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.     

Acoustic signalling of hunger and thermal state by nestling tree swallows

The begging displays used by altricial nestling birds to solicit care from parents include vigorous movements and loud calling. These begging signals have attracted considerable interest, mainly because their intensity seems excessive for the function of transmitting information about nestling need to parents. However, how information on need is encoded in the various components of the signal, especially its acoustic components, is poorly understood. We examined how begging calls of large and small nestling tree swallows, Tachycineta bicolor, changed during a short period of food deprivation and cooling, as a first step in determining the role that various call characteristics played in advertising nestling need. In contrast to previous studies, we examined several call variables, and related them not only to need for food but also need for warmth. When nestlings were deprived of food, their calls increased in rate and length. Large nestlings also increased the amplitude of their calls. When nestlings were cooled during food deprivation, they decreased the frequency of their calls and their call rate. The latter trend was especially evident in small nestlings. Our results suggest that begging calls carry information not only on the overall hunger level of broods, as emphasized in previous studies, but also on the size, hunger and thermal need of individual nestlings. Further tests are needed to determine whether parents use this information and whether begging calls are optimally designed to convey it. Copyright 2001 The Association for the Study of Animal Behaviour.     

IBERIAN AZURE-WINGED MAGPIE CYANOPICA (CYANA) COOKI NESTLINGS BEGGING CALLS: CALL CHARACTERIZATION AND HUNGER SIGNALLING

ABSTRACT

Nestling begging behaviour has long been seen as a signal by which nestlings solicit care from parents and most of the existing evidence provides some support for it being an honest signal. Begging is a multicomponent signal in which both sound and vision components are usually important. Although it is known that begging encodes information about nestling hunger the present knowledge about the specific behavioural features that convey the information is still scarce. The aim of this study was to describe begging calls of Iberian Azure-winged Magpie Cyanopica (cyana) cooki nestlings and examine how information on nestling hunger might be encoded in the begging calls. Nestlings were experimentally submitted to different periods of food deprivation and the call variation within individuals was studied. The young were individually tested and stimulated to beg by simulating parental visits. When subject to increasing food deprivation periods, nestlings increased the response level to simulated parental visits. The study also found that for the studied size differences, nestlings did not differ in their response level. Results confirmed that information on nestlings' hunger might be encoded in parameters of the calling behaviour. When the food deprivation periods increased, nestlings tended to start begging earlier, begged more often, extended their calling bout and increased the call duration, changing both at the level of the call and vocal begging bout. Overall the results support the view of begging as an honest signal, namely that begging should reflect nestling hunger and that only some call features might encode information about hunger.     

How Nestling Tree Swallows (Tachycineta bicolor) Integrate their Responses to Hunger and Signalling by Nestmates

Young animals in a broad range of taxa solicit care from their parents with begging displays, which are used at least partly for competition among brood or litter mates. The effect of other begging offspring on an individual’s own begging display varies across studies, however, increasing its intensity in some, but not changing, or even decreasing it, in others. One possible reason for this discrepancy is that the potential pay‐off for more intense begging depends not only on how intensely an individual’s brood or littermates are begging, but also on how long that individual has been without food. Surprisingly, however, no studies have focused on how begging responses vary when both factors are varied simultaneously. We therefore examined how nestling tree swallows, Tachycineta bicolor, respond to nestmates in relation to both their own hunger levels and the begging intensity of nestmates. During a period of food deprivation, we played focal nestlings parental contact calls either alone (control) or with the begging calls of a nestling deprived of food for 30–50 (low intensity) or 100–110 min (high intensity). Nestlings called for longer in response to the low‐intensity playback, but, surprisingly, not in the high‐intensity playback, in which they instead delayed the onset of their calling. All these responses to nestmates were independent of how long the responding nestling had been deprived of food. Thus, even in the seemingly intensely competitive environment of a passerine brood, offspring do not necessarily respond to nestmates with escalation. This may be because de‐escalation is the best competitive option in some circumstances, or because begging has other functions besides advertisement of individual need and competition over food allocation. Certainly, the results illustrate the need for studies of how nestmate interactions vary across a broad range of contexts.     

