Heron rookery locations and numbers of pairs observed in these rookeries 

 during aerial censuses are shown in table 16. Rookeries v/ere located in mature 

 cottonwood stands in the interior of islands and along edges of stands which 

 occurred on the main banks. The total breeding populations in these rookeries 

 were approximately 222 pairs in 1975 and 176 pairs in 1976. Two new rookeries 

 were discovered in 1976, but three rookeries attended in 1976 were abandoned 

 in 1976. Part of a rookery near the mouth of Armells Creek was destroyed 

 in May of 1976 when high flows eroded the bank where it stood. 



TABLE 16. Aerial censuses of great blue heron rookeries on the lower Yellowstone 



River in 1975 and 1976. 



a Rookeries observed for the first time in 1976. 



^Bank erosion destroyed most of this rookery between May 3 and May 25. 

 c 0n June 24 this rookery was unattended. 

 On June 7 this rookery was unattended. 



Dates of first herons in rookeries varied. The earliest rookery attended 

 in 1975, based on observations recorded during aerial censuses, was near 

 Pompey's Pillar and contained herons on April 1. The first date that eggs 

 were observed in nests in 1975 was April 22, and the first young were observed 

 on aerial censuses in late June. Based on an incubation period of 27 days 

 (Vermeer 1969), an egg-laying interval of two days, and an average clutch size 

 of five eggs, nests initiated in the third week in April would hatch in early 

 June. Some incubating hens were observed in early June 1976, but the first 

 nests had hatched by that time. Brooding at the nest occurs in June and early 

 July. Young herons may leave the nest for the first time at 7.5 weeks of age 

 (Vermeer 1969). Young from clutches initiated during the third week of April 

 could leave the nests by mid- to late July. 



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