Limits on egg predation by Richardson's ground squirrels

Abstract

To test the inference, arising from circumstantial evidence, that Richardson's ground squirrels (Spermophilus richardsonii (Sabine, 1822)) frequently depredate eggs of greater sage-grouse (Centrocercus urophasianus (Bonaparte, 1827)), gape size was measured and the response of free-living squirrels to three sizes of eggs was observed. Maximum gape measured on carcasses was 26 mm and functional gape assessed from tooth imprints in artificial clay eggs was 17 mm. Squirrels left imprints in 46 of 110 clay eggs, but whether tested with domestic fowl (Gallus gallus (L., 1758)) or ring-necked pheasant (Phasianus colchicus L., 1758) eggs that approximated the maximum width of sage-grouse eggs or with much smaller Japanese quail (Coturnix japonica Temminck and Schlegel, 1849) eggs that approximated maximum gape, no squirrels (28 adults and at least 28 juveniles) spontaneously depredated eggs, even after multiple exposures. When re-tested with damaged eggs, 15 of 16 adult females scavenged contents, though usually not on their first exposure. After scavenging damaged eggs, 2 of 12 squirrels opened a few intact eggs, but only quail eggs and usually only if the shell was rough. Although Richardson's ground squirrels are potential scavengers of large damaged eggs and likely they could depredate small eggs, the inference from circumstantial evidence that they are major predators of greater sage-grouse eggs remains unsubstantiated.