Shifting from outdoor to indoor breeding: house martin's (Delichon urbica)defence against house sparrow (Passer domesticus)

House martins (Delichon urbica) usually breed in colonies on the outside wa lls of buildings, but in recent years they also begun to use inside walls. In the present study, we examined why house martins changed their breeding habits during 1983-1996 in western Poland. House martins breeding inside bu ildings were significantly less parasitized by house sparrows (Passer domes ticus) than those breeding more typically, on outer walls (P = 0.02). The m ost probable reason for this is that house sparrows have some difficulty to fly inside buildings effectively. In some cases, the percentage of nests t aken by sparrows inside buildings may be lowered due to human removal of sp arrow nests. Therefore, we concluded, that pressure from house sparrows cou ld be a reason for the change in house martin breeding habits, namely its s hift from outdoor to indoor breeding.