Similarity in the begging calls of nestling Red‐winged Blackbirds

ABSTRACT Although individually distinct begging calls may permit parents to recognize their offspring, birds nesting in dense breeding colonies where fledglings intermingle might benefit from additional adaptations. For example, if the calls of all nestlings in a brood were similar, parents would need to recognize only one brood call instead of the identity calls of each nestling. We recorded nestling Red‐winged Blackbirds (Agelaius phoeniceus) to determine whether their calls function to identify individuals (identity call hypothesis) or broods (brood call hypothesis). We used spectrogram cross‐correlation and dynamic time warping as well as call duration, peak frequency, and frequency range to estimate the similarity of begging calls of nestling Red‐winged Blackbirds. We recorded individual nestlings on day 5 and on day 9 of the nestling period to determine whether calls of individuals were more similar than calls of different nestlings, and whether calls of broodmates were more similar than calls of nestlings from different broods. We found that calls of 8‐d‐old individuals were more similar than calls of different nestlings, but the calls of broodmates were not more similar than those of nestlings from different broods. These results were consistent with the identity call hypothesis. We then compared begging calls of pairs of nestlings recorded separately and together on day 9. We found that the calls of 8‐d‐old nestlings recorded together were more similar than when they were recorded separately. In addition, using playback of begging calls from normal broods and artificial “broods” constructed from the calls of single nestlings, we found that females returned with food sooner in response to the calls of single nestlings (with enhanced call similarity) than to those of normal broods. Our results suggest that similar begging calls may be beneficial for both nestlings and parents, with broodmates fed at higher rates when their calls are more similar and, after fledging, parents needing to recognize only one brood call instead of the identity calls of each fledgling.     

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.      

Chick Begging Calls Reflect Degree of Hunger in Three Auk Species (Charadriiformes: Alcidae)

Begging behaviour is an important element in the parent-offspring conflict; it has been studied in many avian species. However, the majority of the studies have been entirely based on the call counts, and they agreed that vocal activity was a good indicator of chick’s nutritional need and/or condition. Fewer researches were dedicated to the temporal-frequency variables of the begging calls themselves and they showed contrary results. Here begging behaviour in three burrow nested, uniparous species of auks (Alcidae) was studied. These objects provide an opportunity to study the signalling value of begging calls in the absence of important confounding factors such as nestling competition and predation pressure. I recorded calls of individual chicks in two conditions: during natural feeding and after experimental four-hour food deprivation. I found that almost all measured acoustic variables contain information about the chick’s state in all studied species. The hungry chicks produced calls higher in fundamental frequency and power variables and at higher calling rate compared to naturally feeding chicks. The effect of food deprivation on most acoustic variables exceeded both the effects of individuality and species. In all studied species, the frequency variables were stronger affected by hunger than the calling rate and call durations. I suppose that such strong change of acoustic variables after food deprivation can be explained by absence of vocal individual identification in these birds. As parents do not need to check individuality of the chick in the burrow, which they find visually during the day time, the chicks could use all of the acoustic variables to communicate about their nutritional needs.     

Ambient noise and the design of begging signals

The apparent extravagance of begging displays is usually attributed to selection for features, such as loud calls, that make the signal costly and hence reliable. An alternative explanation, however, is that these design features are needed for effective signal transmission and reception. Here, we test the latter hypothesis by examining how the begging calls of tree swallow (Tachycineta bicolor) nestlings and the response to these calls by parents are affected by ambient noise. In a field study, we found that call length, amplitude and frequency range all increased with increasing noise levels at nests. In the laboratory, however, only call amplitude increased in response to the playback of noise to nestlings. In field playbacks to parents, similar levels of noise abolished parental preferences for higher call rates, but the preference was restored when call amplitude was increased to the level that nestlings had used in the laboratory study. Our results show that nestling birds, like other acoustic signallers, consistently increase call amplitude in response to ambient noise and this response appears to enhance discrimination by receivers. Thus, selection for signal efficacy may explain some of the seemingly extravagant features of begging displays.     

