Begging in the absence of parents by nestling tree swallows

Begging by nestling passerine birds has become a model systemfor studies in animal communication. Although most beggingoccurs when parents arrive at the nest to feed (here called"primary begging"), it also occurs between feeding visits andimmediately after parents leave the nest. Begging in thesecontexts (here called "secondary begging") may have relativelylittle influence on the probability of receiving food, but could increase the overall cost of the signal and thus influence nestlingbegging strategies. The purpose of our study was to determinehow often tree swallow (Tachycineta bicolor) nestlings begin contexts other than to parents with food and to examinewhat factors influence the frequency of this begging. Secondarybegging ranged from 7% of measured begging responses at day2 to 30% by day 8 and was more frequent when the interval betweenparental feeding visits was relatively long and when the timeto respond to the arrival of parents with food was short. Increasesin both age and intervisit interval were associated with decreasesin nestling response times, suggesting that secondary beggingmay be related to the speed with which nestlings respond to stimuli. We discuss possible functions of secondary beggingand raise the possibility that it may, in fact, be an error.     

Parental allocation of food to nestling tree swallows: the influence of nestling behaviour, sex and paternity

Parents are expected to invest more in young that provide the greatest fitness returns. The cues that parents use to allocate resources between their offspring have received much recent attention. In birds, parents may use begging intensity, position in the nest or nestling size as cues to provision the most competitive young or those most likely to survive. It may also benefit parents to invest in young differentially by sex or relatedness if the fitness returns of sons and daughters differ or broods are sired by multiple males. We examined the allocation of food to tree swallow, Tachycineta bicolor, nestlings in relation to their begging behaviour, size, sex and paternity. Provisioning by parents was not related to nestling size, sex or paternity. The begging behaviour of nestlings did not differ with respect to sex or paternity. Both parents were more likely to feed nestlings that begged first or were closer to the nest entrance, suggesting that parents allocate food resources in response to cues that nestlings control. As a consequence, brood reduction was facilitated by biased provisioning within the brood in addition to the nestling size hierarchies created by hatching asynchrony. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Avoidance,tolerance, and resistance to ectoparasites in nestling and adult tree swallows

We examined avoidance, tolerance, and resistance strategies of nestling and adult tree swallows Tachycineta bicolor in response to ectoparasitic blowflies Protocalliphora sialia. Tree swallows avoided settling in north‐facing nest boxes early in the breeding season. These boxes were more likely to be parasitized later in the season, suggesting that box selection may facilitate blowfly avoidance. After experimentally manipulating blowfly intensity, we found that nestlings were generally tolerant of parasitism. Parasites significantly reduced nestling blood hemoglobin but had no effect on nestling body mass, primary feather growth, age at fledging, or fledging success. Parents of parasitized nestlings did not increase their food provisioning rate to promote nestling tolerance. Adult female tree swallows demonstrated both tolerance and resistance: blowfly parasitism had no effect on adult hemoglobin and body mass, and those with higher P. sialia‐binding antibody levels had fewer blowfly larvae in their nests. Nestling antibodies were unrelated to blowfly intensity. Despite considerable variation among years, our results suggest that the costs of blowfly parasitism to nestling and adult tree swallows are modest, and limited to blood loss in nestlings. Future work should examine the effects of reduced blood hemoglobin on fledgling survival and the importance of parasite‐specific antibodies.     

Dietary calcium limits size and growth of nestling tree swallows Tachycineta bicolor in a non-acidified landscape

Much previous research has focussed on the role of food supply in determining the growth and the survival of avian offspring. More recently, acid deposition in some ecosystems has demonstrated that in addition to energy, birds also need to acquire sufficient nutrients such as calcium to be successful. Whether procurement of adequate levels of calcium can limit reproductive success in areas that have not been impacted by acid rain remains equivocal. We tested whether calcium affected reproductive success of tree swallows Tachycineta bicolor by feeding extra calcium to nestlings during the brood-rearing period. Our manipulation did not enhance the survival of offspring, however, provisioning of extra calcium resulted in nestlings showing enhanced rates of growth of mass (all nests) and of ninth primary flight feathers (nests with after-second year female parents), compared to control nestlings. Calcium supplementation also resulted in nestlings having longer feathers and tarsi at age 16 days, and there was evidence that some nestlings receiving extra calcium were heavier at 16 days old. As offspring that have faster growth, or that are in good condition at fledging, often survive better after leaving the nest, these results suggest that calcium availability can limit fitness. Our results are noteworthy because our experiment was conducted in an area with abundant soil calcium where acid deposition has not occurred. The role of calcium in limiting the reproductive performance of avian species may therefore be more pervasive than previously thought.     

