DIURNAL, INTRASEASONAL, AND INTERSEXUAL VARIATION IN FORAGING BEHAVIOR OF THE COMMON-YELLOWTHROAT

Foraging characteristics of breeding Common Yellowthroats (Geothlypis tichas sinuosa) on the central California coast varied between early m orning and midday periods, among periods within the nesting season, an d between sexes. From early morning to midday, yellowthroats increased their use of vegetation at heights below 1 m and above 3 m, but reduc ed their use of vegetation at heights of 2-3 m. A midseason increase i n the use of vegetation at heights above 3 m occurred after 3 May. Mal e yellowthroats foraged at significantly greater heights, on average, were more likely to choose flycatching over other maneuvers, and were possibly better adapted, morphologically and behaviorally, to exploit spatiotemporal changes in prey abundance than were females. Singing ma les occurred at greater heights, on average, than either foraging male s or females, but did not differ from foraging birds in their use of s ubstrates or plant species. We evaluated the use of plant species for foraging in comparison with availability within 10 m of foraging indiv iduals and over the entire study area, and found significant diurnal a nd intraseasonal patterns of selection. Common Yellowthroats actively selected arroyo willow (Salix lasiolepis) at both spatial scales of av ailability during all diurnal and seasonal periods. Use of foraging ma neuvers varied intraseasonally but was independent of diurnal period. We found no significant interaction between diurnal and intraseasonal patterns of foraging. Our results suggested that Common Yellowthroats alter their foraging behavior in response to changes in foraging condi tions associated with diurnal and intraseasonal dynamics of freshwater marsh ecosystems. Pooling of foraging data over diurnal or intraseaso nal periods, or between sexes, may mask significant differences that a re important in understanding foraging characteristics.