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 to differ: scrubwren nestlings beg to alarm calls and vocalize when parents are absent

Nestling birds face a dilemma: they can increase parental provisioning by begging more intensively, but by doing so may also increase their risk of predation. Nestlings could deal with this dilemma by reducing begging intensity after parents have warned them of a nearby predator. We therefore tested experimentally whether nestling scrubwrens, Sericornis frontalis, increase begging intensity with hunger but reduce it after adult alarm calls. Single 5- and 8-day-old nestlings were temporarily taken into the laboratory for playback experiments. Over a 90-min period of food deprivation we simulated parental visits every 10 min by playing back adult feeding calls. Hungrier nestlings begged louder and longer to simulated parental visits, but contrary to expectation did not beg less if they had previously heard playback of alarm calls, and even begged to the alarm calls themselves. The results were similar for both ‘mobbing’ and ‘flee’ alarm calls. Nestlings also gave distinctive calls in the 10-min interval between simulated parental visits, and the number of these calls increased with hunger and after playback of alarm calls. We suggest that nestlings acquire the ability to respond appropriately to alarm calls late in the nestling period and that therefore parents might be selected to avoid alarm calling when defending young nestlings.Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Species divergence in offspring begging intensity: difference in need or manipulation of parents?

Conflicts over the delivery and sharing of food among family members are expected to lead to evolution of exaggerated offspring begging for food. Coevolution between offspring begging intensity and parent response depends on the genetic architecture of the traits involved. Given a genetic correlation between offspring begging intensity and parental response, there may be fast and arbitrary divergence in these behaviours between populations. However, there is limited knowledge about the genetic basis of offspring solicitation and parental response and whether these traits are genetically correlated. In this study, we performed a partial cross-fostering experiment of young between pied and collared flycatchers (Ficedula hypoleuca and Ficedula albicollis) and recorded the behaviour of individual offspring and their (foster)parents. We found that nestling collared flycatchers reached a higher phenotypic quality, estimated both as mass at fledging and as intensity of their T-lymphocyte-mediated immune response when raised by heterospecific foster parents. However, although collared flycatchers begged relatively more intensively, we found no evidence of corresponding higher resistance (i.e. lower feeding rate) of adult collared flycatchers than of adult pied flycatchers. Thus, the difference in offspring begging intensity between the two species seems not to be a result of a difference in escalation of the parent-offspring conflict. Instead, the species' divergence in exaggeration of offspring begging intensity 'honestly' matches a difference between the species in offspring need. This interpretation is strengthened by the fact that the difference in begging intensity between the two species increased as the season progressed, coinciding with the higher sensitivity of nestling collared flycatchers to the seasonal decline in food availability. Thus, the behavioural differentiation appears to be a direct consequence of a life-history differentiation (offspring growth patterns).     

Signalling of hunger when offspring forage by both begging and self-feeding

Studies on vertebrate species in which the offspring obtain food only from their parents have shown that offspring begging conveys information on offspring hunger. It is unclear whether begging can also convey information on hunger in partially begging species, in which the offspring, from hatching, obtain food partly through begging for food from the parents and partly through self-feeding. In partially begging species, offspring hunger state could reflect the amount of food obtained by self-feeding in addition to the amount obtained from the parent, and the offspring could respond to hunger by self-feeding instead of begging. To test whether begging reflected the current hunger state of offspring in the partially begging beetle Nicrophorus vespilloides, we subjected larvae to two deprivation treatments: no access to any food for 2 h (i.e. food-deprived larvae) and no food provided by the parent (i.e. self-feeding larvae). Larvae of both treatments spent more time begging than did control larvae, which had access to food both from the parents and by self-feeding. Furthermore, the amount of begging differed between the treatments, with food-deprived larvae spending more time begging than the self-feeding ones. We conclude that, in partially begging species, food obtained through both foraging strategies, i.e. begging for food from parents and self-feeding, contribute to the offspring's current hunger state, and that begging can convey information about it.     

