A breakthrough study published on Friday in the journal Science uses life history theory to explain why tropical songbirds typically raise fewer chicks than temperate songbirds.
WHAT’S LIFE HISTORY THEORY?
Life history theory seeks to explain the ways in which natural selection works to determine a given organism’s anatomy and behaviors. Life history theory suggests that reproductive tendencies, or “life history strategies,” are part of a complex system of species traits, or “life history characteristics.” Parent organisms display reproductive strategies that essentially represent calculated trade-offs based on the anatomical, environmental, and ecological characteristics of their species. (For example, some parents elect not to have another kid because of the first’s socio-economic load).
WE KNEW THEY WERE DIFFERENT, WE JUST DIDN’T KNOW WHY
Ornithologists first documented a geographical variation in songbird clutch size (the number of eggs a bird produces per breeding effort) as early as 1944, when British scientist Reginald Moreau noticed that the good songbirds of England were laying almost twice as many eggs on average (4-6 in a clutch) as the birds he observed during his years working in Africa (2-3 eggs per clutch). Various explanations abound from Moreau’s initial discovery onward as to what exactly accounts for the variation, but none so far have held water with the scientific community for very long.
In this most recent study, evolutionary ecologist Thomas Martin documented life history characteristics of both tropical and temperate songbirds, focusing on closely related species across the latitudinal divide. His team determined that in comparison to the temperate birds, the tropical songbirds grow longer wings, grow them more quickly, and appropriate more resources per egg to nurture their nesting offspring. Tropical songbirds also historically experience greater threats of nest predation than do temperate songbirds.
Martin’s study theorizes that among tropical populations, parent birds produce smaller clutches in order to provide each egg in the clutch with more resources. The increased provision of resources accounts for the differing wing growth patterns, which better equip the tropical songbirds to evade predators and ultimately increase survival rates.
It appears these birds aren’t anti-conformists, after all.
HAND IT TO THE MAN
Others in the field are calling Martin’s study transformative. It represents a large collection of field data that both answers old questions and offers better ways to approach the new ones. Martin uses life history theory to study not just growth in the nesting stage, but also growth and survival tactics throughout fledging and adulthood.
BIRD ENTHUSIAST? SKETCH WHILE YOU BIRDWATCH!
