Biological Conservation 36 (1986) 115-141 Importance of Area and Habitat Heterogeneity to Bird Assemblages in Temperate Forest Fragments Kathryn E. Freemark & H. G. Merriam Biology Department, Carleton University, Ottawa, KIS 5B6 Canada A BSTRA CT Relationships among area, habitat heterogeneity and bird assemblages were examined for 21forest fragments (3 to 7620 ha) in an agricultural landscape near Ottawa, Canada. Habitat heterogeneity within forests was measured by an index of spatial rariability (HH) in plant species and forest structure. HH based on tree species or shrub species increased with forest size. Larger and more heterogeneous forests had more spec&s and pairs of birds, according to multiple regressions for two summers combined. Number of species and pairs of birds at individual points within forests increased with greater HH but not with larger size. Birds also were more patchily distributed in more heterogeneous, but not necessarily large,forests, because of relatively uncommon species. Larger forest size was more important for increasing species number in forest-interior and resident-related classes of birds. Some classes, e.g. forest interior species, were infrequent in smaller forests. Habitat heterogeneity was more important to edge-related classes. To maintain a diverse forest avifauna, regional conser~'ation strategies should maximise both size and habitat heterogeneity of forests. INTRODUCTION Throughout the world, extensive forests continue to be fragmented as population pressure converts land to agricultural, industrial, suburban and urban uses. As forests become more limited, preservation of many 115 Biol. Conserv. 0006-3207/86/$03'50 © Elsevier Applied Science Pubiishers Ltd, England, 1986. Printed in Great Britain