Data from: Pair complementarity influences reproductive output in the polymorphic black sparrowhawk (Accipiter melanoleucus)

How multiple morphs are maintained within populations of colour polymorphic bird species remains a challenging question in evolutionary ecology. In some systems, differential productivity or survival between morphs are thought to play a role. Here we examine key demographic parameters between the two discrete adult morphs that characterise the polymorphic black sparrowhawk Accipiter melanoleucus. Using long-term breeding and survival data from a population on the Cape Peninsula, South Africa, we test for differences in reproductive performance between light and dark morphs, both in isolation and in combination with their partner morph and adult survival between morphs. We found that neither morph had a specific advantage in terms of productivity or survival. Despite this lack of difference between the individual morphs, we did however find that morph combination of adult pairs influenced productivity significantly, with mixed-pairs producing more offspring per year than pairs consisting of the same morph. The body condition of the offspring showed the opposite relationship, with nestlings of mixed-pairs having lower body condition than nestlings of like-pairs. While our results suggest an advantage of mating with the opposite morph, there was no evidence for disassortative mating; instead breeding pair morph combinations were random with respect to the background frequencies of the two morphs. Higher productivity of mixed-pairs may be the result of the complementary nature of care provided by the different morphs. We propose that differential foraging success between black sparrowhawk morphs under varying light conditions allows mixed-pairs to expand their foraging niche. We conclude that emergent pair-level properties may play an important role in promoting and maintaining polymorphism and may be important for other bird species which display bi-parental care.