Male plumage colouration predicts female reproductive investment and nestling survival in a colour dimorphic tropical songbird

In sexually colour dimorphic bird species, males can exhibit phenotypic variation, with males breeding in either dull female-like plumage or brightly coloured plumage. Two contradictory hypotheses predict that the male phenotype variation can influence the female investment in a given breeding attempt. Whereas females usually prefer males in bright coloured plumage, the “differential allocation hypothesis” predicts that females should invest more in their reproductive output when mating with them; while the “compensatory investment hypothesis” predicts that females should invest more when mating with non-preferred males. To test those predictions, we analysed reproductive data for two consecutive breeding seasons (2018–2019 and 2019–2020) of Lined Seedeaters Sporophila lineola. S. lineola is a socially monogamous songbird species in which males exhibit two breeding phenotypes, a black-and-white plumage being the most common, and a less common female-like brownish plumage. Our findings show that females mated with brownish males have a higher reproductive investment (i.e., egg volume) than those mated with black-and-white males. Despite the lower investment of females in egg volume, our results showed that black-and-white males have a higher nestling survival, producing more hatchings and fledglings per season than brownish males, which could suggest a higher fitness. Our findings indicate that investment allocation on the eggs and offspring survival rates can be attributed to the plumage colour of males.