The effect of summer cottages on land bird numbers was studied in the
archipelago of SW-Finland. Bird fauna was very similar on cottage and
non-cottage islands, with the following exceptions: The Wagtail Motaci
lla alba was significantly (P < 0.05) more abundant on cottage islands
. The Oystercatcher Haematopus ostralegus and the Willow Tit Parus mon
tanus were nearly significantly (P < 0.10) more abundant on non cottag
e islands. In the grouping of species based on nesting habits, birds n
esting in nest boxes and other man-made structures were more abundant
on cottage islands. The mean species richness and the Shannon-Wiener d
iversity index H' were significantly greater on cottage islands than o
n non-cottage islands. However, if the data are combined and treated a
s one sample for cottage islands and the other for non-cottage islands
, the differences in species richness and diversity between island gro
ups disappear.