American robin nestlings compete by jockeying for position

Summary We investigated whether nestling American robins (Turdus migratorius) were capable of influencing food distribution in their nests by perceiving that certain sectors of the nest received a relatively high proportion of feedings and positioning themselves accordingly. Feeding observations were obtained from videotape recordings taken at different stages of the nestling period. Parents generally arrived at a predictable location on the nest rim and allocated proportionally more food to nestlings in the central position. The degree of nestling movement was significantly positively correlated with variation in the predictability of parental arrival locations on the nest rim. Furthermore, nestlings moved more in broods suffering brood reduction. This suggests that when competition for food is intense and the location of parental arrival is predictable, nestlings respond by jockeying for access to the most favorable (i.e., central) position in the nest. We conclude that jockeying for position by nestlings can influence the pattern of food allocation by parents, and that hungry nestlings can improve their competitive standing against nestmates by moving to positions where parents are more likely to feed them. Correspondence to: S.B. McRae     

Asymmetric contests at the nest

Hatching asynchrony of nestling birds leads to weight asymmetries, which in turn affect the nestlings’ relative success when competing for feedings brought to the nest. We present a game theoretic model that predicts how weight asymmetry influences the nestlings’ energy on securing feedings, thus determining the caloric value remaining for weight gain as well as the distribution of feedings obtained. The model has a unique Evolutionary Stable Strategy (ESS) profile, in which nestlings in more asymmetric nests exhibit less aggression and hence achieve larger weight gain per feeding. The impetus for this model was data from a long-term study of Arabian babblers (Turdoides squamiceps) that showed a surprising negative correlation between the number of feedings that a nest received and the overall weight gain in the nest. This finding is, however, entirely consistent with our model—in more symmetric nests, the individuals fight more and are consequently hungrier and beg for additional food, are fed more, but still gain less weight due to the higher energetic costs of fighting. The model provides a fundamental explanation also for related findings in other species, in which chicks in asynchronous broods were found to be heavier than those from synchronous broods. In addition, it supports the sibling rivalry hypothesis by which brood asynchrony may diminish aggressive interactions among nestlings, leading to more efficient use of resources.     

Do males and females differ in the feeding of large and small siblings? An experiment with the bluethroat

Males and females have been reported to differ in their feeding of large and small siblings in several species of birds. According to recent hypotheses, this phenomenon may be related to a sexual conflict over avian hatching patterns. We designed an experiment to test for the existence of such a sex difference by manipulating nestling size hierarchies of the bluethroat (Luscinia s. svecica) in two directions; half the broods were “asynchronized” to yield large size-differences within broods and the other half were “synchronized” to yield small size-differences. In all broods, nestlings were categorized as being either large or small according to body mass. We recorded male and female food distribution by video early (day 4 after hatching) and late (day 8) in the nestling period. Males and females did not differ in their distribution of food among different-sized nestlings. With large size-differences, both males and females fed large nestlings nearly twice as often as small ones. In contrast, when the size-differences were small, food was more evenly distributed among nestlings. Early in the nestling period, males fed more nestlings during each feeding visit than did females. Our finding that male and female bluethroats do not differ in the feeding of large and small siblings is in contrast to most previous studies. Variation in costs and benefits to males and females from feeding different-sized nestlings, and restrictions to parental choice due to nestling interactions, may explain interspecific variation. Received: 27 June 1997 / Accepted after revision: 26 January 1998     

Begging calls and parental feeding decisions in tree swallows (Tachycineta bicolor)

We conducted playback experiments to examine how parent tree swallows (Tachycineta bicolor) use nestling begging calls to distribute feedings to individuals within broods. In a first study, we used a paired-choice test to determine if parents discriminated between the taped begging calls of nestlings deprived of food and those of nestlings that had been recently fed. Our results showed that parents directed their first feeding attempt towards model nestlings near speakers playing deprived calls significantly more often than to models near speakers playing fed calls. They also made more feeding attempts overall to models with deprived calls. In the second study, we varied call rate and amplitude to examine which call features parents might use to discriminate begging calls. Parents directed significantly more first feeding attempts and more feeding attempts overall towards non-begging nestlings near speakers playing high call rates than to nestlings near speakers playing low call rates. They did not, however, discriminate between calls differing in amplitude. Previous studies have shown that parent birds use begging calls to regulate overall feeding rates to the brood. Our results suggest that parent tree swallows also use begging calls when feeding individual nestlings and, in particular, prefer calls associated with increased levels of nestling hunger. Received: 14 February 2000 / Revised: 6 October 2000 / Accepted: 16 October 2000     

