Marty L. Leonard

Provisioning rules in tree swallows

Abstract Conflict between parents and offspring may result in offspring exaggerating their needs and parents devaluing their begging signals. To determine whether this occurs, it is first necessary to establish the link between need, begging and parental response. The purpose of our study was to examine these relationships in tree swallows (Tachycineta bicolor). Parents preferentially fed nestlings that begged sooner, reached higher and were closer to the front of the nestbox (Fig. 1). Begging intensity of both individuals and entire broods increased with relatively long periods between feeding visits. Within broods, parents responded to increased begging intensity by increasing their feeding rate, although this effect was relatively weak. Large and small nestlings did not differ in their begging behavior and all nestlings, regardless of size, were fed at similar rates. Despite the overall equity in feeding, male parents preferentially fed larger nestlings while female parents fed smaller nestlings. Nestlings did not increase their begging intensity in response to begging by nestmates. Our results suggest that begging is related to need in this species and that parents respond to variation in begging intensity.

Sex-specific, seasonal foraging tactics of adult grey seals (Halichoerus grypus) revealed by state-space analysis

In many large pelagic animals, observing behavior is limited to observation by radio or satellite telemetry. In many cases, discriminating different behaviors from telemetry data has been a key, but often elusive, goal. Here we use state-space models (SSMs) to fit a correlated random walk (CRW) model that switches between two unobserved behavioral states (nominally foraging and traveling) to 41 male and 43 female adult grey seal (Halichoerus grypus) satellite telemetry tracks. The SSM results reveal markedly different spatial behavior between the sexes, fitting well with sexual size dimorphism and known dietary differences, suggesting that the sexes deal with seasonal prey availability and reproductive costs differently. From these results we were also able to produce behaviorally informed habitat use maps, showing a complex and dynamic network of small, intensely used foraging areas. Our flexible SSM approach clearly demonstrates sex-related behavioral differences, fine scale spatial and temporal foraging patterns, and a clearer picture of grey seal ecology and role in the Scotian Shelf ecosystem.

Sexual segregation of seasonal foraging habitats in a non-migratory marine mammal

Many animal species segregate by sex. Such segregation may be social in nature, or ecological, or both. Grey seals (Halichoerus grypus), like many large mammals, are sexually size dimorphic. In size dimorphic species, allometric differences in morphology, metabolic rate and reproductive costs are likely. Such differences may require the sexes to use different foraging strategies or different habitats. To investigate sexual segregation of habitat in grey seals, we used satellite tracks from 95 (male 46; female 49) adults breeding at Sable Island, Nova Scotia (44 °N, 60 °W) collected from 1995 to 2005. Location estimates were made from satellite fixes using a state-space movement model to estimate true locations and regularize them in time. Location estimates were used to calculate home range kernels of male and female habitat use each month. Month by sex kernel home ranges revealed striking differences and dynamics in habitat use between males and females on spatial scales broader than most terrestrial examples and at temporal and spatial resolutions rarely available for marine species. Differences were most pronounced just before (October–December) and immediately after breeding (February–March). During both periods, males primarily used areas along the continental shelf break, while females mainly used mid-shelf regions. Coupled with previously identified sex-specific seasonal patterns of energy storage, diving and diet, our findings suggest that males and females differ profoundly in their spatial foraging strategies. These differences may serve to maximize fitness by reducing intersexual competition during key foraging periods.

The Evolution of Begging

Parental feeding of young is often accompanied by striking begging displays on the part of offspring. These displays are now widely thought to function as signals of need; in other words, they serve to elicit additional resources from parents by advertising the benefits that offspring thereby stand to gain. Sib-sib and parent-offspring conflicts over resource allocation, however, favour misrepresentation of need. Consequently, signalling models have focused on signal cost as a means to maintain the reliability of offspring displays. Recently, however, the possibility of alternative, cheaper signalling equilibria has emerged. We review costly and cost-free signalling models, and suggest that both face difficulties in accounting for observed begging behaviour. We conclude with the suggestion that these difficulties may best be tackled by developing more realistic models that incorporate more of the complexities of parent-offspring interaction revealed by empirical studies of begging.

Need and nestmates affect begging in tree swallows

Abstract 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.

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.

Oxygen consumption during crowing by roosters: Talk is cheap

The energetic cost of signalling may be important in maintaining the honesty of the signal, yet it is rarely measured directly. Oxygen consumption during crowing by roosters, Gallus gallus domesticus, was measured in a closed-circuit indirect calorimeter. Although there was a positive relationship between crowing and O2 consumption, roosters consumed only 0·005 ml/g/h for each vocalization. Thus at the average crowing rate, O2 consumption rate would be 15% above standing, which is less than the cost of low-level activities such as feeding, drinking and preening, and over 10 times less than the maximum sustainable metabolic rate. These results contrast with previous reports of high energy consumption during calling in insects and frogs, which approach or exceed maximal levels. Other costs, such as predation and social retaliation, are probably more important than energetic costs in maintaining crowing as an honest signal.

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

Abstract 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.

Crowing in relation to status in roosters

The purpose of this study was to examine the relationship between crowing and dominance using domestic roosters,Gallus gallus domesticus. Dominant males crowed significantly more often than subordinate males and often attacked subordinates that crowed. Dominants also produced crows that were higher in frequency than subordinates. In addition, dominant males spent more time near the speaker when crows of dominant males were played than they did when crows of subordinate males were played. Neither subordinate males nor females responded to tapes from males of either status. These results suggest that both crow rate and quality vary with male status and that dominants can and do discriminate between males using crow quality as a cue. Crowing by roosters could thus potentially function as a signal of status.

Do social networks of female northern long-eared bats vary with reproductive period and age?

Social structure, which is a function of the patterns of interactions among individuals, is particularly variable in fission–fusion societies. The underlying factors that drive this variation are poorly understood. Female northern long-eared bats (Myotis septentrionalis) live in fission–fusion societies where females form preferred associations within groups that vary daily in size and composition as individuals switch roosts. The goal of our study was to test the predictions that preferred associations and social networks of female northern long-eared bats vary with reproductive period and age. We also tested the prediction that preferred relationships persist across years despite movements from summer roosts to winter hibernacula. Network analyses revealed that during gestation, females roosted in smaller groups where they roosted more regularly with fewer individuals than during lactation. This variation may reflect different social strategies to mediate higher energetic costs during lactation. Females of all ages roosted more often with younger individuals, which in turn had more direct and indirect associations than all other age classes. Younger individuals may play a role in maintaining connections between individuals, perhaps as a result of younger individuals being more exploratory. Temporal analyses suggested that relationships can persist for years as some pairs roosted together for multiple summers. We suggest that the dynamic nature of fission–fusion societies is associated with individual strategies to increase fitness relative to individual characteristics, in this case reproductive condition and age.