A University of Montana professor who studies birds around the world recently discovered why tropical birds tend to have fewer offspring that seem to grow slower and live longer, slower lives than their northern counterparts. To put it simply, he discovered that it's all in the wings, according to a release.

Thomas Martin, assistant leader of the U.S. Geological Survey Montana Cooperative Fish and Wildlife Research Unit at UM, studied with other scientists the growth and net predation of 20 to 30 co-existing songbird species in Venezuela from 2002 to 2008, and in Malaysia from 2009 to 2014. They also studied songbird nests in Arizona for the past 28 years. Their findings were recently published in Science. 

Tropical songbirds were observed to only raise two young, on average, while temperate species commonly raised four or more. However, more does not always mean better. The researchers found that tropical songbirds grow their wings faster since they are fed more frequently by their parents, who have less offspring to tend to than their temperate counterparts. According to the release, Martin explained that that directly relates to how well offspring can escape from predators both in the nest and after they leave it.

"A bird species' risk of death among life stages, together with growth strategies of young, is a pivotal basis for a major leap forward in understanding latitudinal variation in life history strategies of songbirds," Martin said in a statement. "Provisioning, parental investment and mortality are all related," Martin said. "A later, faster growth spurt of tropical songbirds, together with higher parental effort invested per offspring, aids wing growth and flight capabilities after the young birds leave the nest."

Temperate birds also face the dangers of cold temperatures during winter months and migration--so, ultimately, tropical songbirds are more likely to survive. Martin suggests that this research explains the balance of traits passed on from generation to generation, the release said.

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