Individual Variation in the Strength of Territory Defense in Male Song Sparrows: Correlates of Age, Territory Tenure, and Neighbor Aggressiveness

Abstract

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.

Does a rival’s song elicit territorial defense in a tropical songbird, the Pied Bush Chat (Saxicola caprata)?

“…In other words, if male song increases primarily as a result of territorial defense, then seasonal song activity should be positively correlated with the territorial intrusions and escalated aggressions (Chiver et al, 2015). Such results have been reported in Song sparrows (Hyman, Hughes, Searcy, & Nowicki, 2004;Nowicki, Searcy, Krueger, & Hughes, 2002).…”

Integrating perspectives on vocal performance and consistency

“…A number of studies have found that females preferentially mate with older males, possibly because older individuals have demonstrated viability (by surviving) and are often able to outcompete younger individuals (e.g. Hegyi et al, 2006;Hyman et al, 2004). The fact that older males produce more consistent songs suggests that females can gather information about the age of a male by assessing his vocal consistency and use this information to select potential mates.…”

The ability to perform physically challenging songs predicts age and size in male swamp sparrows, Melospiza georgiana

“…This pattern could represent an overall improvement of breeding skills (i.e. parental competence or obtaining high-quality territories) (Pä rt 2001a, b;Hyman et al 2004), increasing reproductive effort with age (Pä rt 1995), or heritable variation in survival (Mauck et al 2004). Although it is not well understood whether genes or experience contribute more to age-related patterns of reproductive success (Forslund & Pä rt 1995), theory suggests that genes for viability could be an indirect benefit of mating with older males (Kokko & Lindströ m 1996).…”