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.     

Begging, food provisioning, and nestling competition in great tit broods infested with ectoparasites

Ectoparasites are a ubiquitous environmental component of breedingbirds, and it has repeatedly been shown that hematoph-agousectoparasites such as fleas and mites reduce the quality andnumber of offspring of bird hosts, thereby lowering the valueof a current brood. Selection acting on the hosts will favorphysiological and behavioral responses that will reduce theparasites' impact. However, the results of the few bird studiesthat addressed the question of whether parasitism leads to ahigher rate of food provisioning are equivocal, and the beggingresponse to infestation has rarely been quantified. A changein begging activity and parental rate of food provisioning couldbe predicted in either direction: parents could reduce theirinvestment in the brood in order to invest more in future broods,or they could increase their investment in order to compensatefor the parasites' effect on the current brood. Since the nestlingsare weakened by the ectoparasites they may beg less, but onthe other hand they may beg more in order to obtain more food.In this study we show experimentally that (1) hen fleas (Ceratophyllusgallinae) reduce the body mass and size of great tit (Parusmajor) nestlings, (2) nestlings of parasitized broods more thandouble their begging rate, (3) the male parents increase thefrequency of feeding trips by over 50%, (4) the females do notadjust feeding rate to the lowered nutritional state of nestlings,and (5) food competition among siblings of parasitized broodsis increased. Ultimately the difference in the parental feedingresponse may be understood as the result of a sex-related differencein the trade-off of i0vesting in current versus future broods.     

Nest mate size, but not short-term need, influences begging behavior of a generalist brood parasite

Hosts of generalist brood parasites often vary with regardsto their life-history traits, and these differences have thepotential to influence the competitive environment experiencedby brood-parasitic nestlings. Although begging by brood parasitesis more exaggerated than their hosts, it is unclear if generalistbrood parasites modulate their begging behavior relative tohost size. I examined the begging behavior of brown-headed cowbird(Molothrus ater) nestlings when competing against nest matesthat differ in size and under different levels of short-termneed. Cowbird nestlings begged on nearly all feeding visits,responded to adults as fast as (or faster than) their nest mates,and typically begged more intensively than their nest mates.Latency to beg, time spent begging, and maximum begging postureof cowbirds were similar during supplementation and deprivationtreatments, indicating begging intensity was not influencedby short-term need. Time spent begging by cowbirds varied amonghosts of 3 different sizes when short-term need was standardized,suggesting that nest mate size strongly influenced begging behavior.Cowbirds obtained more food when competing against an intermediate-sizedhost due to lower provisioning rates of small hosts or becauseof increased competitive ability of large host nestlings. Overall,cowbirds obtained the greatest volume of food per unit timespent begging when competing against intermediate hosts, butthis value approached that of the small host when adjusted formodal brood size. These results demonstrate that cowbirds adjusttheir begging relative to the size of the hosts against whichthey compete but not to levels of short-term need.     

Features of begging calls reveal general condition and need of food of barn swallow (Hirundo rustica) nestlings

Altricial offspring of birds solicit food provisioning by complexbegging displays, implying acoustic and visual signals. Differentcomponents of begging behavior may function as reliable signalsof offspring state and thus reproductive value, on which parentsbase optimal parental decisions about allocation of criticalresources (e.g., food). We experimentally manipulated componentsof general condition of nestling barn swallows (Hirundo rustica)by (1) altering brood size by cross-fostering an unbalanced number of nestlings between pairs of synchronous broods andthus manipulating the level of within-brood competition forfood, (2) injecting some nestlings with a harmless immunogen,simulating an infection, and (3) preventing part of the nestlingsfrom receiving food for a short period while establishing controlgroups. We recorded rate of begging response by individual nestlings as parents visited the nest and recorded begging calls usinga DAT recorder to analyze six sonagraphic features of vocalizations.Our factorial experiment revealed that nestlings deprived offood begged more frequently when parents visited the nest comparedto their non—food-deprived nest mates. Food deprivationincreased duration of syllables forming begging calls, whereas brood size enlargement resulted in increased latency of responseto parental calls. Heavy nestlings in good body condition vocalizedat a relatively low peak frequency. To our knowledge, thisis the first study in which begging rate and sonagraphic structureof begging calls are shown to reliably reveal a diverse setof components of offspring general state, on which parental decisions may be based.     

Begging in Common Redstart Nestlings: Scramble Competition or Signalling of Need?

