But at three sites along the Columbia, the herons shared their nesting grounds with bald eagles. Because the researchers hoped to establish a correlation between the two species, they made a special effort to collect eggs at these sites during the same years that another research team was taking samples of bald eagle eggs. Nest selection criteria went beyond shared locations with eagles. The scientists chose heron colonies with large, established populations. Colonies with 50 or more nests, where a sample of 10 to 15 eggs each year likely would not be missed.
Herons nest in tall trees.
To collect heron eggs she enlisted the help of some professional tree climbers and a couple of biologists trained to climb trees. She also collected some herself. During the two field seasons Thomas collected 140 eggs. About half of the eggs ended up in the incubator at a U.S. Fish and Wildlife Service lab, labeled and cataloged by site. Hatching rates varied from site to site.