Jeremy Hyman

Individual variation in the strength of territory defense in male song sparrows: Correlates of age, territory tenure, and neighbor aggressiveness

In many species, the ability to defend a territory is essential for a male to obtain any reproductive success at all, and even among territorial individuals, variation in the strength of territory defense could have a significant impact on how much reproductive success is obtained. Previous studies have documented consistent individual differences in the vigor with which male song sparrows (Melospiza melodia) defend their territories, as measured by the strength of their reactions to territorial intrusions simulated through song playback. Variation in the strength of defense could reflect intrinsic differences among individuals in their resource holding potential (RHP), or variation in extrinsic factors. In this study, we examined whether variation in the strength of territory defense corresponds to differences in intrinsic factors such as the age or experience of the territory owner, the extrinsic factor of the level of aggression shown by neighbours, or both. Results indicate that males that previously held territories on the study site, regardless of whether they were holding the same territory as the previous year, show higher levels of territory defense than males that are new to the study site, and, assuming that returning males are older males, suggest that age is more important than experience on a specific territory in determining strength of territory defense. In addition, we found evidence that males with high levels of territorial aggression tend to be spatially clustered. The pattern observed suggests that a combination of intrinsic and extrinsic factors contribute to the expression of individual differences in territorial aggression.

Differences in measures of boldness even when underlying behavioral syndromes are present in two populations of the song sparrow (Melospiza melodia)

One commonly studied behavioral syndrome is the correlation between aggression and boldness. Studies in song sparrows (M. melodia) have found greater aggression and boldness in urban populations and a correlation between aggression and boldness in rural populations, but not within urban populations. In previous studies, boldness was measured as flight initiation distance (FID), which may reflect habituation by urban birds to human presence. In this study, we measured boldness using playbacks of heterospecific alarm calls and investigated whether higher boldness is a general trait of urban birds and whether the same pattern of correlations between aggression and boldness would be seen. We conducted trials involving FID, alarm call playbacks and conspecific song playbacks on 25 birds from both an urban and a rural site. The results showed that urban birds were bolder, as measured by FID and response to alarm calls. Boldness and aggression were correlated in rural birds with each method of measuring boldness but were correlated in urban birds only when using alarm call playbacks. Our results suggest that a behavioral syndrome exists in both urban and rural populations but that urban birds are able to decrease their response to human disturbance.

Personality influences alarm calling behaviour in song sparrows

Alarm calls are a communication system in which the interests of senders and receivers often overlap, such that there may be little benefit to deception for senders, and yet, the potential cost of ignoring an alarm call can lead to a high number of false alarms. Increased frequency of false alarms may also be influenced by variation in personality traits such as boldness, which underlie all responses to dangerous or threatening situations. In this study, we investigated alarm calls in song sparrows (Melospiza melodia), and found a behavioural syndrome linking territorial aggression and boldness, such that more aggressive males respond more strongly to both conspecific and heterospecific alarm calls. In addition, those males that respond more strongly are also more likely to produce more alarm calls in response to playback. These results suggest that the strength of response to an alarm call by an individual could result from both the information in the signal and the personality of the receiver, and that the information in the signal may be influenced by the personality of the sender.