Intruder pressure explains more of the variation in territory size than fish abundance for Red-necked Grebes Podiceps grisegena breeding on small boreal lakes

Abstract

In the Red-necked Grebe (Podiceps grisegena), breeding pairs use both overt aggression and ritualistic behavior to defend territories for breeding and feeding. Examining the influence of factors determining territory size simultaneously continues to be stressed in studies of territorial systems. The behavior of grebes breeding on small boreal lakes (15-240 ha) was examined in relation to several environmental variables through three stages of the breeding season (pre-nesting, nesting, and post-nesting). Territory size was shown to be inversely related to grebe density and fish abundance during the pre-nesting and nesting stages of the breeding season. In addition, grebe density was more strongly associated with territory size than was fish abundance. The costs and benefits that balance economic decisions for breeding grebes rigorously defending areas are discussed.