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

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Abstract
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.