Does competition with residents affect the distribution of migrant territories?

Abstract

Herrera (1978) found an inverse relationship between relative densities ofresident and migrant passerines in European breeding bird assemblages. He hypothesized that the decreasing proportion of migrants to the south would be due to interspecific competition with residents. The putative competition between resident titmice (Parus montanus and P. major) and migrant conguilders (Fringilla coelebs and Phylloscopus trochilus) was tested by a playback experiment in northern Finland. If competition was important between individual birds, it could reduce overlap between territories . The spatial configuration of the two migrant species was studied in relation to the nests of the two residents during the breeding season in late May - early June, in 1989 and 1990. The experiment was carried out at 13 (year 1989) and 17 (year 1990) tit-nest plots. To control habitat effects, the tape was also played at control plots where the tit pair had bred in the previous year and at randomly chosen plots. The data were analysed using log-linear models, which test for dependencies between categorical variables . In 1990, there were far fewerpositive responses at the study points than in 1989, possibly due to changes in the population densities of the migrants. We found that migrants did not avoid settlement near or in resident territories . There was a tendency that migrants aggregated more at the vicinity of tit-nests compared with the random- and control plots. This result is consistent with the `heterospecific attraction' hypothesis generated by Mönkkönen et al . (1990) .