Begging behaviour by the young affects parental food distribution among nestlings of altricial birds. We present an analysis of two types of begging behaviour (assuming the front nest positions and gaping) based on videotaped natural nestling feeding in European common redstart (Phoenicurus phoenicurus). We test whether these types of begging support the predictions of two mathematical models: scramble competition with competitive asymmetries between nestlings [Anim. Behav. 27 (1979) 1210] or honest signalling model [Nature 352 (1991) 328]. None of the measured variables of nestling or parental behaviour were affected by body weight differences between siblings. In contrast, both gaping and nest positioning were affected by individual differences in nestling hunger. In agreement with the honest signalling model, hungrier nestlings gaped with higher probability and started to gape sooner after the arrival of the parent than did their less hungry nestmates. Those nestlings with the shortest latency to gape also received food more often. Nest positioning was related to nestling hunger in a way unforeseen by the existing models. The intervals between nestling position changes were several times longer than the intervals between parental feeding visits, and parents preferred to feed nestlings in front positions, so nestlings in front positions were always less hungry than nestlings in back. Hence the pattern of movements influenced the feeding decision in favour of the more satiated nestlings and acted against the effect of gaping. Nestling movement seemed to be caused by the less hungry nestlings moving actively from front to rear positions. Low mortality of individual nestlings within broods that survived to fledging and small within‐brood variation in fledging weights indicated low competition among nestmates. We suggest that there are two behavioural mechanisms that contribute to the equalization of fledging weights in common redstart nestlings: the signalling of need through gaping and the regular turnover of nestlings at front positions.     

Hatching asynchrony, nestling competition, and the cost of interspecific brood parasitism

All parental hosts of heterospecific brood parasites must paythe cost of rearing non-kin. Previous research on nest parasitismby brown-headed cowbirds (Molothrus ater) concluded that competitivesuperiority of the typically more intensively begging and largercowbird chick leads to preferential feeding by foster parentsand causes a reduction in the hosts' own brood. The larger sizeof cowbird nestlings can be the result of at least two causes:(1) cowbirds preferentially parasitize species with smallernestlings and lower growth rates; and/or (2) cowbirds hatchearlier than hosts. I estimated the cost of cowbird parasitismfor each of 29 species by calculating the difference betweenhosts' published brood sizes in nonparasitized and parasitizednests and using clutch size to standardize values. In this analysis,greater incubation length and lower adult mass, surrogate measuresof the hatching asynchrony and size difference between parasiteand hosts, were both related to greater costs of cowbird parasitismwithout bias owing to phylogeny. To establish causality, I manipulatedclutch contents of eastern phoebes (Sayornis phoebe) and examinedwhether earlier hatching by a single cowbird or phoebe egg reducesthe size of the rest of the original host brood. As predicted,greater hatching asynchrony increased the proportion of theoriginal phoebe brood that was lost. This measure of the costof parasitism was partially owing to increased hatching failureof the original eggs in asynchronous broods but was not at allrelated to the size differences of older and younger conspecificnestmates. However, proportional brood loss owing to an earlierhatching conspecific was consistently smaller than brood lossowing to asynchronous cowbirds in both naturally and experimentallyparasitized phoebe nests. These results imply that althoughhatching asynchrony is an important cause of the reduction ofhost broods in parasitized clutches, competitive features ofcowbird nestlings remain necessary to explain the full extentof hosts' reproductive costs caused by interspecific brood parasitism.     

Sex differences in provisioning rules and honest signalling of need in Manx shearwaters, Puffinus puffinus

Sex differences in provisioning rules have been found in a number of bird species, but the reasons remain unclear. Studies of begging in species with single-chick broods exclude the influence of nestling competition and may provide especially useful models for the study of signalling during parent-offspring conflict. We tested whether sex differences in provisioning rules occur in a species with an obligate brood size of one, the Manx shearwater. We found that chicks conveyed information about their body condition through begging, but male and female parents responded differently to that information. Females varied meal sizes according to the begging intensity of the chick and adjusted subsequent trip duration according to the chick's body condition after feeding, but males did not alter meal size or adjust trip duration. We discuss these findings in the context of recorded differences in the contributions to food provisioning by male and female parents, and we discuss why females may respond more sensitively than males to changes in chicks' nutritional requirements.     

Barn swallow (<Emphasis Type="Italic">Hirundo rustica</Emphasis>) parents use past information to decide on food provisioning to the brood,but not to decide on allocation within the brood

The begging behavior of birds is thought to influence the allocation of food within the brood (allocation function) or to increase the total amount of food delivered to the brood (delivery function). Considering the nature of the feeding activity of passerines, in which hundreds of feeding events occur in a single day, parents may not necessarily decide the amount of resources allocated to each nestling/brood during a single feeding event. To examine this possibility, the relationship between begging intensity and parental responses to allocation and delivery functions was tested and modeled at multi-temporal scales in the barn swallow (Hirundo rustica). Comparison of models revealed that barn swallow parents differed in the temporal scales at which they respond to nestlings with respect to these two functions. While barn swallow parents decide which nestling to feed at a focal feeding event according to chick begging of the focal feeding event, they integrated past information on total begging effort to determine delivery rates to their offspring during the 14 and 6 min prior to the focal feeding event in males and females, respectively. These findings highlight that it is important to investigate parental response to begging at appropriate temporal scales when analyzing begging functions.     

