roles in a highly promiscuous parrot: the Greater Vasa Parrot Caracopsis vasa

We describe the unusual mating system of the Greater Vasa Parrot Caracopsis vasa . The dull black plumage of males and females is similar but females are significantly larger than males. Females are promiscuous and copulated with at least five different males. Copulations were either short (1–3 s) or very long (mean 35.9 min), and long copulations involved a copulatory tie facilitated by the male's enlarged cloacal protrusion. Multilocus DNA fingerprinting of 17 broods showed that all were of mixed paternity, and that some broods had three fathers. Males never visited nests directly, but during the incubation and chick-rearing periods females came off the nest and were fed regurgitated fruit by multiple males. Four had band-sharing coefficients that suggested they were unrelated. Males copulated with and provided food for several widely separated females simultaneously. During the chick-rearing period females defended a territory around the nest from conspecific females, developed conspicuous orange skin on the head (through feather loss), and uttered loud, complex vocalizations that we refer to as 'song' from prominent perches near the nest. Males showed none of these traits. Females with high song rates attracted more males and as a result received more food than other females. Play-back experiments in which female song rates were either increased or decreased, attracted more or fewer males respectively. We propose that female song, conspicuous head colour and territoriality have all evolved as a result of competition between females for the food provided by males. The selective pressures favouring this highly unusual breeding system in the Greater Vasa Parrot are unclear but some sort of ecological constraint, such as food availability, may be important.     

Experimental food supplementation reveals habitat-dependent male reproductive investment in a migratory bird

Environmental factors can shape reproductive investment strategies and influence the variance in male mating success. Environmental effects on extrapair paternity have traditionally been ascribed to aspects of the social environment, such as breeding density and synchrony. However, social factors are often confounded with habitat quality and are challenging to disentangle. We used both natural variation in habitat quality and a food supplementation experiment to separate the effects of food availability—one key aspect of habitat quality—on extrapair paternity (EPP) and reproductive success in the black-throated blue warbler, Setophaga caerulescens. High natural food availability was associated with higher within-pair paternity (WPP) and fledging two broods late in the breeding season, but lower EPP. Food-supplemented males had higher WPP leading to higher reproductive success relative to controls, and when in low-quality habitat, food-supplemented males were more likely to fledge two broods but less likely to gain EPP. Our results demonstrate that food availability affects trade-offs in reproductive activities. When food constraints are reduced, males invest in WPP at the expense of EPP. These findings imply that environmental change could alter how individuals allocate their resources and affect the selective environment that drives variation in male mating success.     

BREEDING BIOLOGY OF HOUSE MARTINS DELICHON URBICA IN RELATION TO AERIAL INSECT ABUNDANCE

The breeding biology of House Martins nesting in artificial boxes was examined in relation to aerial insect abundance and weather. The insect food supply was continuously monitored with a suction-trap at 40 ft above ground level. Aerial insect abundance started to rise from the winter level in April and reached a high stable state in June. Insects remained abundant until September; the subsequent decline continued at least until the end of October. The size of first clutches (mean size 3.87) was correlated with the abundance of aphids in spring, though not with temperature. It is suggested that this was an adaptive response because high food levels in May were associated with high levels in the main nesting period. Clutch-size declined through the season but was not matched by changes in food abundance. Second clutches (mean size 2–95) were most frequent following a high food level in June. No correlation was found between egg-weight and preceding food conditions, nor with season, clutch-size or order of laying within clutches. If food was scarce on the day of laying of the first egg, these clutches suffered a suspension of laying of subsequent eggs. Some of the young in these nests showed a reduced growth rate. The duration of incubation (mean 14.6 ± 1.1 days) was not apparently correlated with food abundance, however it must have been potentially extended by extreme shortages because incubation duties were neglected at such times. Infertility was probably the main cause of hatching failure (14.2%). A model was used to predict the date of onset of laying. It was based on the assumption that nestling tissue was produced with the same energetic efficiency as eggs. The model indicated that House Martins could collect enough food to lay eggs earlier than the observed date in most years. The high probability of food shortage before the laying period, however, appeared to discourage laying at this time. The observed onset of laying coincided with the appearance of aphids in the air, probably because they comprised an abundant and stable food source for egg formation. Breeding generally occurred in the period of highest food abundance. Pairs rearing only a single brood each year did so in July when food levels were highest. The growth of first brood nestlings was more closely associated with food levels than any other factor investigated, while in second broods, rainfall was found to have the greatest influence. The total variance in growth explained by environmental factors in the first broods (41%) was greater than in second broods (22%). This relative independence of second broods from adverse environments probably arose from more abundant food and the feeding of some of these broods by the young of first broods. An exceptionally late brood showed an extended nestling period indicating deteriorating conditions later in the breeding season. It is proposed that the spread of laying may reflect differences in the feeding efficiency of nesting House Martins, because the growth of two early first broods was more rapid at given food levels than two late broods. Laying patterns conformed to a distribution based on a progressive threshold mechanism. The more efficient pairs may be able to attain breeding condition and lay at low levels of food abundance, and hence breed earlier in the season. Subsequent layings are facilitated by the rise in food levels in early summer. Mortality within the nest was low (5.8%) and associated with food shortage. Nestling periods (mean 30.6 ± 2.3 days) were not shown to be correlated with food abundance; they were more dependent on brood-size. Recruitment into House Martin populations is thus widely influenced by food supply, particularly through an influence on clutch-size, the occurrence of second clutches and nestling mortality.     

