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.