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.     

The adaptive value of parental responsiveness to nestling begging

Despite extensive theoretical and empirical research into offspring food solicitation behaviour as a model for parent-offspring conflict and communication, the adaptive value of parental responsiveness to begging has never been tested experimentally. Game theory models, as well as empirical studies, suggest that begging conveys information on offspring state, which implies that parental investment can be better translated to fitness by responding to begging when allocating resources rather than by ignoring it. However, this assumption and its underlying mechanisms have received little or no attention. Here we show by experiments with hand-raised house sparrow (Passer domesticus) nestlings that a 'responsive parent' will do better than a hypothetical 'non-responsive' mutant (that provides similar food amounts, but irrespective of begging). This is neither because food-deprived nestlings convert food to mass more efficiently, however, nor because responsiveness reduces costly begging. Rather, responsiveness to begging is adaptive because it reduces two opposing risks: one is wasting time when returning too soon to feed already satiated nestlings and the other is repeatedly overlooking some nestlings as a result of the stochastic nature of a random, non-responsive strategy. This study provides the first experimental evidence for the adaptive value of parental responsiveness to offspring begging.     

Fine-tuned modulation of competitive behaviour according to kinship in barn swallow nestlings

Kin selection theory predicts that, in species where progeny members compete for limiting parental care, individual offspring should be more prone to monopolize parental resources as their genetic relatedness to brood competitors decreases. Mixed parentage among broodmates may arise as a consequence, for example, of extra-pair fertilization or brood parasitism events. In this experimental study of barn swallows (Hirundo rustica), we reciprocally partially cross-fostered hatchlings between broods and compared the behaviour of pairs of related and unrelated broodmates in a competitive context, both under normal food provisioning regime and after mild food deprivation. We found that scramble competition for food mediated by visual and vocal solicitation displays (begging) is inversely related to relatedness among competitors, independent of their level of satiation. Nestlings may modulate their competitive behaviour according to vocal cues that vary with their origin and allow kin recognition. We also uncover direct fitness costs to both parents and offspring arising from mixed parentage in a brood, in terms of increased parental workload and reduced survival of the nestlings. Such previously neglected costs may select for reduced frequency of extra-pair fertilizations and brood parasitism in species with extensive parental care.     

Hatching Asynchrony Decreases the Magnitude of Parental Care in Domesticated Zebra Finches: Empirical Support for the Peak Load Reduction Hypothesis

Parent–offspring conflict over the supply of parental care results in offspring attempting to exert control using begging behaviours and parents attempting to exert control by manipulating brood sizes and hatching patterns. The peak load reduction hypothesis proposes that parents can exert control via hatching asynchrony, as the level of competition amongst siblings is determined by their age differences and not by their growth rates. Theoretically, this benefits the parents by reducing both the peak load of the offspring's demand and their overall demand for food and benefits the offspring by reducing the amplification of their competition. However, the peak load reduction hypothesis has only received mixed support. Here, we describe an experiment where we manipulated the hatching patterns of domesticated zebra finch Taeniopygia guttata broods and quantified patterns of nestling begging and parental feeding effort. There was no difference in the begging intensity of nestlings raised in asynchronous or experimentally synchronous broods, yet parental feeding effort was lower when provisioning asynchronous broods and particularly so when levels of nestling begging were low. Further, both parents acted in unison, as there was no evidence of parentally biased favouritism in relation to hatching pattern. Therefore, our study provided empirical support for the prediction that hatching asynchrony reduces the feeding effort of parents, thereby providing empirical support for the peak load reduction hypothesis.     

Differential Begging and Locomotory Behaviour by Early and Late Hatched Nestlings Affecting the Distribution of Food in Asynchronously Hatched Broods of Altricial Birds

Distribution of food to early and late hatched nestlings was studied in asynchronously hatched broods of the great tit Parus major, the blackbird Turdus merula, and the fieldfare T. pilaris. Food distribution is related to the locomotory and begging behaviour and positions in the nest of these nestlings. Late hatched (small) nestlings were found to beg more often per feed than bigger nestlings and move more towards favoured positions in the nest to counteract selective feeding of bigger young. The functional significance of these differences in the behaviour of early and late hatched nestlings are discussed. It is argued that they are adaptive by 1) ensuring that each nestling survives when food supplies are ample, and 2) by mediating an optimal brood reduction when food is insufficient to raise the entire brood. The roles of asynchronous hatching, and selective feeding which follows from differential behaviour of early and late hatched young are discussed in relation to food conditions during the breeding season.     

