Intraspecific nest parasitism and anti-parasite behaviour in swallows, Hirundo rustica

Intraspecific nest parasitism was relatively frequent (overall 16·5% of 261 nests) among swallows, Hirundo rustica, during four consecutive years. Parasite eggs were laid by neighbouring females. Those females, but not their mates, visited strange nests during the egg-laying period of the female owner. Parasite eggs were laid during and immediately after the egg-laying period of the parasitized female. Parasitism tended to be frequent in years with large populations and was more frequent in large colonies (of 17–32 pairs) than in groups of 1–4 pairs. The same females were parasites more frequently than expected during both first and second broods. Parasite females had more fledglings per breeding attempt than either non-parasite hosts or females not being parasitized. Females guarded their nest sites less during egg laying than during pre-laying and incubation because of their large feeding demands when producing eggs. Nest guarding rates of colonial females were significantly higher during and following egg laying than those of solitarily nesting swallows. Parasitized females had lower nest guarding rates than other females. Parasite eggs were usually evicted by female nest owners during pre-laying, but never during egg laying and incubation.