Does Climate or Habitat Affect the Frequency of Cooperative Brood Rearing in Canada Geese?

Canada geese ( Branta canadensis ) are unusual because individuals use either of two different brood-rearing behaviors: cooperative broods (two or more merged broods attended by more than three parents) or two-parent families. We tested whether cooperative broods form in response to habitat or climatic conditions by examining variation in cooperative brood frequencies among Canada geese nesting in Connecticut from 1982 to 1996. Percent of goslings raised in cooperative broods ranged from 0 to 100% at a given site in different years, but the pattern of annual variation was different at each site. The sites were in close proximity to each other and had similar climates; thus, the differences in annual variation among sites was not likely to be a response to climatic conditions. Cooperative brood frequencies also varied among sites in each individual year, but sites with the most gang brooding in one year often had the least the next. Such would not be expected if gang brooding occurred in response to non-ephemeral habitat characteristics. Sites where gang brooding occurred and where it did not have similar food resources and predation risks. These findings failed to support the hypotheses that gang broods form in response to food competition or predation. Sites where gang broods occurred had more parent geese and more goslings than sites where they did not occur. Furthermore, the proportion of goslings raised in gang broods was correlated with the number of goslings and parents at the site. Our results support the hypothesis that gang broods form from the inadvertent mixing of goslings. This single factor, however, was not sufficient to account for all of the observed variation in gang brooding frequencies.     

Experimental tail elongation in male Barn Swallows Hirundo rustica reduces provisioning of young, but only in second broods

At the beginning of the breeding season, the outermost tail feathers of 31 male Barn Swallows Hirundo rustica were either shortened by 20 mm, elongated by 20 mm or left unmanipulated. In first broods, the number of feeding bouts (per nestling per hour) by males and females did not differ significantly among experimental groups. However, in second broods, males with elongated tails fed their nestlings less often than males with shortened or unmanipulated tails. Male tail elongation may have been detrimental to flight, making capture of insects more difficult. Females paired to long-tailed males did not compensate for this reduction in feeding by males and their nestlings received less food. Neither feeding rates nor brood size differed significantly between first and second broods. Variation in abundance of large insects at different times of the day (high around noon and low in the morning and afternoon) matched variation in feeding rates, supporting the importance of large insects in the Barn Swallow diet. Different feeding rate patterns in first and second broods by males with elongated tails could have at least three explanations: (1) such males were able to adjust their parental effort to some extent, temporarily compensating for an imposed handicap; (2) they were more sensitive than males in the other experimental groups to late season food shortage (a decrease in numbers of large insects, the main Barn Swallow prey, was observed as the season progressed); and (3) they were more sensitive than males in the other experimental groups to the deterioration in their physical condition after first broods.     

