It turns out that the shapes of birds may be predictable. Scientists have taken a closer look at birds that live on islands and have found that these creatures follow a predictable trend in evolution.
"The search for general trends in evolution of animal shape, size and color, often comes up empty," said Natalie Wright, one of the researchers of the new study from the University of Montana. "Evolution tends to be unpredictable, leading toward different forms in different places. So it was gratifying to discover this trend among island birds."
Many types of birds become flightless after they colonize islands that have no predators. In fact, flightlessness has evolved more than a thousand times, though it tends to evolved from only certain types of ancestors. In this case, the researchers wanted to see whether there are predictable evolutionary trends that apply to all island birds.
In this latest study, the researchers focused on how flying birds evolved on islands. More specifically, they looked at whether these birds had similar changes to those seen in flightless island species.
So what did they find? Flying birds evolved smaller flight muscles and longer legs on islands, which is what occurred in flightless birds. However, it occurred to a lesser extent. On islands with fewer predators, this trend was more pronounced.
"I would be shocked if a hummingbird or tanager or fruit-dove ever lost the ability to fly entirely," Wright said. "It's central to how they make a living. But these birds are still reducing the size of their flight muscles when relief from predators allows them to do so."
The researchers found that a richness in raptor species and mammalian predators best explained variation in flight-muscle size and leg length. In other words, predation actually drives the pattern.
"Without predators selecting for the maintenance of large, powerful flight muscles, birds can save energy by reducing these muscles," Wright said. "Instead, they develop longer legs as they rely more heavily on leg-powered locomotion. These longer legs may act as a longer lever as the birds jump during takeoff, making up for their smaller muscles."
The findings are published in the April 2016 edition of the journal Proceedings of the National Academy of Sciences.
