Nest Success and Nesting Habits of Eastern Kingbirds and Other Flycatchers

-Patterns of nest placement and its relationship to nest success in the Eastern Kingbird (Tyrannus tyrannus) were studied in populations breeding in New York and Kansas. Data were augmented with information on nest placement in other open-nesting tyrannids in order to examine the hypothesis that these flycatchers place their nests chiefly so as to conceal them from predators. Nesting success was significantly greater in New York than in Kansas but was relatively high in both populations, as is apparently true of North American breeding flycatchers in general. Geographic variation in nest placement in the Eastern Kingbird was relatively small and statistical comparisons of failed and successful nests indicated that nests placed at mid-heights in the tree, and about midway between the center of the tree and canopy edge, were most successful. These nest sites were also the most commonly used sites. Increased vegetative cover around the nest and a greater number of supporting branches for the nest were also associated with success, and it is their interaction with nest height and distance from the canopy edge that is apparently most important in determining success. Most tyrant flycatchers nest several meters or more above the ground, and interspecific variation in nest placement matches the range of sites used by Eastern Kingbirds. Aggressive nest defense is apparently characteristic of the family, and average nest height is also positively correlated with length of the nestling period. These facts support the predator-avoidance hypothesis and suggest that nest placement habits and aggressive nest defense are means by which opennesting birds can reduce rates of nestling mortality. Nest site selection in birds can be an important determinant of reproductive success by affecting losses caused by predators and weather, the two most important causes of nest failure (Nolan 1963, Ricklefs 1969a). Except for the well-known differences in the breeding biology of cavityand open-nesting birds (von Haartman 1957, Lack 1968), few studies of temperate-zone breeding passerines have related aspects of nest placement to the species' overall breeding habits. If species with long nesting periods placed their nests in cryptic, easily defended or inaccessible sites, they could counteract the increased probability of predators finding the nest before the young fledge. This hypothesis has yet to be examined for temperate-zone breeding passerines, yet is presumably the basis for the great diversity of nesting habits in tropical breeding land birds (Ricklefs 1969a). As part of a geographic comparison of the breeding biology of the Eastern Kingbird (Tyrannus tyrannus), I collected data on nest placement and success. Eastern Kingbird nests are relatively conspicuous and are typically placed on horizontal branches close to the canopy edge (Davis 1941, Bent 1942, Pettingill 1973). Nestling kingbirds develop slowly (Murphy, in press) and eggs and nestlings are exposed in the nest for up to 39 days. I therefore predicted that nest placement would be important in determining the reproductive success of Eastern Kingbirds. Furthermore, since slow nestling growth and long periods of nest occupancy are typical of most tyrannids (Ricklefs 1976; Murphy, in press), nest placement should be especially important in this family and reflect the need to reduce nest loss. I t sted these hypotheses using data on nest placement and success in Eastern Kingbirds, and comparative data from the literature. STUDY AREA AND METHODS I studied the breeding biology of Eastern Kingbirds in western New York and eastern Kansas from May through August, 1979 and 1980, espectively. The center of the New York research site was located midway between the towns of Eden and Angola, Erie Co. (42040'N, 78057'W) and was approximately 16 km from Lake Erie. In Kansas, I concentrated research in a area 6.5 km west of Lawrence, Douglas Co., near Clinton Lake (38057'N, 95019'W). The stud site in New York encompassed the adjacent lands 0.5 km on either side of 45 km of country roads, plus several larger isolated areas (range of 2-4 ha). My study site in Kansas included the adjacent lands 0.5 km on either