Maternal Condition Affects the Offspring Testosterone Concentration in Early Nestling Period: An Experimental Test on Tawny Owls

This study addressed the question of whether maternal condition during egg laying or the rearing environment has a greater effect on offspring testosterone levels. We tested this in field experiments on a population of tawny owls Strix aluco in Duna‐Ipoly National Park, Hungary. In the experiments with females of poor condition, when broods were supplied with extra food none of the nestlings died, whereas in control broods, which were not supplied with extra food, some nestlings did die. Large differences in testosterone levels were correlated positively with hatching order both in experimental and control broods. However, it was only in control broods that the later‐hatched nestlings with low testosterone concentrations, died. In the experiments with females of good condition, the males were removed and females and their broods were supplied with restricted amounts of food. In these broods starving nestlings, whose growth had stopped, were considered as having died and were removed from the nest and hand‐reared. In control broods all nestlings fledged, and both in experimental and control broods testosterone concentrations were more even between siblings. Both types of trials confirmed a maternal influence on offspring testosterone concentration: large between‐sibling differences in concentrations in the broods of females of poor condition, where some nestlings died, could not be reduced with increased food supply, and the more even concentrations in the broods of females in good condition, where all nestlings survived, could also not be increased by restricting the food supply.     

Parental versus offspring control on food division within the brood: the role of hatching asynchrony

Using an individual-based simulation model we study how different mechanisms of food division among multiple offspring influence nestling number and quality, as well as parental effort. We consider the combination of different scenarios of food availability (feeding conditions), hatching asynchrony and food division. If parents have full control on how to divide food among offspring, asynchronous broods have higher breeding performance than synchronous ones in a wide range of feeding conditions, giving theoretical support to empirically proved benefits of hatching asynchrony. If parents accept the outcome of sibling competition there is a threshold in feeding conditions below which asynchronous broods produced more fledglings and the reverse was true above the threshold. Interestingly, parents relying on the outcome of nestling competition do not necessarily differ in breeding performance from those which have full control over food allocation. Our study combines hatching asynchrony, provisioning behaviour of parents, jostling behaviour of nestlings and feeding conditions as a network of interacting processes of enormous interest to fully understand the parent–offspring conflict.     

Food availability and the male's role in parental care in double-brooded Treecreepers Certhia familiaris

The aim of this work was to examine differences in paternal and maternal care in a double-brooded, monogamous species, the Treecreeper Certhia familiaris, in relation to food availability. As a measure of parental care, we recorded the hourly feeding activity of parents when the nestlings from their first and second breeding attempts were 7 and 12 days old. Feeding frequency of the first brood increased with the age of the nestlings and also with the brood size when 12 days old. While the feeding activities of the females were similar with respect to the first and second broods, the males were less active and failed to provide any food to their nestlings in 15 cases out of 28 second broods. In spite of this, the fledglings from the second broods were heavier than those in the first. Such a pattern of male behaviour was possible without being a disadvantage to the chicks because the food supply increased during the breeding season and the female could provide food for the young alone. Thus paternal care was particularly important in times of poor food supply, i.e. during the first brood, where the extent of these males' activity in feeding the 7-day-old nestlings was positively correlated with the average mass of the nestlings. Our results support the idea that the male of monogamous, altricial bird species often makes important contributions to raising the young, especially during periods when it is difficult for the female to do so alone. Males show flexibility in their pattern of parental care, and male Treecreepers change their contribution to the first and second broods within the same season.     

Brood size and begging intensity in nestling birds

Theoretical models suggest that sibling competition should selectfor conspicuous begging signals. If so, begging intensity mightbe expected to increase with the number of competitiors. Thepurpose of our study was to examine the relationship betweenbegging intensity and brood size using nestling tree swallows(Tachycineta bicolor) as our model. Over 2 years, we videotapedbegging behavior in unmanipulated broods of different sizes.We found that begging intensity increased with brood size. Theaverage weight of nestlings in each brood did not vary withbrood size, but feeding rate per nestling decreased with broodsize, suggesting that nestlings in larger broods begged moreintensively, possibly because they were hungrier. We also conductedan experiment to examine the effect of nest mates on beggingin different-sized broods. We found that nestlings with similarweights, previous competitive environments, and food deprivationbegged more intensively in large broods than in small broods.Overall, our study indicates that begging intensity increaseswith brood size in tree swallows. This relationship may resultfrom interactions among brood mates rather than from lower feeding rates to individual nestlings in larger broods.