Experimental heating reveals nest temperature affects nestling condition in tree swallows (Tachycineta bicolor)

Investment in one life-history stage can have delayed effects on subsequent life-history stages within a single reproductive bout. We experimentally heated tree swallow (Tachycineta bicolor) nests during incubation to test for effects on parental and nestling conditions. Females incubating in heated boxes maintained higher body condition and fed nestlings at higher rates. We cross-fostered nestlings and found that young nestlings (4-7 days old) incubated in heated nests had higher body condition and body mass, regardless of treatment status of their rearing parent. However, older nestlings which were fed by heated females maintained higher condition and body mass regardless of treatment status of their incubating parent. These results indicate that investment in one life-history stage can have multiple pathways of carry-over effects on future life-history stages.     

Cross‐fostering reveals sources of variation in innate immunity and hematocrit in nestling tree swallows Tachycineta bicolor

Innate immunity plays an important role in protecting birds early in development. The strength of innate immunity in nestlings appears to be affected by heritable and early maternal effects, as well as later environmental effects. To explore factors affecting innate immunity, we conducted a partial cross‐fostering experiment in nestling tree swallows Tachycineta bicolor. The ability of nestling blood plasma to kill Escherichia coli in vitro was influenced by heritable or early maternal effects and was unrelated to body condition of nestlings or brood size of the foster nest. The relative importance of heritable and early maternal effects versus environmental effects on nestling hematocrit (packed volume of red blood cells) and body condition were also examined. Hematocrit was significantly influenced by the nest in which nestlings were reared, though it was not related to nestling body condition. Body condition was significantly influenced by the nest in which nestlings were reared, though it was also weakly influenced by heritable or early maternal effects. In summary, heritable and early maternal effects are stronger predictors than rearing conditions of innate immunity in nestling tree swallows. Furthermore, hematocrit is largely environmentally determined but is not related to body condition or innate immunity, as measured in this study.     

Does experimentally altered plumage ornamentation of female passerines influence nestling quality? A test in female tree swallows

It is widely recognized that female animals in a variety of taxa display ornamental traits, such as elaborate plumage, but elucidating whether such traits evolve by selection or genetic correlation remains a challenge because more ornamented females are often found to produce low‐quality offspring. While resource trade‐offs between the production of ornaments and offspring may underlie this negative relationship, it is not an adequate explanation for species where the timing of production of ornaments and offspring does not overlap. Instead, costs associated with engaging in agonistic interactions with conspecifics, which maintains the honesty of signals of quality, may also reduce resources available to invest in offspring. In this study, we enhanced and reduced the plumage brightness of female tree swallows (Tachycineta bicolor) relative to controls to test whether social costs associated with displaying bright plumage is a potential mechanism underlying the observation that more ornamented females produce low‐quality nestlings. Our results showed that nestlings reared by females in the enhanced plumage brightness treatment were structurally smaller, having shorter lengths of the combined head and bill than nestlings in the reduced plumage brightness treatment, and tended to grow their head and bills more slowly than nestlings in the reduced and control plumage brightness treatments. Nestlings in the enhanced plumage brightness treatment also tended to gain mass more slowly than nestlings in control treatment. Overall, that females with enhanced plumage brightness produced structurally smaller nestlings provides evidence that social costs paid by females may lead to the production of low‐quality offspring and should be considered in future studies.     

Parasites favour intermediate nestling mass and brood size in cliff swallows

A challenge of life‐history theory is to explain why animal body size does not continue to increase, given various advantages of larger size. In birds, body size of nestlings and the number of nestlings produced (brood size) have occasionally been shown to be constrained by higher predation on larger nestlings and those from larger broods. Parasites also are known to have strong effects on life‐history traits in birds, but whether parasitism can be a driver for stabilizing selection on nestling body size or brood size is unknown. We studied patterns of first‐year survival in cliff swallows (Petrochelidon pyrrhonota) in western Nebraska in relation to brood size and nestling body mass in nests under natural conditions and in those in which hematophagous ectoparasites had been removed by fumigation. Birds from parasitized nests showed highest first‐year survival at the most common, intermediate brood‐size and nestling‐mass categories, but cliff swallows from nonparasitized nests had highest survival at the heaviest nestling masses and no relationship with brood size. A survival analysis suggested stabilizing selection on brood size and nestling mass in the presence (but not in the absence) of parasites. Parasites apparently favour intermediate offspring size and number in cliff swallows and produce the observed distributions of these traits, although the mechanisms are unclear. Our results emphasize the importance of parasites in life‐history evolution.     

