Lazy Bustards

When it comes to great bustards (Otis tarda) in central Spain, it seems to be better to be a lazy bustard than a migrating bustard, according to a study published recently in Conservation Biology. Scientists found that migratory bustards have a higher mortality rate than their sedentary counterparts—in some cases 3.5 times higher—and the sedentary bustards have increased their numbers quite substantially over the last 15 years.

Carlos Palacín and Juan Alonso have studied a large population of bustards since 1997, and the breeding groups they observe consist of both migratory and sedentary bird. When they started, male migratory birds were dominant, making up 86 percent of all males. But the team was concerned about the threatened birds because migrating animals are in trouble in general. “That some of the most spectacular migrations have decreased or disappeared due to human activities has led some to believe that animal migration is an endangered phenomenon that could eventually disappear,” according to the new paper. Is this true of the bustards? From 1997 to 2012, the scientists tagged almost 200 male birds to find out.

The team knew that some male bustards migrate to escape the summer heat and that the choice to migrate depends on a number of things—environmental and genetic factors, and given the birds’ intelligence, social learning plays a role, too. In fact, during their juvenile stage—the first three years of their lives—males may experiment with migrating, but once mature, they choose one way or the other. If they become migratory, they will not adjust their routes once they mature, and these birds travel an average of 90 kilometers (56 miles) each season. In contrast, the sedentary males move an average of four kilometers (2.5 miles) each year.

The team discovered that collisions with power lines were the leading cause of death in the bustards, and that migratory birds naturally were more at risk—over three times as much. In addition, the sedentary birds’ population increased over time, from 17 percent in 1997, to 45 percent in 2012. The migratory birds fell from 86 percent from 1997-1999 to 44 percent of the population in 2006-2010. Unlike other migratory species, the team found no correlation in reduced migration with temperature increase or habitat destruction. “Therefore,” they write, “we conclude that casualties at power lines may be inducing a change in the migration pattern of great bustards.”

The Academy’s bird expert Jack Dumbacher explains, “What is interesting is that the species is actually EVOLVING—and that non-migratory behavior is spreading, both directly through natural selection, but also behaviorally through learning. And the selective agent is mainly human power lines—which are killing the migratory birds as they migrate. So could it be that the things that cause anthropogenic deaths to migrants (windows, power lines, and more) are actually strongly selecting against migration—rather than selecting for birds that can avoid the hazards.”

While it may appear to be beneficial being a lazy bustard, this could hurt the birds’ population in the long run, whether through increased competition for resources, gene flow, or more. The scientists recommend altering the power lines to make it easier for the large birds to maneuver around them. This could save all of those bustards, lazy and migratory alike!

Image: Wadofglue/Wikipedia