Begging signals more than just short-term need: cryptic effects of brood size in the pied flycatcher (Ficedula hypoleuca)

The begging of nestling birds is known to reliably signal short-term nutritional need, which is used by parents to adjust rates of food delivery and patterns of food distribution within broods. To test whether begging signals reflect more than just short-term feeding history, we experimentally created 18 "small" (4-nestling) and 18 "large" (8-nestling) broods in the pied flycatcher (Ficedula hypoleuca). Compared to small broods, large broods were provisioned by parents at a greater rate, but at a lower visit rate per nestling and with no obvious differences in load mass per visit. However, lower rates of food mass delivery per nestling in large broods did not result in any measurable reduction in nestling growth (i.e. "long-term need") or in any increase in the begging effort per individual nestling whilst in the nest. Mid-way through the nestling period we also used hand-feeding laboratory trials to assess in more detail individual begging behaviour and digestive performance of the three mid-ranking nestlings from each brood. More food items were required at the start of each trial to satiate nestlings from large broods, but despite this initial control for "short-term need", nestlings from large broods went on to beg at consistently higher rates and at different acoustic frequencies. Large brood nestlings also produced smaller faecal sacs, which were quantitatively different in content but did not differ in frequency. We suggest that different nutritional histories can produce cryptic changes in nestling digestive function, and that these can lead to important differences in begging signals despite controlling for short term need.     

An experimental study of competition for food between male and female nestlings of the red-winged blackbird

Summary Ten experimental broods of red-winged black-bird (Agelaius phoeniceus) nestlings, experimentally manipulated to contain two males and two females of similar age, were observed with the aid of video cameras to determine (a) which nestling characteristics were most important in influencing its chances of being fed and (b) if males and females differed with respect to these characteristics. Nestlings that could reach highest while begging were most successful at obtaining food from parents during individual feeding bouts. However, while there was a tendency for nestlings that begged earlier to be offered food first, a nestling's position in the nest during these bouts did not influence its feeding success. Males begged more than often females and were offered more food by parents. Males were also more likely to be fed when they begged. This was most likely because males, being larger than females, were able to reach higher while begging. Neither sex was more likely to beg sooner or occupy certain positions in the nest. However, while males that had been transferred from other nests received less food than natal males, there were no differences in food acquisition between transferred and natal females. Differences in the relative success of males and females under different conditions may help explain differences in fledgling sex ratios observed in many dimorphic species.     

Food availability and immune capacity in serin (Serinus serinus) nestlings

Parental feeding effort is an important determinant of chick development in birds. Quality and amount of food provided to each nestling, however, might affect the development of the immune system. Food availability around the nest site may account for part of the difference in body condition and immune capacity of nestlings. We tested this idea in the socially monogamous serin (Serinus serinus), investigating growth and immune capacity of nestlings in relation to food availability around the nest site. We used a set of immunological and serological assays, including antibody responses to sheep red blood cells (SRBCs), the T lymphocyte cell-mediated immune response (PHA), granulocyte:lymphocyte ratio (G:L), haematocrit and sedimentation rate. The results suggest that serins raised under poor environmental conditions were less immunocompetent. The immune response to PHA and SRBCs significantly increased with food availability. Haematocrit values were positively, sedimentation rate and the G:L ratio negatively correlated with food availability. Similar results were obtained when examining chick development in relation to the immunological and serological variables. However, there was a negative relationship between parental feeding rates and food availability, as well as between parental feeding rates and chick development. We conclude that body condition and immune capacity of nestlings are strongly correlated with the availability of food around the nest site.     

Reproductive effort of colonial and solitary breeding tree sparrows Passer montanus L.