Genetic and environmental effects on morphology and fluctuating asymmetry in nestling barn swallows

A barn swallow Hirundo rustica partial cross‐fostering experiment with simultaneous brood size manipulation was conducted in two years with contrasting weather conditions, to estimate heritable variation in tarsus, tail and wing size and fluctuating asymmetry. Environmental stress had contrasting effects depending on trait type. Significant heritabilities for tarsus, tail and wing size were found only in enlarged broods irrespective of year effects, while tarsus asymmetry was significantly heritable in the year with benign weather conditions irrespective of brood size manipulation effects. Tail, wing and composite (multicharacter) asymmetry were never significantly heritable. The environment with the higher heritability generally had higher additive genetic variance and lower environmental variance, irrespective of trait type. Heritability was larger for trait size than for trait asymmetry. Patterns of genetic variation in nestlings do not necessarily translate to the juvenile or adult stage, as indicated by lack of correlation between nestling and fledgling traits.     

Repeatability of extra-pair mating in tree swallows

Models of sexual selection assume that female mating preferences are heritable and, thus, repeatable for individual females across multiple mating episodes. Previous studies of the repeatability of female preference have examined individuals in captivity and focused presumably on social mate choice. However, extra-pair mating is widespread and can also influence sexual selection. We examined the repeatability of extra-pair mating in a wild population of tree swallows (Tachycineta bicolor) by experimentally inducing females to lay two clutches in rapid succession within the same season. We found that the proportion of extra-pair young and the number of extra-pair sires were highly repeatable for individual females. However, the repeatability of specific extra-pair sires was low. We suggest that this unusual pattern of mating may be due to females maximizing the heterozygosity of their offspring.     

Survival of extrapair and within-pair young in tree swallows

In monogamous species, it is generally accepted that males seekextrapair matings to increase their reproductive success withoutadditional parental investment; however, the benefits of extrapairmatings to females are much less clear. One possibility isthat females obtain genes for enhanced offspring viabilityfrom the extrapair sires. If this is the case, then the increasedviability of extrapair young may be evident throughout the periodof embryonic development as well as later in life. Tree swallows(Tachycineta bicolor) have one of the highest known levelsof extrapair mating in birds, and females have substantialcontrol over the paternity of their offspring. We used moleculartechniques to determine the parentage of nestlings and unhatchedembryos to examine the possibility that female tree swallowsgain viability benefits for their extrapair offspring. Althoughboth extrapair paternity and mortality of embryos and nestlingswere high (89% and 54% of broods respectively), we found nodifference in the viability of within-pair and extrapair youngprior to fledging. In addition, extrapair young were not morelikely to be male. There was no bias in the sex of young atfledging, but unhatched embryos were more likely to be male.Our results do not support the idea that female tree swallowsengage in extrapair mating to increase offspring viability,at least early in life.     

Sex differences in begging vocalizations of nestling barn swallows, Hirundo rustica

Parents of sexually reproducing species should adjust their investment in production of sons and daughters in relation to the relative costs and reproductive value of offspring of either sex. Sex allocation mediated by differential allocation of care such as food provisioning, however, requires that parents can identify offspring sex. We analysed sex differences in offspring begging calls that may serve as a cue for parents to discriminate between sons and daughters. A combination of three sonagraphic variables of begging calls of nestling barn swallows allowed us to classify them according to sex at day 16, but not at day 12 after hatching, suggesting that sex differences in begging calls arise during the nestling period as the time of fledging approaches. Hence, parents may be able to discriminate between sons and daughters by auditory cues, which would enable differential allocation of food between offspring during the late nestling and early fledging stages. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.      

Intraspecific competition and the maintenance of monogamy in tree swallows

Intraspecific competition for access to breeding resources maylimit male mating success typically monogamous birds. We examinedthe potential for intraspecific competition to limit polygynyin tree swallows at Beaverhill Lake, Alberta, Canada. In thispopulation, polygynous males raisedmore fledglings than monogamousmales, and there was little or no cost to females from nestingpolygynously. Under these conditions one might expect polygynyto be more common than that observed(8% of males). We foundthat females were most aggressive toward conspecific intrudersearly in the breeding season. This aggression was associatedwith (1) females settling farther apart than expected underrandom settlement, (2) later settlement by secondary than bymonogamous females, and (3) no relationship between female settlementdate and male territory size instead of the negative correlationexpected if females settled randomly without competition. Earlyin the season, males also settled farther apart than expectedif they had settled randomly, and among males with two or morenest boxes on their territory, males with widely separated nestboxes were more likely to be polygynous. Monogamy is probablythe most common pairing association in this population becauseintraspecific competition for nest sites prevents most malesfrom gaining a territory with nest sites far enough apart topermit two females to breed without one female excluding theother. Females appeared to be defending an area surroundingtheir nest box to limit nest usurpation or intraspecific broodparasitism, rather than to limit any loss of male parental carefrom polygyny.     

