Abstract
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.
Highlights
Territory size is negatively correlated with density of food for a variety of animals during both breeding (Pitelka et al 1955, Stenger 1958, Holmes 1970, Cody and Cody 1972, Lyon 1976) and nonbreeding seasons
In Red Grouse territory size is positively correlated with an index of aggression by owners
Watson ( 1964) and Burger ( 1974) showed that territory size was negatively correlated with visibility in Red Grouse and Franklin's Gulls (Lams pipixcan) respectively
Summary
Title TERRITORY SIZE IN WESTERN GULLS - IMPORTANCE OF INTRUSION PRESSURE, DEFENSE INVESTMENTS, AND VEGETATION STRUCTURE. TERRITORY SIZE IN WESTERN GULLS: IMPORTANCE OF INTRUSION PRESSURE, DEFENSE INVESTMENTS, AND VEGETATION STRUCTURE. Myers et al (1979) investigated this question with territoriality of wintering Sanderlings (Calidris alba) They showed that the negative correlation between ' Manu~cript received 11 October 1978: revised accepted. The present study investigates variation in size of breeding territories in the low-density colony of Western Gulls (Lams occ:ide11ralis) on Santa Barbara Island. This system may offer insights into territory s ize that differ from those of the above investigations because Western Gulls do not forage on these territories. We made observations to evaluate three hypothetical relationships between costs of territorial defense and territory s ize. I) Variation in territory size could be caused by variation in intrusion rate throughout the colony. This argument has been developed theoretically by several authors
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