Researchers have long puzzled over why tropical songbirds have fewer offspring than temperate songbirds living at higher latitudes. According to a new Science study, clutch size is a function of chick growth and parental investment.
The life history strategies of songbirds living at different latitudes fall along a slow-fast gradient. Slow strategies are characterized by slow growth, fewer offspring, high parental investment, and long life, while fast strategies are characterized by the opposite. But what drives this pattern remains unclear. To investigate why tropical songbirds raise fewer young, Thomas Martin from the University of Montana analyzed the growth rates of nestlings of 20 to 30 species from each of these three locations: temperate Arizona and tropical Malaysia and Venezuela.
North temperate species, like this Hermit Thrush, often raise four or more young, compared with the typical two young raised by tropical species. Thomas E. Martin
Tropical songbirds typically only raise two chicks while temperate species might raise four or more. Martin found that while tropical and temperate songbirds grow at similar overall rates, tropical nestlings grow longer, more developed wings, and they do this fast too. That way, they don’t end up spending more time in the nest where they’re the most vulnerable to predators. While advantageous, this rapid growth is resource intensive. Nurturing these chicks is a significant investment requiring a lot of provisioning per chick, and the only way this is possible is through reduced clutch size.
Martin thinks that northern temperate species don’t show similarly fast wing growth because there are fewer resources available to them. Instead, they focus on having more, “lower-quality” offspring – which is adaptive since temperate birds tend to die (as both juveniles and adults) at higher rates than tropical birds. Mortality risk does vary with different life stages. Surviving winter months and migration, for instance, can be quite challenging for temperate adults.
"Provisioning, parental investment and mortality are all related," Martin says in a statement. "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."