PARASITE LOAD REDUCES SONG OUTPUT IN A PASSERINE BIRD

Secondary sexual characters may have evolved because they allow females to assess the ability of conspecific males to resist parasites. In this study the prediction that the song output of swallow, Hirundo rustica, males will reflect their parasite load was tested. Male swallows sang more if they nested in colonies and were reproducing early during the season, and if their nests held few or no haematophagous tropical fowl mites, Ornithonyssus bursa. In one experiment the mite loads of swallow nests during the laying of the first clutch were manipulated and the song output of males during the fertile period of their mates prior to both the first and the second clutch was recorded. Song output did not differ between parasite treatments for first clutches (i.e. before treatment), but did so for second clutches, song output being inversely related to parasite abundance. In another experiment both mite loads of swallow nests and clutch sizes were manipulated simultaneously during the laying of the first clutch. There were no differences in song output between experimental groups for the first clutch, but mite load and the clutch size-mite load interaction affected song output during second clutches. It is likely that song output was reduced by the mites because, as a result of the anaemia they cause, they will reduce the working capacity of their hosts. © 1991 The Association for the Study of Animal Behaviour.