Removal of Territory Holders Causes Influx of Small-Sized Intruders in Passerine Bird Communities in Northern Finland

were taken in the field from every bird captured. On both plots the removal caused a significant increase in the number of individuals captured and in capture rate (ind/100-net-hours) as compared with the control year results on the same plot. The removal also resulted in a decreasing trend in the course of experiment in tarsus and wing length when detected across all species. These trends became more pronounced after the identified females had been omitted from the data. The results suggest that breeding numbers in northern migrant bird populations are limited by territoriality. Furthermore, it is suggested that northern bird populations, guilds and communities may track environmental variation because of existence of floating populations, flexible breeding strategies, and non-synchronic population declines. Therefore, competition both within and among populations may occur more commonly than usually thought.