Andrew G. Horn

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.

Design of Playback Experiments: The Thornbridge Hall NATO ARW Consensus

Playback is an experimental technique commonly used to investigate the significance of signals in animal communication systems. It involves replaying recordings of naturally occurring or synthesised signals to animals and noting their response. Playback has made a major contribution to our understanding of animal communication, but like any other technique, it has its limitations and constraints.

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.

Calling in the Face of Danger: Predation Risk and Acoustic Communication by Parent Birds and Their Offspring

Abstract Birds raise their young under constant risk of predation, which shapes how parents and young communicate acoustically. Nestling begging calls extract care from parents but expose them to eavesdropping by predators, which selects for cryptic signal design. However, for largely unknown reasons, nestlings often call even when parents are absent and thus unavailable for defense. Nestlings can give distress and defensive calls, but their efficacy is unknown. Parental alarm calls warn young of danger in some species, and young can show adaptive changes in response according to their age-related vulnerability to specific predators. Parents often signal their arrival at the nest with provisioning calls, which might reduce mistaken begging and increase the efficiency of feeding, and thereby minimize detection by predators. Acoustic interactions between offspring and parents can lead to young either remaining silent until prompted by a parents provisioning call, or begging indiscriminately to subtle cues of arrival while remaining alert to parental alarm calls. Young can also assess danger independently of parents, which could in turn affect parental decisions about giving alarm calls. All these behaviors offer fertile ground for studying how animals trade off acoustic communication with the risk of eavesdropping.

The role of posturing and calling in the begging display of nestling birds

Nestling birds produce a multicomponent begging display that has visual (e.g. posturing) and vocal (e.g. call rate) elements. Most work on the function of the display has focused on each component separately. However, understanding the evolution of complex displays such as begging requires knowledge of how the components function collectively. The purpose of our study was to determine how postural intensity and calling rate together influence parental feeding decisions in tree swallows, Tachycineta bicolor. We compared how begging components responded to a manipulation in which pairs of nestlings were either free to approach the parent when it arrived to feed (unconfined treatment) or confined to the back of their nestbox by a Plexiglass partition (confined treatment). We found no significant differences in postural intensity between treatments, but calling rate was significantly higher in the confined treatment. In both treatments, postural intensity, but not calling rate, correlated with hunger. Both components positively and independently correlated with the likelihood of a nestling being fed, although the correlation with postural intensity was stronger. Previous work suggested that both posture and call rate advertised hunger in nestling tree swallows. Here, call rate was not associated with hunger, but rather was affected by nestling position. These results suggest that calling may serve an additional role in helping nestlings in disadvantaged positions attract parental attention. The results also suggest that calling may have a complex relationship with hunger, position and nestmates.

Ambient noise increases missed detections in nestling birds

Ambient noise can mask acoustic cues, making their detection and discrimination difficult for receivers. This can result in two types of error: missed detections, when receivers fail to respond to the appropriate cues, and false alarms, when they respond to inappropriate cues. Nestling birds are error-prone, sometimes failing to beg when parents arrive with food (committing missed detections) or begging in response to stimuli other than a parents arrival (committing false alarms). Here, we ask whether the frequency of these errors by nestling tree swallows (Tachycineta bicolor) increases in the presence of noise. We found that nestlings exposed to noise had more missed detections than their unexposed counterparts. We also found that false alarms remained low overall and did not differ significantly between noise and quiet treatments. Our results suggest that nestlings living in noisy environments may be less responsive to their parents than nestlings in quieter environments.