With their famous flight, male blue-black grassquits are sometimes known as "johnny jump ups".

These leaping Lotharios display to females by flying vertically, beating their wings at high speed, rotating their bodies and exhibiting their white underwing patches. This is all usually done while calling and each leap is completed in about a half a second.

But sexual signals in grassquits can be placed under three main categories: leap display, song and plumage colouration.

And it was the importance of the latter that this study, published in the journal Animal Behaviour, highlighted.

“Males that possess higher ultraviolet and blue colouration in their plumages provided food less often to their offspring compared to poorly ornamented males,” said lead researcher Pedro Diniz from the Laboratório de Comportamento Animal at the University of Brasilia, Brazil.

The blue-black grassquit (Volatinia jacarina) is a small bird in the tanager family Thraupidae, and is found across Central and South America.

Currently there is evidence that sexual signals in these birds, which are indicative of male quality, are costly to produce and influence female choice.

“Previous studies with grassquits have shown that bluer males, in terms of coverage or colour reflectance of their plumages, are high quality males. These are males that are usually older thus more experienced, in better body condition, and less infested with ectoparasites compared to less ornamented males," Mr Diniz explained.

“Although we still do not have conclusive evidence that females exhibit a preference for male plumage colouration in grassquits, we suggest that females may consider this quality when choosing a mate.” 

It seems that, at least in grassquits, colouration is more important than song and motor display for parental care decisions.

The discovery that fine-feathered males made worse fathers in terms of providing food was not that surprising to the team because in socially monogamous bird species, as Mr Diniz explained, attractive males tend to pursue extra pair copulations instead of investing in parental care.

However, according to Mr Diniz, there were some unexpected findings relating to parental investment in raising the young.

Making a song and dance

Recent studies suggest that dynamic signals, for example leap displays, should be more important than static ones such as plumage colouration for female mate choice. But the team's findings indicated that song and leap display had no influence relative to paternal or maternal care.

“We were surprised to find that song and leap display were not effective predictors of how much a male or a female should feed their offspring.

“It seems that, at least in grassquits, colouration is more important than song and motor display for parental care decisions,” said Mr Diniz.

And particular note was made by the team of how females behaved in relation to male colouration.

“Interestingly, female parental effort was high when they were paired to males with high blue-black plumage coverage but at the same time, females did not compensate for the poor job of their bluer partners with high UV and blue reflectance,” Mr Diniz told BBC Earth.

The team hope to further study the complex roles of multiple and multimodal sexual signals in blue-black grassquits.

“We need more studies to clarify which is more important for grassquit mothers when deciding how much to invest in their chicks: the quality of their partners as dads or their attractiveness as partners, which can be inherited by the male offspring,” Mr Diniz said.

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