National parks and reserves 54 Identification 44 Botany 33 Birds 31 History 30 Natural history 27 Juvenile literature 23 more ... Management 21 Ecology 20 Congresses 16 Wild flowers 15 Birds of prey 14 Reptiles 14 Description and travel 13 Environmental aspects 13 Handbooks, manuals, etc 13 Mammals 13 Trees 13 Wildlife conservation 13 California condor 12 Endangered species 12 Indians of North America 12 Pictorial works 12 Research 12 Insects 11 Wilderness areas 11 Wildlife management 11 Historic sites 10 Natural resources 10 Amphibians 9 less ...