Prey choice and nestling hunger: an experiment with pied flycatchers,Ficedula hypoleuca

Prey choice variation was studied in a multiple-prey loader, the pied flycatcher, Ficedula hypoleuca, by manipulating the body condition of the adults and the hunger level of the nestlings. Parents with high and low body weight brought back similar prey when they were feeding an experimental brood of five hungry nestlings, but parents with high body weight fed the brood at a higher frequency than those with low body weight. Parents with high body weight took more small prey when they were feeding five hungry nestlings than when they were feeding two well-fed nestlings. The latter result is consistent with a recent model of central place foraging, which takes into account the costs of staying away from the nest. Alternatively, the parents may have changed the currency for energy maximization, and hence the criteria for prey choice; e.g. a shift from net to gross energy maximization would imply a less selective prey choice.     

Cost of reproduction, T-lymphocyte mediated immunocompetence and health status in female and nestling barn swallows Hirundo rustica

We investigate the trade-off between reproductive effort, health status and T-lymphocyte acquired immunity in female and nestling barn swallows Hirundo rustica using a brood size manipulation experiment. Maternal and total feeding effort increased with experimental brood size. Parents did not fully compensate for the increased food demand of the enlarged broods and as a consequence the per capita feeding rate of nestlings decreased with increasing experimental brood size. Body mass and a measure of T-cell mediated immunity in 12 days old nestlings also decreased with increasing experimental brood size. Different leucocyte concentrations and the heterophile/lymphocyte ratio – an index of stress – of nestlings did not change in relation to experimental brood size, suggesting that within brood competition did not affect stress to nestlings. The brood size manipulation had a significant effect on maternal T-cell mediated immunity, measured by the phytohemagglutinin skin test, but not on maternal body mass, haematocrit or differential or total white blood cell counts. Our results seem to support the prediction that under mild work stress females respond first by reducing the energetically expensive acquired immunity. Different leucocyte types and the heterophile/lymphocyte ratio appear less sensitive to parental workload.     

Short-term fluctuations in cellular immunity of tree swallows feeding nestlings

We examined cellular immunity of adult tree swallows feeding nestlings under variable weather conditions. Birds received an injection of phytohaemagglutinin (PHA), which causes a local swelling, reflecting the strength of T-cell-mediated immunocompetence. There was a negative relationship between the immune response and the number of nestlings in the brood (range 3-6 young) which suggests that parental effort suppresses the immune function. However, there was also a strong effect of ambient temperature and food abundance (aerial insects) on immune response. Parents that received the PHA injection during cold weather and at low food abundance showed a suppressed immune response compared to birds treated during more favourable conditions. They also lost more body mass during the 24 h inoculation period, and their offspring showed reduced growth. When controlling for ambient temperature and food abundance in a multivariate analysis, there was no longer any significant effect of brood size on the parents' immune response. Three of 39 pairs deserted their broods after PHA injection. All three desertions took place when the mean ambient temperature fell below 13°C. The PHA response is known to have both heritable and environmental components; our study emphasizes its condition-dependency. Previous studies of other passerine birds have shown that high levels of parental effort may have an immunosuppressive effect. Our study indicates that weather conditions may override the effects of natural variation in parental effort, and that the PHA response is particularly influenced by short-term fluctuations in energy balance.     

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.     

The relationship between plumage coloration and aggression in female tree swallows

Intrasexual competition is an important selective force that can favor the evolution of honest signals of fighting ability. Research has focused predominantly on male birds, but many female birds also possess plumage ornaments that could mediate the outcome of competitive interactions. We examined the relationship between blue and white structural coloration and aggression in female tree swallows Tachycineta bicolor. Tree swallows are secondary cavity nesters and females show delayed plumage maturation which may be related to intense competition for nest sites. We compared plumage reflectance of second‐year (SY) and after‐second year (ASY) females, and within‐individual changes in plumage reflectance of ASY females in two successive years. We assessed aggression by placing a caged SY female 2 m from the nest box of an ASY female and quantified the ASY female's response in relation to her own plumage coloration. We found substantial differences in plumage reflectance of SY and ASY females, but found that ASY females became greener with duller white coloration in the second year. This may be due to poor weather that made reproduction particularly costly in the first year of the study and suggests coloration is influenced by condition. We found no relationship between dorsal coloration and aggressiveness. Rather, brighter white females spent more time on their nest box and within 2 m of the intruder than females with dull breasts. Our findings suggest that brighter white ASY females may perceive a SY female as less of a threat or that there may be a trade‐off associated with aggression and parental care that is related to white brightness.