Why should Greylag Goose Anser anser parents rear offspring of others?

Abstract

The benefits and costs of rearing large broods were studied in a population
of neck collared Greylag Geese in Scania, southernmost Sweden.
Families were divided into four groups according to brood size: 1-4, 5-8,
9-12 and >12 young. Gosling survival showed a significant positive relationship
with brood size on arrival at the rearing area. First-year and
third-year local survival, as well as breeding recruitment, showed a significant
positive relationships with brood size at fledging. For adults
rearing large broods, no costs were detected during the year following
that when they fledged >8 young; both survival rate and reproductive
success exceeded the long-term averages.