Body size, habitat quality, and territory defense in Bachman’s sparrow

Many wild populations of animals conform to the ideal despotic distribution (IDD) in which more competitive individuals exclude less competitive individuals from high quality resources. Body size and aggressiveness are two important traits for resource defense, and they positively covary so that larger individuals are usually more aggressive. Using Bachman’s sparrows, we tested the hypothesis that larger birds are more aggressive and are thus able to compete for the best quality territories. We found that larger males were more aggressive, and more aggressive birds fledged at least one young. However, we did not find consistent relationships between aggressiveness and habitat characteristics. Our results suggest that Bachman’s sparrows meet most of the predictions of the IDD. Frequent ecological disturbances, such as fires, might disrupt the IDD or make it difficult to detect. Additional studies are needed to test for relationships between ecological disturbances and territorial behaviour.

Despotic aggression in pre-moulting painted buntings

Aggression in territorial social systems is easy to interpret because the benefits of territorial defence mostly accrue to the territorial holder. However, in non-territorial systems, high aggression seems puzzling and raises intriguing evolutionary questions. We describe extreme rates of despotism between age classes in a passerine bird, the painted bunting ( Passerina ciris ), during the pre-moulting period. Aggressive encounters were not associated with aggressors gaining immediate access to resources. Instead, conspecifics, and even other species, were pursued as though being harassed; this aggression generated an ideal despotic habitat distribution such that densities of adult males were higher in high-quality sites. Aggression was not a by-product of elevated testosterone carried over from the breeding season but, rather, appeared associated with dehydroepiandrosterone, a hormone that changes rates of aggression in non-breeding birds without generating the detrimental effects of high testosterone titres that control aggression in the breeding season. This extraordinary pre-moult aggression seems puzzling because individual buntings do not hold defined territories during their moult. We speculate that this high aggression evolved as a means of regulating the number of conspecifics that moulted in what were historically small habitat patches with limited food for supporting the extremely rapid moults of painted buntings.

Evolutionary conservation advice for despotic populations: habitat heterogeneity favours conflict and reduces productivity in Seychelles magpie robins

Individual preferences for good habitat are often thought to have a beneficial stabilizing effect for populations. However, if individuals preferentially compete for better-quality territories, these may become hotspots of conflict. We show that, in an endangered species, this process decreases the productivity of favoured territories to the extent that differences in productivity between territories disappear. Unlike predictions from current demographic theory on site-dependent population regulation (ideal despotic distribution), we show that population productivity is reduced if resources are distributed unevenly in space. Competition for high-quality habitat can thus have detrimental consequences for populations even though it benefits individuals. Manipulating conflict (e.g. by reducing variation in habitat quality) can therefore prove an effective conservation measure in species with strong social or territorial conflict.

A temporally stable spatial pattern in the spawner density of a freshwater fish: evidence for an ideal despotic distribution

We explore the causal basis for a temporally stable spatial pattern in the density of spawning individuals of a freshwater fish. Based on a comparatively long-term set of data spanning one decade, reproductively active brook trout (Salvelinus fontinalis) inhabiting a near-pristine river on Cape Race, southeastern Newfoundland, occupy areas of either high or low density. Compared with their low-density counterparts, high-density aggregations are typically more than 30 times denser despite occupying only 4% of the available habitat. High-density areas are characterized by slower flow and suspected groundwater or bog-water seepage, attributes likely to increase the probability of offspring survival in Freshwater River. Disparity in density between high- and low-density aggregations declined as total population size increased, a pattern consistent with the predictions of the ideal free distribution. The larger body sizes of trout in the high-density aggregations may prevent others from occupying the most preferred spawning habitat. This spatial pattern in spawner density is consistent with that predicted by an ideal despotic distribution, although we cannot discount the influence that Allee effects might have on the distributional patterns of spawning individuals at low population sizes.