Diel rhythms of tick parasitism on incubating African penguins.

1. Ticks (Ornithodoros capensis Neumann) were most abundant on incubating host African penguins (Spheniscus demersus Linnaeus) at 24.00 hours and least abundant during 09.00-12.00 hours during 4 day periods in May and October at 3 h intervals. 2. Ticks were three times as abundant in May, the start of the breeding season, as in October, at its end. 3. Air temperature and humidity appear less important than light levels in determining tick activity. 4. The degree of tick parasitism of breeding seabirds is best studied at night.