Experimental evidence of 'floaters' in two isolated populations of an Afrotropical forest bird

Abstract

We studied the presence of 'floaters', i.e. non-territorial individuals roaming occupied territories awaiting future breeding opportunities, in two separate White-starred Robin (Pogonocichla stellata) populations at the Taita Hills, Kenya, using a removal experiment. Previous vegetation analyses at this site had showed that 'population Ngangao' occupied a high-quality forest fragment, and 'population Chawia' occupied a low-quality one. Using a combination of radio telemetry, dawn calls, visual observations and targetted playbacks in one 16ha plot per fragment, 52 individuals of different territorial status were identified. Ngangao hosted 14 resident and 13 floating males, while Chawia hosted eight residents and two floaters. Population Chawia (but not Ngangao) hosted unmated territorial males. Floating males in Ngangao took over three of seven (i.e. 43%) vacated territories within a few days of the residents' removal. In both populations, no resident neighbouring males expanded their territories or moved to take over the vacant territories following experimental removal of resident males. These results suggest that (1) territorial behaviour is an important factor limiting the number of breeders in Population Ngangao; (2) other factors such as poor habitat quality and female scarcity, most likely associated with the lower bird densities in Population Chawia, can limit numbers of breeders before territorial exclusion becomes a concern.