NO EVIDENCE FOR INBREEDING AVOIDANCE IN A NATURAL-POPULATION OF SONG SPARROWS (MELOSPIZA-MELODIA)

We studied mate choice and inbreeding avoidance in a natural populatio n of song sparrows (Melospiza melodia) on Mandarte Island, Canada. Inb reeding occurred regularly: 59% of all matings were between known rela tives. We tested for inbreeding avoidance by comparing the observed le vels of inbreeding to those expected if mate choice had been random wi th respect to relatedness. Independent of our assumptions about the av ailability of mates in the random mating model, we found that the expe cted and observed distributions of inbreeding coefficients were simila r. as was the expected and observed frequency of close (f greater than or equal to 0.125) inbreeding. Furthermore, there was no difference i n relatedness of observed pairs and those that would have resulted had birds mated instead with their nearest neighbors. The only evidence t o suggest any inbreeding avoidance was a reduced rate of parent-offspr ing matings as compared to one random mating model but not the other. Hence, despite substantial inbreeding depression in this population, w e found little evidence for inbreeding avoidance through mate choice. We present a simple model to suggest that variation in inbreeding avoi dance behaviors in birds may arise from differences in survival rates: in species with low survival rates, the costs of forfeiting matings t o avoid inbreeding may exceed the costs of inbreeding.