California Gulls and Exotic Eggs

Abstract

The recent paper by Sugden (Condor, 49, 1947:93-96) referring to exotic eggs in nests of California Gulls (Larus caZifornicus) suggested the recording of some of my own experiences with these birds.The purpose of this paper is to indicate more fully the range of the California Gulls' feeding impulses during the nesting season.In July of 1947, while conducting field work in Idaho, I had an opportunity to see at first hand the unusual habit of gulls bringing other birds' eggs to their nests.Through the generous cooperation and assistance of Mr. W. Rodgers, manager of the Deer Flat Refuge in southwestern Idaho, I was able to observe a large California Gull colony (six to seven thousand pairs) on a small island at the east end of the Deer Flat Refuge.The eggs were nearly all hatched, and there were hundreds of downy young scattered over the entire nesting site.It was only a matter of a few minutes until the gulls more or less accepted my presence and began sorting out their young.Adult gulls arrived continuously with food, most of which was a mass of regurgitated material that was almost unrecognizable.In two instances, however, bits of rodent remains were apparent.As I watched, an adult suddenly dropped to within five yards of me and began feeding a ' threeor four-day old chick.To my surprise a Cinnamon Teal (Quwqueduh cyanopera) egg was regurgitated intact (see fig. 23).The moment that the egg rolled out upon the ground the young began pecking at it, evidently having learned the appropriate response in past feedings of a similar nature.The adult struck the egg two or three times with its bill and the contents were quickly eaten by .bothadult and young (see fig. 24).I soon discovered that other birds' eggs were being brought into the colony and fed to the young, although the old birds were eating them, also.The eggs brought in were mostly those of Cinnamon Teal, Ring-necked Pheasant (Pkasianus cotchicus), Coot (Fulica americana), Black-necked Stilt (Himantopus mexicanus), and Eared Grebe (Colymbus nigricoZZis) .Fifteen eggs in all were seen, but the amount of yolk stains around the nesting sites indicated an extensive egg collecting campaign on the part of the gulls.Eggs were sometimes carried in the bill, but more often they were swallowed and then regurgitated at the nesting site (see fig. 25).The gulls are naturally colonial in their habits, and their subsequent response to competition for limited nesting sites is extremely vigorous to the point of being detrimental to the welfare of their own progeny.A great many young birds up to a week or so old were found dead on the island.In many cases fatalities were due to the vicious treatment received from adult birds.An adult would not tolerate.another young bird other than its own to enter its local sphere of territorial influence.During such defense of the territory, the adult birds responded as vigorously to a four-day old chick as to another adult.On numerous occasions this resulted in critical injury to the young.Such vigorous response of the adults has been attributed to a nervous irritation caused by a