Do male hoots betray parasite loads in Tawny Owls?

Bird song structure may honestly reveal the health and vigour of individual males to potential mates and competitors. If this is the case then song ma y reflect the level of parasitic infections in males. We initially examined the relationship between blood parasite infections and the time taken to r espond by 22 male Tawny Owls to a broadcast hoot. We then examined the call structure (total length and frequency) in relation to parasite infection, an index of owl condition and an index of food abundance. Owls with higher parasite loads responded more slowly to an intruder, although this relation ship was not significant once condition and vole abundance were controlled for. We found no relationship between call length and any of the measured v ariables. However, the high frequency and the range of frequencies used in calls decreased with increasing parasite load. Thus, there was the potentia l for individuals to assess male parasite load from the speed of response a nd the structure of the call. Experimental tests of these relationships are now required.