Birdwatching--in Texas wild-life refuges--is the sometimes-intriguing, often-tedious background for this generally shaky mystery debut. Primary bird person: Sarah Deane, who comes to Texas, on vacation from her Boston teaching job, to meet fellow-teacher Philip Lentz--an ardent birder and a possible fiancÃ‰. But Philip never shows up. And soon his strangled body is found in the brush of the Donna Clara refuge by 13-year-old Jeff Goldsmith, a wunderkind traveling with his aunts on ""Ready Tours."" So the sleuthing begins, with local Lieutenant Perez aided by Sarah and by Dr. Alex McKenzie, a friend of Philip's who happens to be in the area. (He's investigating the illegal traffic in Celluwell, a would-be cancer cure.) Another corpse surfaces; there are conflicting reports on bird-sightings; young Jeff is in peril. But despite an easygoing style and the brisk opening chapters, the pace sags badly as bird lore and Sarah's psychological problems take over--with the so-so mystery fading away. Heaven for birders, then, but a bit of a bore for anyone else.