Foraging time activity study of breeding songbirds in a successional white spruce community

Abstract

I investigated the amount of time spent on foraging by birds in a successional white spruce community, during semi-monthly time blocks, in the context of flying, singing and other activities. Five hypotheses were tested: 1) cumulative amounts of time spent on foraging and other activities by birds vary throughout the breeding season and among species; 2) bird species differ in the mean amount of time spent per event on different activities; 3) birds spend more cumulative time in the study area foraging on spruce than on other plants; 4) birds capture more prey on spruce than on other substrates; and 5) prey capture rates differ among species. All bird actions on the study area that were observed with binoculars were counted, timed and recorded. Data for White-throated Sparrow, Song Sparrow, Lincoln’s Sparrow, Clay-coloured Sparrow and Savannah Sparrow were analysed. Cumulative foraging times among periods were not significantly different in 1991, but differed for White-throated Sparrow in 1992. Mean foraging times differed only in late July 1992 for White-throated Sparrow and Clay-coloured Sparrow. Foraging, except for Savannah Sparrow, occurred mostly on white spruce, but use of tree species differed among periods in both years. Spruce budworms were most commonly taken between 15 June and 15 July in 1991 and 1992. Prey capture rates differed significantly among bird species in late June 1991, in late June 1992 and early July 1992. Therefore, each hypothesis was supported, but not in each time period. Such variations in bird activities during the breeding season reflect breeding stage and changing ecological conditions. 1 concluded that sparrows foraged similarly when spruce budworm was most easily obtained. These results were obtained only by the method of dividing the breeding season into time blocks, and by recording time durations rather than counts of events. I recommend that including variables in time as well as space be the new norm in studies o f avian life history dynamics.