Suitable habitat distribution for the Long-tailed Tit (Aegithalos caudatus) as indicated by the frequency of occurrence-a longterm study

Abstract

Data from 22 seasons were used to analyse the occurrence of Long-tailed Tits (Aegithalos caudatus) in relation to the habitat distribution in 26 one km-squares on the island Aasla in the SW archipelago of Finland. The one km-squares most frequently used by Long-tailed Tits had considerably higher proportions of deciduous/mixed (all species) and alder forest, whereas the number of habitat patches in the km-squares were less correlated to bird occurrences. A threshold for the frequent presence of Long-tailed Tits showed at 15-20% deciduous/mixed forest in km-squares in a logistic regression model, which agrees with the level suggested from a study in a Swedish landscape where the total proportion of deciduous/mixed forest was much lower. Further, the occurrence pattern of Long-tailed Tits over time suggested a density dependent relationship . The six km-squares occupied in low density seasons had a mean proportion of deciduous/mixed forest twice as high and a mean proportion of alder forest three times as high as the 20 others . Long-tailed Tits were also present in these six kmsquares in all seasons with higher population densities, as predicted by a density dependent habitat selection. The importance of long-term monitoring, scale considerations and the use of population densities for habitat or landscape assessments are discussed.