Territory Size in Western Gulls: Importance of Intrusion Pressure, Defense Investments, and Vegetation Structure

Variation in territory size in a low—density colony of Western Gulls (Larus occidentalis) was analyzed in terms of costs and benefits accruing to owners. Spatial variation in intrusion rate per unit area best explains variation in territory size; the highest intrusion rate and smallest territories occurred near undefended gathering spots. Killing of chicks by neighbors was more frequent on small territories than on large territories. However, owners of large territories spent more time in territorial defense and more time per act of defense than owners of small territories. Intrusion elicited aggression from owners more frequently on small territories than on large territories, and intruders remained on large territories for longer periods of time. Territory size was positively correlated with the amount of visually obstructive vegetation. This correlation was apparently a result of gathering spots occurring in areas with little cover.