PATTERNS OF ADOPTION IN RING-BILLED GULLS - WHO IS REALLY WINNING THE INTERGENERATIONAL CONFLICT

Although at variance with Darwinian theory, adoption of unrelated offspring has been reported in a variety of ground-nesting colonial seabirds. Pierotti and Murphy (1987, Anim. Behav., 35, 435-444) suggest that an inter-generational conflict exists between disadvantaged chicks and unrelated adults, a conflict that often results in wandering chicks successfully soliciting investment from foster parents. Of 47 ring-billed gull, Larus delawarensis, chicks that departed their natal broods over four summers of field work, 34 remained in recipient broods for 3 or more days and received parental care (i.e. were successfully adopted), and 13 remained in recipient broods for less than 3 days ('runners'). Chicks in broods that were subsequently subject to abandonment were fed at a significantly lower rate (on a per chick basis) than chicks in control broods (which neither adopted nor had a chick depart) or broods that subsequently adopted a chick. In most cases, adoptees were older than the oldest resident chick in the accepting brood, but runners were younger than resident chicks. Pairs whose oldest chick was less than 7 days were more likely to adopt foreign offspring than pairs with older chicks. Twenty-six (76%) of the 34 adopted chicks survived to at least 24 days (fledged). Pairs that adopted a chick realized a lower annual breeding success (chicks fledged/pair) than control pairs or pairs that had a chick depart their brood. Disadvantaged chicks that gain permanent acceptance into foreign broods 'win' the inter-generational conflict at the expense of the foster parents.