A total of 250 nestboxes were arranged in five plots in a suburban area of Budapest, Hungary (19°04E, 47°41N). In each plot, 25 were placed at 50 m intervals to simulate solitary breeding and 25 3–5 m apart to simulate colonial breeding. Length of nest building period, feeding frequency, nestling mortality, nestlings' diet, productivity and parental condition were compared for colonial and solitary breeding tree sparrows Passer montanus. Parents with long nest-building periods, including the majority of first-year females, produced fewer young than parents which built over short periods. Parents fed nestlings morefrequently and nestlings had lower mortality in second than first broods; whether or not a third brood was reared was determined by the costs invested in first and second broods. Females that laid a third clutch had reared fewer young in first and second broods and were heavier than females that reared many young in two broods. Colonial birds had higher feeding frequencies, more similar diets and suffered lower nestling mortality than solitary parents for first broods, but they fed less frequently, diets were less similar, and nestling mortality was higher in second and third broods. It is suggested that colonial breeders benefited from the social stimulation of simultaneous feeding in first broods, but the advantage of synchronicity in feeding declined in second broods and the sparser breeding spacing of solitary parents was more advantageous for feeding in second and third broods. Birds that changed nest spacing between broods had fed nestlings less frequently and had higher nestling mortality before changing than birds which retained their spacing. Parents which changed from colonies to solitary nests fed more frequently with lower nestling mortality in the next brood than parents which retained colonial nests for their second (and third) brood. Solitary parents did not show such a relationship. The rearing of three broods caused higher weight loss in colonial than solitary parents.Correspondence to: L. Sasvári     

The effect of rearing environment on blue structural coloration of eastern bluebirds (<Emphasis Type="Italic">Sialia sialis</Emphasis>)

We used a brood-size manipulation to test the effect of rearing environment on structural coloration of feathers grown by eastern bluebird (Sialia sialis) nestlings. Ultraviolet (UV)-blue structural coloration has been shown to be sexually selected in this species. Our experimental design took advantage of the growth of UV-blue wing feathers in nestlings that are retained as part of the first nuptial plumage. We cross-fostered nestlings to create enlarged and reduced broods with the purpose of manipulating parental feeding rates and measured the effect on nestling growth and plumage coloration. Brood size influenced feeding rates to offspring, but the effect varied with season. In general, male nestlings reared in reduced broods were fed more often, weighed more, and displayed brighter structural plumage compared to nestlings reared in enlarged broods. Female nestlings appeared to experience less adverse affects of brood enlargement, and we did not detect an effect of brood-size manipulation on the plumage coloration of female nestlings. Measures of plumage coloration in both males and females, however, were correlated to hatching date and nestling mass during feather development. These data provide empirical evidence that environmental quality can influence the development of the blue structural coloration of feathers and that males may be more sensitive to environmental fluctuations than females.     

Parental feeding rate in relation to begging behavior in asynchronously hatched broods of the great tit Parus major

Summary Variations in the begging behaviour of the nestlings of altricial birds can provide the parents with information about the nestlings' nutritional needs and thus influence the parental feeding rate. In a series of four experiments, the stimulus situation encountered by great tits on their feeding visits to the brood was manipulated to explore its effect on feeding rate.A higher feeding rate was observed under the following conditions: (1) after a period of food deprivation, as compared with both normal conditions and satiation through artificial feeding; (2) in periods when recorded begging calls were played during feeding visits, as compared with control periods; (3) after temporary removal from the nest of heavier, as compared with lighter, siblings. The lighter nestlings in the brood benefitted more —in terms of gain in weight — from the increase in parental feeding rate following the playing of begging calls than did the heavier nestlings. Differences in weight spread within broods did not affect the amount of food the parents brought.We conclude that parental feeding rate is affected not simply by the begging of the hungriest nestling, but rather by the behaviour of all the nestlings in the brood, which makes possible an adjustment of the feeding rate to the average hunger level of the brood. The effects of hatching asynchrony and sibling competition on parental feeding rate are discussed.     

