For the first time in recorded history, Yellowstone National Park trumpeter swans added no young to their flock last summer. The decline in cygnets parallels a decrease in the adult population from almost 500 last year to 277 this year. Ruth Shea, of the Idaho Fish and Game Department, believes a major cause is competition between trumpeter swans that summer in Canada and those that stay year-round. When the more numerous Canadian birds winter on the Yellowstone region's geothermal lakes, they may spread disease and exhaust feeding grounds. To alleviate crowding, Idaho state biologists are trapping Canadian swans in the winter and trucking them to warmer areas to the south and west. But Yellowstone ornithologist Terry McEneaney says moving the birds could add to the problem. Biologists may inadvertently trap resident swans, which look similar to Canadian swans. Life isn't easy for resident swans; they face harsh winters, lead poisoning, competition with mute swans introduced on ranches around the park, as well as flooding and human disturbance of nesting sites. The Park Service recently outlawed fishing with lead sinkers and erected signs and barriers to keep the public away from nesting areas. For more information, contact the Idaho Fish and Game Department at 208/334-1931 or the National Park Service at 307/344-7381.