Abstract
Black-tailed Gull territorial defense was compared between plots with tall (2-3 m high) and sparse vegetation. While the frequency of defense against neighboring breeders did not differ between these plots, defense against intruders was 8 times greater in plots with sparse vegetation than in those with tall vegetation. When we removed vegetation from a plot with tall vegetation we found that gulls sometimes landed in this newly opened area, whereas landings did not occur in areas with tall vegetation. Such behavior indicates that tall vegetation prohibits intruders from landing and thus reduces the frequency of territorial defense.