Nestling American robins compete with siblings by begging

Summary The evolution of intense begging by dependent nestling birds has recently been the subject of several theoretical papers. The interesting problem here is that nestlings should be able to communicate their nutritional status to parents in ways that are less costly energetically and less likely to attract predators. Thus, conspicuous begging behaviour is thought to have evolved as a result of either competition among nestmates or the manipulation of their parents to provide more food than would otherwise be favoured by selection. We studied sibling competition for parental feedings in the American robin (Turdus migratorius). We demonstrate that the probability that an individual nestling received food was related to several indices of begging. When we experimentally prevented parents from feeding part of their brood, both the intensity of begging and the number of feedings subsequently received by food-deprived nestlings increased. Furthermore, the begging intensity of those nestlings that were not food-deprived also increased in response to the begging of their hungrier siblings.Offprint requests to: R. Montgomerie     

Food delivery and sibling competition in experimentally even-aged broods of the cattle egret

Summary The age of nestlings was made even in 12 nests of cattle egrets (Bubulcus ibis) by exchanging young chicks or eggs in a breeding colory in Japan. The growth of chicks in such synchronously hatching broods (SHBs) grew almost as fast as first-hatched chicks in 38 control (asynchronously hatching) broods (AHBs). Four last-hatched chicks in AHBs but no chicks in SHBs died of starvation. The frequency of parental nest-visits with food, that of begging food by chicks, and food mass eaten by chicks were greater in SHBs than in AHBs during the first half of the nestling period and similar thereafter. Dominance rank within each SHB was formed through ritualistic fights among siblings. It was correlated with neither growth rates nor winning ratios in food contests, but in some SHBs the subordinate chicks were attacked more frequently by dominant siblings during food contests than in AHBs. The most subordinate chick in one SHB died as a direct result of such attacks. Sibling aggressions were more frequent in SHBs than in AHBs.     

Siblicide in the cooperatively breeding laughing kookaburra (Dacelo novaeguineae)

I describe siblicide in the laughing kookaburra (Dacelo novaeguineae), a reverse size-dimorphic, cooperatively breeding kingfisher. Clutches were usually of three eggs, and nestlings hatched asynchronously, with intervals of 2–72 h between successive eggs. Siblicide occurred in two temporally and mechanistically distinct episodes. The youngest nestling died in one-third of all nests within days of hatching as a result of aggression from its elders. Kookaburra nestlings attacked each other using a hook on their upper beak – a rare example of a morphological specialisation for sibling rivalry. In one-fifth of all nests, the youngest nestling starved to death much later, without overt aggression, when nestling growth rates were highest. I examined the effects of food availability and competitive disparities between nestlings on the incidence of both types of siblicide. The probability of late, starvation-mediated mortality was negatively correlated with the number of male helpers. Early, aggressively mediated siblicide occurred in nests characterised by a suite of correlated variables that I call the ”kookaburra siblicide syndrome”: (1) no male helpers attended the nest, (2) the third-hatched nestling was much smaller than the second-hatched nestling, (3) the first and second nestling to hatch were male and female, respectively, and (4) there was a short hatch interval between the first two nestlings. The kookaburra siblicide syndrome variables could be inter-correlated if they were all related to the female’s condition at the onset of incubation. Females in poorer condition may be less likely to have male helpers, more likely to lay small third eggs, and more likely to hatch the first two eggs relatively synchronously because of nutritional constraints during the onset of incubation. These females may further promote siblicide by modifying the sexes of the first two nestlings. If a female hatches soon after an older but eventually smaller brother, dominance between the first two nestlings could be destabilised. I suggest this leads to escalated aggression in the nest and the death of the third nestling, which is least able to defend itself. Received: 17 December 1999 / Received in revised form: 8 May 2000 / Accepted: 20 May 2000     

Influence of kinship on helping behavior in Galápagos mockingbirds

Summary The social organization of the Galápagos mockingbird (Nesomimus parvulus) in unusual in that groups frequently include more than one breeding pair (plural breeding), and helping behavior is flexible: some birds neither breed nor help, while others do both. To investigate the influence of kinship on helping behavior, I categorized each bird as a helper or non-helper with respect to each nest within its group where it had an opportunity to help. The incidence of helping varied with relatedness: more birds helped when nestlings available to be fed were close relatives than when not. This result was independent of a higher incidence of helping among males than among females and of variation with age among males. Proportionally more nonbreeding than breeding males helped, but breeding and nonbreeding females helped equally infrequently; breeders helped most often after their own nests failed. The incidence of helping was highest among birds with opportunities to feed offspring of breeders that had fed the potential helper as a nestling, suggesting a mechanism for kin discrimination based on associative learning. Juveniles with opportunities to choose among alternative recipients preferentially fed closely related nestlings, but insufficient information was available to determine if adults also did so. Kinship did not influence the rate at which nestlings were fed by helpers. Juveniles fed nestlings at lower rates than did adult helpers, but helping effort was otherwise unaffected by age, sex, or relatedness. Limitation of help to former feeders functions as a mechanism for directing aid to relatives in a plural breeding system where degrees of kinship vary among potential recipients within the same group.     

