Territorial breakdown of black-capped chickadees, Poecile atricapillus, in disturbed habitats?

The propensity of males to behave territorially may be greatly affected by the perceived quality of the habitat. In suboptimal habitats, the cost of defence may be prohibitively large compared to the associated benefits, and territorial behaviour may be expected to decline. We tested this hypothesis in a population of black-capped chickadees breeding in adjacent habitats: one habitat consisted of an 85-ha patch of early seral forests regenerating after clear-cut logging (disturbed site), and the other consisted of an 85-ha patch of mature mixed-wood forests (undisturbed site). Breeding success of pairs in the disturbed site was significantly lower than in the undisturbed site, suggesting a difference in the relative quality of the two habitats. During the spring of 2000 and 2001, we mapped the actively defended areas and song posts of colour-marked chickadees and found that pairs occupying the disturbed site were subject to more territorial intrusions than pairs in the undisturbed mature woodlands. Five pairs of chickadees in the disturbed habitat and five pairs in the undisturbed habitat were radiotagged and we conducted repeated focal observations on the movement patterns of the birds. All five pairs in the disturbed habitat regularly intruded into areas actively defended by neighbouring birds; only one of the five pairs in the undisturbed habitat ever intruded onto the known territory of another pair during an observation period. Our results suggest that the quality of the disturbed habitat may have been sufficiently low so as to make normal levels of exclusive defence cost inefficient. Conversely, birds in disturbed habitat may have intruded more frequently into neighbouring territories because resource levels in their own exclusively defended territory were insufficient to meet energetic requirements during the breeding season.