Life History Trends and Migratory Behavior of Warblers in West-Central Kansas

Standard mist-netting procedures were used to assess trends of migratory warbler populations in west-central Kansas. Four species were selected based on their high capture frequency in historical banding records. Orange-crowned Warblers (Vermivora celata), Nashville Warblers (V. ruficapilla), Yellow Warblers (Dendroica petechia), and Wilson's Warblers (Wilsonia pusilla) were compared in frequency distribution, timing, sex class patterns, age class patterns, and migratory behavior. The banding station at Fort Hays State University (active from 1966 to 1992) was reconstructed and birds were netted during autumn migration in 1996 and 1997. Current data (1996-1997) were summarized with historical records (1966-1986) to evaluate warbler population trends. The total combined data set consisted of 3,280 individuals. Of 287 warblers captured in 1996 and 1997, only six were recaptured, suggesting most individuals did not stay in the sample area for more than 72 hours. Frequency distribution of the four warbler species fluctuated over time with well -defined high and low points, but without clear patterns in population numbers. Orange-crowned Warblers were captured in the greatest numbers and appeared to be the most common migrating warbler on the study site. Most warbler species were captured in September with the exception of Orange-crowned Warblers, which were netted almost exclusively in October. Available sex class data showed that adult males were captured more often than females for both Nashville and Wilson's Warblers. Age class comparisons exhibited a definite pattern with juvenile birds captured in considerably higher numbers than adults. This trend was most apparent in the historical banding records. In general age classes of all four warbler species seemed to migrate synchronously. Warbler migratory behavior can be classified as fallout, based on comparisons between daily weather patterns and fat deposits in captured birds. Migrating warblers probably do not select the riparian woodlands along Big Creek as stopover habitat, but may be forced down in response to physical stress. High capture days were usually associated with cold fronts and precipitation. During the onset of cold fronts, associated with a decrease in barometric pressure, target species were consistently captured with visible fat deposits. Given the relatively high levels of stored fat, other factors such as weather may stimulate birds to interrupt migration in a fallout pattern. Warbler migratory behavior in the interior portion of the continent may be an episodic phenomenon controlled by factors other than fat depletion.