Effects of feeding frequency on nestling begging and digestion

Nestling begging has the potential to provide parents with honest information about both short- and long-term nutritional needs, yet the importance of previous feeding experience remains largely untested in empirical studies. We examined the effect of two experimental feeding rates on nestling begging in Southern Grey Shrikes Lanius meridionalis using differences in load size to equalize the total volume of food received. There was variation in the pattern of begging behaviour between six pairs of siblings during a hand-feeding trial, although individuals maintained a similar begging intensity throughout a 9-h feeding period. Both treatment groups showed elevated begging responses during a terminal deprivation period, but nestlings fed small food items at frequent intervals demonstrated higher begging responses after a period of deprivation than did siblings fed large food items infrequently. As nestlings fed frequently with small food items had greater levels of undigested protein present in their faeces than birds fed large items infrequently, we suggest experimentally induced variation in digestive efficiency may account for the observed differences in begging behaviour. The possible role of learning, the adaptive significance of trade-offs between feeding rate and digestive efficiency, and a possible conflict of interests between parents and offspring are discussed.     

Begging in the absence of sibling competition in Wilson’s storm-petrels, Oceanites oceanicus

Begging displays of nestlings in multichick broods can signal both hunger and competitive ability. 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. However, there is no evidence that chicks signal hunger by begging in the absence of sibling competition. I tested predictions of signalling models in a species with single-chick broods, the Wilson's storm-petrel. Chicks used two types of begging calls, ‘rhythmic’ calls and ‘long’ calls. I found that chicks conveyed information about their current body condition by begging. When their body condition was low, chicks increased the number and frequency of long begging calls, as well as the frequency of rhythmic calling. Parents responded to increased begging by regurgitating larger meals. The study thus demonstrates that the begging system can work in the absence of nestling competition. Chicks also called in the absence of their parents, but in this context they used only rhythmic calls and there was no correlation with current body condition. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Acoustic parameters of begging calls indicate chick body condition in Wilson’s storm-petrels <Emphasis Type="Italic">Oceanites oceanicus</Emphasis>

Begging behaviour as a key element in the parent–offspring conflict has been studied in many avian species. These types of studies have nearly exclusively been based on call counts, and it is still not entirely clear whether begging calls themselves contain any information. We studied begging behaviour in Wilson’s storm-petrel Oceanites oceanicus, a small procellariiform seabird. This species provides the opportunity to study the signalling value of begging calls in the absence of potentially confounding factors such as nestling competition, previous feeding experiences and predation pressure. We applied a new method using a semi-automatic spectrogram analysis software that measures the acoustic parameters of begging calls. Our analysis revealed that the frequency parameters of begging calls reflect chicks’ current body condition, with chicks in poorer condition uttering calls at higher frequencies. Chicks uttering higher pitched calls also received larger meals. Our study shows that certain acoustic parameters of begging calls can indicate the state of a chick in Wilson’s storm-petrels.     

Parental effort and response to nestling begging in the house sparrow: repeatability,heritability and parent–offspring co‐evolution

Parental effort has a direct impact on individual fitness. Theoretical models exploring how parental effort evolves to cope with offspring demand and sexual conflicts may differ in the assumptions they make in respect to the genetic heritability of parental behaviours. Only a few attempts, however, have been made to estimate the heritability of parental behaviours and their possible co‐evolution with offspring solicitation behaviour. Analysing parent and offspring behaviours in four generations of cross‐fostered broods of house sparrows, we found that parental effort (food delivery rate) was repeatable across consecutive broods and heritable across generations. In contrast, parental response to experimentally induced changes in nestling begging was neither repeatable across broods nor heritable across generations or correlated to nestling begging. Thus, the results give no indication for genetic covariance between begging intensity and parental response, but provide the first cross‐fostering‐based evidence for the heritability of parental investment levels across generations.     

Barn owl (Tyto alba) siblings vocally negotiate resources

Current theory proposes that nestlings beg to signal hunger level to parents honestly, or that siblings compete by escalating begging to attract the attention of parents. Although begging is assumed to be directed at parents, barn owl (Tyto alba) nestlings vocalize in the presence but also in the absence of the parents. Applying the theory of asymmetrical contests we experimentally tested three predictions of the novel hypothesis that in the absence of the parents siblings vocally settle contests over prey items to be delivered next by a parent. This 'sibling negotiation hypothesis' proposes that offspring use each others' begging vocalization as a source of information about their relative willingness to contest the next prey item delivered. In line with the hypothesis we found that (i) a nestling barn owl refrains from vocalization when a rival is more hungry, but (ii) escalates once the rival has been fed by a parent, and (iii) nestlings refrain from and escalate vocalization in experimentally enlarged and reduced broods, respectively. Thus, when parents are not at the nest a nestling vocally refrains when the value of the next delivered prey item will be higher for its nest-mates. These findings are the exact opposite of what current models predict for begging calls produced in the presence of the parents.     

