Anim. Behav., 1995, 50, 607-621 Patterns and consequencesof egg destruction among joint-nesting acorn woodpeckers WALTER D. KOENIG*, RONALD L. MUMMEt, MARK T. STANBACKf & FRANK A. PJTELKA* *Hastings Reservation and Museum of Vertebrate Zoology, University of California, Carmel Valley tDepartment of Biology, Allegheny College IDepartment of Zoology, University of Washington, Seattle (Received 7 March 1994; initial acceptance I June 1994; final acceptance 7 December 1994; MS. number: A6!#!?R) Abstract. Reproductive competition among joint-nesting female acorn woodpeckers, Melanerpes jormicivorus, culminates in the removal and subsequent destruction of eggs laid by co-breeders. Females destroying eggs gain both by synchronizing egg laying and by having more of their own eggs incubated in the final clutch. Here results are summarized based on observations of 19 joint nests. Egg destruction accounted for the loss of 38% of all eggs laid. Either or both joint-nesting females destroyed eggs at the samenest, and a female that destroyed eggs during one nesting attempt sometimes had her own eggs destroyed during the next. Egg destruction was not moderated by the genetic relatednessofjoint-nesting femalesand was apparently unrelated to behavioural dominance, but substantially lowered per female success of three- compared with two-female nests. Females did not discriminate between eggs; thus egg destruction ended when both females laid normal eggs simultaneously in the nest cavity. However, egg destruction ended most frequently when a female did not destroy an egg laid by her co-breeder 1 day prior to laying her own first egg. Such failures to destroy eggs entail short-term fitness costs that may be compensated by the potential for reciprocity across breeding attempts. Abnormally small ‘runt’ eggs, rarely found in other species of birds, were commonly laid as the first egg of the clutches of one or both joint-nesting females. The laying of runt eggs may allow females to destroy normal eggs laid simultaneously by their co-breeders. Egg destruction is a major constraint on the degree to which females nest jointly. More generally, egg destruction and its associated behaviour patterns demonstrate that reproductive interference can be as complex and destructive within groups of close relatives as between unrelated individuals. 0 1995 The Association for the Study of Animal Behaviour Acorn woodpeckers, Melanerpes formicivorus, are unusual cooperative breeders. Not only do groups contain up to 10 non-breeding nest helpers of either sex, but the mating system is polygynan- drous: within a group, as many as four or more co-breeding males share mates (Stacey 1979; Koenig et al. 1984) and up to three females nest jointly (Mumme et al. 1988). Within each sex, co-breeders are usually either siblings, a parent and one or more offspring, or a combination of the two (Koenig & Mumme 1987). Thus, groups Correspondence: W. D. Koenig, HastingsReservation, University of California, 38601E. Carmel ValleyRoad, CannelValley, CA 93924, U.S.A. R. L. Mumme is at the Department of Biology, Allegheny College, Meadville, PA 16335,U.S.A. M. T. Stanback is at the Department of Zoology, University of Washington, Seattle, WA 98195,U.S.A. are generally composed of close relatives, with only the breeders of the opposite sex being unrelated to one another. Approximately 25% of acorn woodpecker groups contain two or more joint-nesting females that cooperate in various forms of communal behaviour including storing acorns and rearing young. However, joint-nesting females also engage in egg destruction, whereby they destroy their co-breeder’s eggs, usually prior to laying their own. Eggs are removed from nests and deposited on a nearby branch where they are consumed by several different group members, frequently including both the female that laid the egg and the bird that removed it. Removed eggs are not defended from other group members, nor is there aggression among birds eating eggs.These observations suggest that nutritional benefits are 0003-3472/95/090607+ 15 $12.00/O d 1995The Association for the Study of Animal Behaviour 607