Signalling of Nutritional Need by Magpie Nestlings

The relationship between begging behaviour, chick nutritional state, and parental distribution of food within broods was studied in 4- and 5-chick magpie Pica pica broods under natural conditions. Three components of the begging display (duration, latency, and posture) were highly correlated with each other and also with the emission and duration of begging calls. Begging performance was strongly influenced by the food intake of nestlings during the preceding 1-h interval, indicating that begging may reliably reflect the nutritional need of nestlings. Daily growth during the preceding day, as well as average cumulative food intake by the brood during the preceding 24 h, seemed not to affect begging in a similar way. Begging signals employed by hungrier nestlings involved a higher degree of muscular activity, thus supporting the prediction that nestlings in greater need should employ more costly signals. Overall, those nestlings who begged more tended to obtain more food, but the relationship between feeding success and begging behaviour was weak due to a high variation between broods in the way that parents seemed to respond to variations in begging behaviour. Possible causes for this variation, and its implications for the evolution of reliable begging displays, are discussed.     

Reliable Information Content and Ontogenetic Shift in Begging Calls of Grey Warbler Nestlings

The most critical assumption of communication models regarding parent–offspring conflict is that food solicitation displays of genetic offspring are honest signals to elicit beneficial parental care. A critical requirement of honesty is the reliable change of perceivable aspects of begging calls with physiological needs. We experimentally tested whether and how the acoustic structure and begging call rate of individual Grey Warbler Gerygone igata nestlings change with hunger level and age. We also examined a rarely documented component of chick begging calls, namely the temporal dynamics of acoustic modulation after nestlings heard parental feeding calls. Begging call structure narrowed in frequency range and, surprisingly, decreased in amplitude as chick hunger levels increased. We also found that begging calls changed with chick age, with the frequency increasing and the duration decreasing for older chicks. These results indicate that the acoustic properties of nestling Grey Warbler begging calls are complex and may be used to signal several aspects of nestling traits, including hunger level and age (or size, a correlate of age). Overall, begging calls of Grey Warbler chicks appear to be honest, implying that parents are likely to benefit from relying on the acoustic features of their progeny’s calls which predict chick need. Our results have important implications regarding the reliability and information content of nestling solicitation signals for the brood parasite shining cuckoo Chrysococcyx lucidus exploiting Grey Warbler parental care, in that these begging‐call mimetic specialist cuckoos might also need to match closely the dynamics of acoustic features of their host chicks’ calls.     

Why brown-headed cowbirds do not influence red-winged blackbird parent behaviour

Brown-headed cowbirds, Molothrus ater, frequently parasitize red-winged blackbirds,Agelaius phoeniceus . The presence of a brood parasite, unrelated to both host nestlings and parents, has provoked speculation regarding within-brood food allocation and parental provisioning. This study is the first to compare directly the effect of brood parasitism on host parent and offspring behaviour in younger and older broods. We videotaped 28 unparasitized red-winged blackbird broods and compared them to 22 parasitized broods. Red-winged blackbird nestling begging appears largely unaffected by cowbird parasitism. The presence of the cowbird in the nest affected neither the latency nor duration of host nestling begging, but stimulated more frequent begging by red-winged blackbird nestlings following food distribution. Begging by cowbirds was unique in two ways: (1) cowbirds maintained a consistent begging effort throughout the nestling period (but did not receive a consistent food share); and (2) cowbirds begged longer and more frequently following the allocation of food. Persistent begging by the cowbird following the allocation of food has implications for the division of parental care, if by doing so the brood parasite is able to provoke the foster parent to increase provisioning, at the expense of brooding. We found no evidence for the adjustment of parental care. Neither the foraging rates nor the lengths of the parental feeding visits differed markedly between parasitized and unparasitized broods. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Coadaptation of offspring begging and parental provisioning: A role for prenatal maternal effects?

