Abstract Aim To study densities of eight groups of resident forest bird populations across Europe to examine their association with abiotic (temperature, precipitation) factors.
Methods Densities of residents (and migrant birds, which were used as a control group) were extracted from published breeding bird censuses. For each census location we obtained geographical co-ordinates (latitude and longitude), temperature and precipitation variables describing both breeding and non-breeding seasons. Resident densities were first examined separately in relation to co-ordinates and principal component axes, which were extracted from temperature and precipitation variables. The relative impact of each explaining factor (co-ordinates and principal component axis) on resident densities was checked by using residual examination and partial correlation.
Results Densities of resident birds were in general negatively correlated with both latitude and longitude and temperature and precipitation factors, but latitude and temperature proved to be the strongest individual factors along which resident densities varied. The higher the latitude or the lower the temperatures, the lower were the densities of most resident birds. Partial correlation analysis suggested that of those two factors, latitude was the dominant one. Both the density of resident and migrant birds decreased towards the north, but the decrease of residents was steeper, and in the north they comprised only a small fraction of breeding bird numbers, whereas further south they commonly comprise half of the breeding bird numbers.
Main conclusions The best explanatory factor for resident densities was latitude. For this reason, it was difficult to separate the relative effect of each individual factor, because latitude partly describes all the original variables (temperature and precipitation). The results suggested that precipitation was of less importance and that the interplay between latitude and ambient temperatures was not the sole factor behind the strong effect of latitude. We suggest that the coupled effect of harshening climate and decreasing amount of available energy with increasing latitude is limiting the population sizes of resident birds. We also propose that the effect of those factors is most apparent in northern Fennoscandia, where resident densities are suggested to be lower than would be expected on the basis of summer time carrying capacity.