Load-lightening in cooperatively breeding birds and the cost of reproduction

Helpers at the nest of cooperatively breeding birds often ‘lighten-the-load’ of the breeders, by allowing them to decrease their effort. When the shape of the relationship between survival cost and provisioning effort is concave-up then a helper incurs only a small increment in mortality, for a given amount of help, but the breeder benefits from a larger fall in mortality. A convex-up relationship for pre-breeders may explain why some groups (such as larids) do not breed cooperatively. Load-lightening could provide significant extra gains to the helper's future indirect fitness by increasing the lifespan and life-time reproductive success of related breeders.