Functional Roles in Mixed-Species Foraging Flocks: A Field Manipulation

Abstract

the winters of 1995-1996 and 1996-1997 in woodlots within the agricultural landscape of Union and Delaware counties, Ohio. These topographically flat deciduous forest fragments consisted primarily of oaks (Quercus spp.), ashes (Fraxinus spp.), shagbark hickory (Carya ovata), sugar maple (Acer saccharum), and American beech (Fagus grandifolia). Each woodlot was completely surrounded by cultivated fields, lacked any connection to other woodlands, and was so small that it contained only one mixed-species flock. During the study period, such flocks were never observed to cross open ground into neighboring woodlots. In addition to the species mentioned above, the study woodlots usually contained one or two Red-bellied Woodpeckers (Melanerpes carolinus) and occasionally a Hairy Woodpecker (Picoides villosus) or Brown Creeper (Certhia americana). Sixteen woodlots were used in the experiment. Before each field season, four of the eight woodlots to be used in that year were randomly assigned to the control group and four to the parid-removal group. Control woodlots averaged 5.3 _+ SD of 2.2 ha in size, and removal woodlots averaged 5.5 -+ 2.1 ha. Each woodlot was used only once. From mid-December to mid-January each year, we trapped and mist netted birds attracted to feeders and wire-mesh suet cages. All Downy Woodpeckers and White-breasted Nuthatches were fitted with USFWS bands and uniquely colored plastic leg streamers for individual identification. Chickadees and titmice in control woodlots were similarly marked, but those in treatment woodlots were removed and released approximately 50 km away in suburban Columbus, Ohio. Observations of flocking behavior took place during two-hour visits to woodlots conducted between 0800 and 1600. To balance sampling across time of day and season, we alternated visits to treatment and control sites. Each woodlot was visited 10 times, or until new parids began arriving in the treatment woodlots in late February during the juvenile dispersal period (T. C. Grubb pers. obs.). During each visit, we assigned the location of individually marked birds to one of the 25 x 25-m blocks that formed a grid that covered the entire area of the woodlot.