Recent studies on birds have shown that offspring begging and parental provisioning covary at the phenotypic level, which is thought to reflect genetic correlations. However, prenatal maternal factors, like yolk testosterone, may also facilitate parent-offspring coadaptation via their effects on offspring begging and development. In fact, maternal effects are thought to adjust offspring phenotype to the environmental conditions they will experience after birth, which are in turn strongly dependent on the levels of parental provisioning. Using cross-fostering experiments in canaries, we tested the role of maternal effects on parent-offspring coadaptation from two different approaches. First, we analyzed whether females deposit yolk testosterone in relation to their own or their partner's prospective parental provisioning, measured as the rate of parental feeding to foster nestlings. Second, we investigated whether females deposit yolk testosterone in relation to costs they incurred when raising a previous brood, as this likely impinges on their capacity to provide parental care in the near future. However, from the results of both experiments we have no evidence that canary females deposit yolk testosterone in order to match offspring begging to the levels of care they and/or their partners provide. We therefore found no evidence that yolk testosterone facilitates parent-offspring coadaptation. In addition, our results suggest that the functional consequences of yolk testosterone deposition may relate to hatching asynchrony since it primarily varied with egg laying order.     

Parent-absent begging: evidence for sibling honesty and cooperation in the spotless starling (Sturnus unicolor)

Begging in avian nestlings is a highly conspicuous behaviorwith important implications for the study of parent–offspringconflict. In some species, nestlings also call for long boutsin the absence of parents, and it has been proposed that thisbehavior is used by nestlings as a means of negotiating accessto food. We studied this phenomenon in the spotless starling(Sturnus unicolor). We found that parent-absent calls were acousticallydistinct from parent-present calls. Observations showed thatthe probability of parent-absent begging increased with nestlingage and brood size, whereas it decreased with increasing bodycondition. This result was confirmed by an experiment that showedthat nestlings produced higher parent-absent begging rates whenfood deprived than when satiated. Finally, we carried out aplayback experiment to test the reaction of nestlings to parent-absentbegging by fellow nestlings. Principle components analyses yielded2 independent components of begging: 1) a general begging componentand 2) a second factor that measures the relative contributionof communicative begging over competitive begging. Nestlingsexposed to playback decreased their general begging levels andsimultaneously increased the relative contribution of communicativeover competitive begging. This behavior may favor needy nestlingsto obtain impending feedings while keeping high levels of foodsolicitation from parents and is consistent with a cooperativestrategy among nestlings. Future research should consider theactual response of parents to these signals.     

Azure‐winged Magpies Cyanopica cyanus trade off reproductive success and parental care by establishing a size hierarchy among nestlings

It is common in birds that the sizes of nestlings vary greatly when multiple young are produced in one nest. However, the methods used by parents to establish size hierarchy among nestlings and their effect on parental provisioning pattern may differ between species. In the Azure‐winged Magpie Cyanopica cyanus, we explored how and why parents controlled the sizes of nestlings. Asynchronous hatching was the main cause of size hierarchy within the brood, although the laying of larger eggs later in the laying sequence reduced this effect. Parents with asynchronous broods produced more eggs and fledged more nestlings than those with synchronous broods but their brood provisioning rates, food delivery per feeding bout and feeding efficiency did not differ. We performed a cross‐fostering experiment to synchronize some asynchronous broods. Provisioning rates of asynchronous broods were lower than those of synchronized broods, but the daily growth rates and fledging body mass of their nestlings were not different. Our findings indicate that parents of asynchronous broods can achieve higher reproductive success than those of synchronous broods based on the same parental care, and the same reproductive success as those of synchronized broods based on less parental care. It appears that parent birds can better trade off reproductive success and parental care by establishing a size hierarchy among nestlings.     

Adaptations of Polygynous Breeding in the Bobolink, Dolichonyx oryzivorus

In some species of polygynous songbirds, males assist in feedingnestlings of their first (primary) female but ignore offspringof subsequent (secondary) mates. A number of adaptations importantin minimizing the disadvantages of a secondary status in femaleBobolinks (Dolichonyx oryzivorus) have been discovered. In Bobolinks, clutches of secondary females are significantlysmaller than those of primary females, thereby adjusting broodsize to correspond better with food delivery capabilities ofsecondary hens. Commencement of incubation with deposition ofthe penultimate egg allows additional brood size adjustment,for the resulting hatching asynchronism promotes efficient cullingof the late-hatching chick in times of food shortage, withoutendangering the entire brood. Secondary females differ fromprimary pairs in food exploitation techniques by capitalizingon insects found close to the nest and by showing less discriminationin food selection. Such behavior permits compensation in therate of food delivery trips to secondary nestlings so as toequal the tempo maintained by both members of the primary pair,yet results in quantitative disparities in food brought to primaryand secondary nestlings. Flexibility in male feeding responses,resulting in partial male assistance at secondary nests in situationswhere secondary brood size is exceptionally large, is also importantin helping to maximize the reproductive performance of birdsengaging in this mating strategy.