Parental investment and quality of insurance offspring in an obligate brood-reducing species, the American white pelican

The last-hatched chick, or B-ofispring, of American white pelican* typically survives only as "insurance" when its elder sibling fails. Life-history theory suggests that parents should invest relatively less in these disadvantaged insurance offspring. For an insurance strategy to be effective, however, reduced investment may be constrained by the need to maintain potential insurance offspring in a viable condition until at least 5-6 days of age, after which they are rarely needed. In agreement with the lifehistory prediction, egg size, resultant hatching mass, and growth rates at two-chick nests were significantly lower for B-offjpring. When hatched in the laboratory, B-eggs were also slightly but significantly less efficient at converting egg size into hatching mass. Despite these differences, B-chicks that were reared as singles, free from sibling competition from nan-hh-ig onward, showed no decrement in survival or growth rate. When A-chicks were removed from nests with underweight 3or 6-day-old B-chicks, a minority (21 %) of B-chicks failed to recover, but mean growth rates of survivors increased rapidly to control levels. Results suggest that although parental investment in B-offspring is reduced, it is usually adequate to produce and mai"*'" potential insurance offspring in viable condition during the time that they are most likely to be needed as replacements for failed elder siblings. Kty wonts: insurance offspring quality, parental investment, AUatnus trythrorhynthos. [Bthav Ecol 8:378— 383 (1997)]