Maternal effects on begging behaviour: an experimental demonstration of the effects of laying sequence, hatch order, nestling sex and brood size

Differential resource allocation by females across the laying sequence has been hypothesised as a mechanism through which females could either compensate nestlings that hatch last in asynchronous broods or promote brood reduction. In this study we artificially incubated eggs and cross-fostered offspring to manipulate nestlings’ position in the hatching order, to identify whether the competitive ability of nestlings is dependent on position in the laying sequence. In both control and experimentally reversed broods, first hatched chicks had a higher survival than last hatched siblings. Yet, nestlings that hatched from eggs laid in the second half of a clutch begged with a greater intensity than nestlings hatched from eggs laid in the first half of a clutch. In natural broods, the greater begging competitiveness of nestlings from later-laid eggs led to a moderation of sibling competition and these nestlings achieved the same body size and weight as nestlings from eggs laid in the first half of the clutch. The lack of a substantial difference in the size and condition of surviving nestlings in respect to laying order suggests that differential resource allocation across the egg-laying sequence partially compensates for hatching last in asynchronous broods and reduces the negative effects of the nestling size hierarchy. The effect of laying order, brood size and experimental treatment also differed for male and female nestlings. Our study highlights the need to be aware of the complex and subtle effects of nestling sex and laying sequence when investigating genetic and environmental influences on individual fitness.     

Provisioning rules and chick competition in asynchronously hatching common terns (Sterna hirundo)

Interactions between nestling birds and their parents are models for examining parent–offspring communication and sibling competition. Most studies have focused on species where young are restricted to a nest. However, offspring of many species are mobile and fed by parents for an extended period post-hatch. These chicks mobility may provide an opportunity to examine the role of signalling and physical competition on parental feeding decisions. We examined parental provisioning rules in relation to offspring behaviour and hatching order (i.e., competitive ability) in a species with mobile young, the common tern. We determined that about 95% of feedings were directed to the first chick to reach the parent when it landed with food. We developed a probabilistic model to predict the likelihood of a chick reaching the parent first, and thus receiving food. Our model showed that begging intensity, feeding history, and the interaction between begging intensity and relative proximity to the parent best predicted which chick would arrive first. Increased begging was associated with arriving first significantly more when a chick was relatively further from the parent than when it was closer than its siblings. Independently of these factors, larger, earlier-hatched chicks were more likely to be fed than smaller, later-hatched chicks. Additional analyses showed that parents landed closer to more intensively begging chicks, however, increased begging did not explain the advantage of earlier-hatched chicks because begging intensity did not vary with hatching order. Instead, earlier-hatched chicks were more likely to outrun later-hatched siblings and reach the parent first.     

Food supply differentially affects sibling negotiation and competition in the barn owl (Tyto alba)

In contrast to most birds, nestling barn owls (Tyto alba) vocalise not only when parents are at the nest but also in their absence. Calls produced in their absence have been shown to facilitate sibling negotiation over the impending food resource. Since nestlings vocalise more vigorously in the presence of parents, they may be calling not to negotiate resources but to compete amongst each other over parental food distribution. A critical issue is to determine whether offspring need differentially affects sibling negotiation and sibling competition, that is vocalisation in the absence and presence of parents. To answer this question, I manipulated the food supply of 26 broods by adding or removing prey items. In the absence of parents, food-added broods vocalised at a significantly lower level than food-removed ones. In contrast, once a parent arrived at the nest, the vocalisation level was not significantly related to the manipulation of brood food supply. This suggests that in the absence of parents, it is more important for food-removed nestlings to vocalise intensely, and that in their presence, the relationship between begging and offspring need tends to vanish. In other words, brood food supply may affect sibling negotiation to a larger extent than sibling competition.     

