Extinction and colonization of birds on habitat islands

We used point-count and transect surveys to estimate the distribution and a bundance of eight scrub-breeding bird species in 34 habitat fragments and t he urban matrix in southern California. We then calculated local extinction and colonization rates by comparing our data with surveys conducted in 198 7. We classified factors that influence extinction and colonization rates i nto two types: (1) extrinsic factors, which are characteristics of the habi tat fragments such as area, age, and isolation and (2) intrinsic factors, w hich are characteristics of the species that inhabit fragments, such as bod y size and population density. Over the past decade, at least one species w ent locally extinct in over 50% of the fragments, and local extinctions wer e almost tu,ice as common as colonizations. Fragment size and, to a lesser extent, fragment age were the most important extrinsic factors determining extinction and colonization. Density indices of scrub bird; were the most i mportant intrinsic factors determining extinction rates, predicting the num ber of sites occupied, the probability of local extinction, relative area r equirements, and time to local extinction.