Nine years (1989/90-2002) of data on a asynchronous pelagic seabird, the White-tailed Tropicbird (Phaethon lepturus), breeding on Aride Island, Seychelles, were used to examine: (1) annual variation in productivity and, (2) the importance of large scale (El Niño Southern Oscillation) factors in explaining productivity levels. In 2001 and 2002, we examined the influence of local scale factors (parasitism by hard ticks Amblyomma loculosum and intra- and inter-specific competition for nest-sites) on productivity. Productivity between years only ranged from 0.21 to 0.37 chicks/breeding pair, but it was nevertheless significantly and inversely correlated with the El Niño index. No influence on productivity was caused by local factors. The frequency distribution of chick deaths varied significantly with chick age in 2001 but not in 2002. Most of the chick deaths were from starvation, in particular for chicks older than 20 days. As opposed to synchronous seabird species, there were no years with total breeding failure in the White-tailed Tropicbird. This may be partly explained by their dispersed feeding in the open ocean.
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Vol. 28 • No. 4