Habitat constraints and spatial bias in seabird colony distributions

What governs the size and location of seabird colonies has long intrigued p opulation ecologists. Previous analysis of the distribution of colonies of four European seabirds revealed a spatial bias - large colonies occurred fa rther apart than expected by chance alone - suggesting that intraspecific c ompetition for food supplies during breeding may regulate colony size. Here we use computer intensive statistics to show that a similar spatial bias e xists in three burrow-nesting seabirds - Cassin's anklet Ptychoramphus aleu ticus, rhinoceros auklet Cerorhinca monocerata, and ancient murrelet Synthl iboramphus antiquus - nesting off the Pacific coast of Canada. Local habita t constraints explain much of the existing spatial bias: large colonies can not fit on small islands, and large islands suitable for colonies tended to occur far apart. However, a residual spatial bias still remained for ancie nt murrelets and Cassin's auklets (but not rhinoceros auklets) after habita t constraints are built into the analysis, for which intraspecific food com petition remains a plausible explanation.