Tropical songbirds get their growth spurt late

Delayed development helps fledglings survive after leaving the nest

whitehead's broadbill and hermit thrush

MOUTHS TO FEED  Tropical birds like the Whitehead's broadbill (left) tend to raise fewer nestlings at a time than birds in temperate regions, like the hermit thrush (right). The reason may lie in how those babies grow, a new study suggests.

Both: T. Martin 

Scientists have long puzzled over why tropical songbirds lay fewer eggs than their temperate-zone counterparts. A new study suggests that it may have to do with how baby birds grow.

Thomas Martin of the University of Montana in Missoula compared nestling development in 72 songbird species from Arizona, Venezuela and Malaysia. While the Arizona birds grew quickly in the early days after hatching, the tropical birds were late bloomers, only showing speedy growth later on. But this late bout of rapid development gave tropical fledglings an advantage: Their wings grew faster than those of birds in milder climates, leaving them better equipped to escape hungry predators. 

Fueling that extra wing growth means more work for bird parents. With fewer eggs to care for, adults can provide more food per hatchling, Martin reports August 28 in Science

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