Oology Cards set #4

On Monday I only entered data for Heermann's Gulls (Larus heermanni). These birds are found through out the continental U.S. and Mexico as well as southern most Canada. 90% of the roughly 150,000 birds breed on a small island in the Sea of Cortez called Isla Rasa. This island was declared a sanctuary in 1962 but not before egg collectors had taken hundreds of eggs. I was entering data from 1925 to 1930 and during this time one man, Griffing Bancroft collected 441 eggs. This may not seem like a lot compared to the total number of breeding pairs on the island which he estimates at as many as 200,000. But, if one considers that he recorded "most pairs [as having] only 1 or 2 eggs" and that he took only clutches of 3 he effectively removed the genetics for large numbers of offspring. Add onto this that each time collectors went to the island they disturbed all the nesting pairs which caused many to abandon their eggs. This cycle of removal of eggs and disturbance of nesting pairs led to a drop in population. The species is currently increasing in number and listed as Near Threatened by the IUCN.
The breeding adults of the species are unlike most other gulls in that their main body color is a sooty grey rather than white. The juveniles and non-breeding adults are solid sooty grey and all ages possess their distinctive bright red-orange beak with a black spot. These birds often pirate the catch of Brown Pelicans (Pelecanus occidentalis).

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