SPECIATION THROUGH TEMPORAL SEGREGATION OF MADEIRAN STORM PETREL (OCEANODROMA-CASTRO) POPULATIONS IN THE AZORES

Madeiran storm petrels Oceanodroma castro breed on three small islets in the Azores: Vila, off Santa Maria, and Praia and Baixo, off Gracios a. Analysis of data on brood patch, incubation periods, chick body siz e and recaptures of adults provides evidence of the existence of two d istinct populations (hot- and cool-season) breeding annually on Baixo and Praia, out of phase by four to five months and overlapping in colo ny attendance during August and early September; on Vila only the cool -season population is present. Analyses of adult morphology indicate h ighly significant phenotypic differentiation between the sympatric hot - and cool-season breeders, whereas an almost complete phenotypic unif ormity characterizes allopatric breeders within the same season. The h ot-season birds are 10% smaller in egg and body mass but have longer w ings and tails than cool-season birds. The two groups were readily sep arated by discriminant analysis. The preference to breed in the cooler season is interpreted as a consequence of greater food availability i n that period. Morphological differentiation between seasonal populati ons is interpreted as an adaptative response to different environmenta l conditions in the two seasons. The hypothesis is given that the hot- season population has evolved from the cool-season population owing to density-dependent constraints on crowded colonies, forcing birds to t ime-share nest sites. These populations may represent a case of sympat ric speciation through temporal partitioning of reproduction and may b e better treated as sibling species.