False feedings at the nests of carrion crows<Emphasis Type="Italic"> Corvus corone corone</Emphasis>

False feedings, when individuals visit the nest but refrain from feeding the chicks, occur in some cooperative species and have been interpreted in the white-winged chough (Corcorax melanorhamphos) as active deception by helpers towards the rest of the group. In a cooperatively breeding population of carrion crows (Corvus corone corone) 81.5% of the individuals that provided nestling care showed various kinds of false feedings: arriving at the nest with no food, consuming part or all the food brought to the nest, or taking back from a chicks gape the food that had just been delivered. False feedings occurred on average during 16.3% of nest visits, with some individuals performing them at very high rates (up to 64% of nest visits). False feedings occurred at similar rates in unassisted pairs and groups with helpers, and breeding females showed false feeding at significantly higher rates than other group members. Furthermore, individuals showed false feedings regardless of whether they were alone on the nest or in the presence of other group members, and false feedings did not provoke aggression by the rest of the group. False feedings are not likely to represent deceptive help in the carrion crow. We suggest that crows evaluate the chicks condition during nest visits and that false feedings occur as result of a trade-off between their own hunger and the chicks needs.Communicated by W.A. Searcy     

Parental favouritism strategies in the asynchronously hatching European Roller (<Emphasis Type="BoldItalic">Coracias garrulus</Emphasis>)

In altricial birds, resource allocation during early developmental stages is the result of an interaction between parental feeding decisions and scramble competition between nestmates. Hatching asynchrony in birds leads to a pronounced age hierarchy among their offspring. Therefore, whenever parents exert control over resource allocation parents feeding asynchronous broods should simultaneously assess individual offspring internal condition and age. In this study, we first studied whether the highly ultraviolet (UV) reflective body skin of nestlings in the asynchronous European Roller (Coracias garrulus; roller hereafter) relates to nestling quality. In a second stage, we experimentally studied parental biases in food allocation towards senior and junior sibling rollers in relation to a manipulation of UV reflectance of the skin of their offspring. Heavier roller nestlings had less brilliant and less UV saturated skins than weaker nestlings. In our experiment, we found that parents with large broods preferentially fed nestlings presenting skin coloration revealing small body size (i.e. control nestlings) over nestlings presenting skin coloration revealing large body size (i.e. UV-blocked nestlings). Within the brood, we found that parental food allocation strategy depended on nestling age: parents preferentially fed senior nestlings signalling small body size, but did not show preference between control and UV-blocked junior nestlings. These results emphasise that parent rollers use UV cues of offspring quality while balancing the age of their offspring to adjust their feeding strategies, and suggest that parents may adopt finely tuned strategies of control over resource allocation in asynchronous broods.     

Need and nestmates affect begging in tree swallows

We conducted an experiment on nestling tree swallows (Tachycineta bicolor) to examine predictions from signalling models for the evolution of conspicuous begging behaviour. Specifically, we examined the relationship between (1) nestling begging intensity and hunger, (2) begging intensity and parental provisioning and (3) begging intensity and nestmate condition. Forty broods of 9-day-old nestlings were removed from their nests for 1 h and assigned to one of the following three treatments: (1) all nestlings in the brood deprived of food (n = 13), (2) all nestlings in the brood fed (n = 11) or (3) half the nestlings in the brood deprived and half fed (n = 16). Videotapes before and after the treatments showed that begging intensity increased in broods in which all of the nestlings had been deprived and decreased in broods in which all of the nestlings had been fed. Deprived nestlings in the half-and-half treatment did not change their begging intensity in response to treatment, while fed nestlings in this treatment group showed a decrease in begging intensity. Parent tree swallows increased their feeding rate to deprived broods and decreased their rate to fed broods. Within broods, parents decreased their feeding rate to fed nestlings, but showed no significant change in feeding to deprived nestlings. Our results suggest that begging intensity is influenced by hunger and that parents appear to respond to variation in begging intensity. The begging of nestmates also appears to influence begging independently of need. These results are consistent with predictions derived from signalling models of begging. Received: 20 June 1997 / Accepted after revision: 19 January 1998