Unlike many other woodpeckers that feed on woodboring insects and larvae, Acorn Woodpeckers (Melanerpesformicivorus) feed primarily on acorns, insects caught by flycatching, and sap (Bent 1939, MacRoberts 1970, MacRoberts and MacRoberts 1976). They also supplement their diets with an occasional small lizard (Koenig, pers. comm.), fruit, oak catkins, and wild oat seeds (MacRoberts and MacRoberts 1976). Acorn Woodpeckers are also known to eat eggs that have been removed from two-female communal nests (Mumme et al. 1983). Other unusual predatory behavior includes a single report of predation on a Western Wood-Pewee (Contopus sordidulus) nest (Bryant 1921) and the mutilation of two Red-breasted Sapsucker (Sphyrapicus ruber) nestlings (Shuford 1985). These incidents, however, appear to be opportunistic; the Acorn Woodpecker does not regularly prey on other vertebrates. We report here on an instance when nest predation by Acorn Woodpeckers may have harmed a colony of Cliff Swallows (Hirundo pyrrhonota) nesting in the Stanford Quadrangle at Stanford University, Stanford, California. The colony consisted of 56 mud nests concentrated under overhanging shingles from the roof of the east-facing main archway of the quadrangle. An additional 10 to 15 nests were dispersed throughout the colony. The nests were constructed in corners bordered by roof supports and shingles, ranging from 3 to 8 m off the ground. We observed eight Acorn Woodpecker visits to this colony during the week of 28 May through 3 June 1985, between 08:00 and 09:00. Though attempts at predation sometimes failed when mobbing Cliff Swallows induced the Acorn Woodpecker to flee, in four (50%) of these visits the woodpecker succeeded in stealing an egg from a nest. In another instance, an Acorn Woodpecker also appeared to have taken a nestling. Acorn Woodpeckers were nesting in two palm trees less than 25 m from the Cliff Swallow colony. After removing an egg, the woodpecker would fly towards its nesting site with the egg still intact in its beak. Unfortunately, we were unable to discover what the woodpeckers did with the Cliff Swallow eggs, though ' Received 4 December 1985. Final accepta ce 27 October 1986. SHORT COMMUNICATIONS 177