BIG BIRDS

Almost 40 species appear in this attractive overview of ``the world's biggest flying animals,'' which begins with a succinct look at the birds' varied conformations and habitats and the effects of large size on their adaptations and abilities- -since, paradoxically, flight requires both light weight and ``strong muscles, which can be quite heavy,'' each species represents some kind of evolutionary compromise. The bulk of the book presents glimpses of representative species: a few salient characteristics are given, accompanied by handsome color photos, both portrait close-ups and shots of the birds in their environments. Size, range, and prevalence or rarity are frequently (but not uniformly) mentioned. A useful and inviting concept book. Index. (Nonfiction. 6-10)