A bird in the hand of an unlicensed hunter may devalue those in the bush, fostering lawlessness and disrupting balance in nature's animal world. This is Mr. McCoy's message and it's by no means disguised in this story of the important jobs done by America's professional conservationists. He uses every tool at his command to stress the urgency of wildlife protection, from opening with a suspenseful detection episode to repeating himself persistently, from honoring the officers' great dedication to showing horror photos of desecration, from enjoining everyone's cooperation to slinking away from problems of ethics--and this last is something of a disappointment. For conflicts do exist re the conservation of birds of prey and the farmer's right to defend his stock and crop, re the sacrifice of one specier the preservation of another, re the rule that ""sleuths"" must track offenders in their distinctive orms despite the consequent camouflage difficulty. Yet the book is surely knowledgeable and out-compasses most conservation juveniles: chapters include the looters and the laws, protectors and managers, air-land-water poachers, the universal responsibility factor, and a small glossary and complete index. There is a tendency to talk down and the majority of photographs and illustrations are gratuitous; otherwise its competence is unequivocal. McCoy's service here is as well-intended and much appreciated as his other books have been.