Are acoustical parameters of begging call elements of thin-billed prions related to chick condition?

Chicks of burrowing petrels use begging calls to advertise their hunger levels when parents arrived at the nest. In a previous study, adult thin-billed prions Pachyptila belcheri responded to higher begging call rates of their single chick by regurgitating larger meals. We tested whether acoustic parameters of begging call elements may also be involved in signalling. To describe variation in begging, we determined begging session parameters, namely the duration, number of calls and the mean and maximum rate of calling. We then digitised calls and carried out a semi-automatic extraction of six acoustic parameters of call elements, including mean and maximum acoustic frequency, the length of call elements and the location of the maximum frequency and amplitude within calls. Chicks showed strong individual differences in all parameters. While the session parameters were correlated with body condition and with the meal size the chick received, none of the acoustic parameters were related to body condition and provisioning. A cross-fostering experiment showed the same pattern, as only session parameters changed related to an experimentally altered body condition, while acoustical cues appear to play no role in signalling hunger levels. We suggest that this may be explained by the absence of sibling competition in these birds. As parents do not need to decide which chick to feed, immediate information on condition at the time of adult arrival may not be required.     

Female Attraction to Higher Pitched Male Enticement Calls in Barn Swallows

Current sexual selection studies demonstrate that female mate preference almost always favors exaggerated male traits, such as colorful plumage and elongated tails. However, such studies focused mainly on secondary sexual characteristics, i.e., traits that are fully developed at maturation, while ignoring others, which precludes generalization of the findings. We studied the expression of the male enticement call, a nestling‐like courtship trait, in relation to female attraction and to the expression of secondary sexual ornaments in the Japanese barn swallow Hirundo rustica gutturalis. First, we showed that the peak amplitude frequency of male enticement calls was, on average, lower than that of nestling food‐begging calls, indicating that higher pitched calls are more similar to nestling food‐begging calls than are lower pitched calls. Second, we conducted an experimental playback of male enticement calls during the courtship period with the female's mate removed, and found that females were more attracted to heightened‐pitch calls than to control calls. Lastly, the peak amplitude frequency of male enticement calls was negatively related to the ornamentation, as measured by throat coloration. Taken together, these findings suggest the following: (1) females are attracted to a more nestling‐like trait, and (2) similarity to nestling food‐begging calls increases with decreasing secondary sexual ornaments. Nestling‐like male traits that are not secondary sexual characteristics should be considered when studying the general pattern of female attraction to male traits.     

Factors Affecting Vocalization in Tengmalm’s Owl (Aegolius funereus) Fledglings during Post-Fledging Dependence Period: Scramble Competition or Honest Signalling of Need?

Begging behaviour of nestlings has been intensively studied for several decades as a key component of parent-offspring conflict. There are essentially two main theories to account for intensity of food solicitation among offspring: that intensity of begging is related to some form of scramble competition between nest mates or that it offers honest signalling of need to parents. The vast majority of studies which have addressed begging behaviour have been based on observations of, and experiments on, nestlings and have not considered begging behaviour, during the post-fledging period. Begging vocalizations in this post-fledging phase of dependence have rarely been studied, despite the importance of vocalizations as a communication method between offspring and parents, particularly for nocturnal species. We radiotracked 39 fledglings of the Tengmalm’s owl (Aegolius funereus) in two years with different availability of prey: 2010 (n = 29 fledglings) and 2011 (n = 10 fledglings) and made 1320 nightly localizations in which we recorded presence or absence of begging calls. Within years, the most important measures related to the probability of vocalization were body condition at fledging, time of night, number of surviving siblings, age and weather conditions. Begging intensity increased with age in both years; however, in the year with low prey availability fledglings vocalized significantly more often. The main factor causing these differences between years was probably the different availability of prey, affecting breeding success, post-fledging behaviour, and thus also both short- and long-term needs of offspring. We believe that our results suggest honest signalling of their fledgling’s need.