A new study published in May 2020 in Ecography asks whether seasonal migration influences breeding range size in North American songbirds. Teresa Pegan, a graduate student in ecology and evolutionary biology and her advisor, Professor Benjamin Winger, published “The influence of seasonal migration on range size in temperate North American passerines.”

“Seasonal migration is associated with tremendous flight ability, which may give migratory birds opportunities that other species lack (such as the ability to cross oceans or mountains),” said Pegan. “On the other hand, seasonal migration also has a slim margin of error: failure to follow strict timing and routes in migration can lead to disaster for birds because of the consequences of ending up too far north when a snow storm comes, or too far over the ocean when running out of energy. These different aspects of migration have led to two hypotheses for the effect of seasonal migration on range size: 

“The first hypothesis predicts that migratory birds should have very large breeding ranges. Many species of animals, including poorly-flying birds in the tropics, are restricted to particular regions by their “dispersal ability”: they might not be able to cross mountains or rivers, for example, even if there is a suitable place to breed on the other side. By contrast, migratory birds can fly almost anywhere, so their breeding ranges could encompass all regions with suitable habitat for them. 

“The second hypothesis predicts the opposite: smaller ranges for long-distance migrants, with suitable breeding areas potentially left unoccupied. Even though long-distance migratory birds can fly well, they don’t actually have the flexibility to fly everywhere due to the time limitations associated with migration and dangers of straying from the migratory route. These limitations may select against individuals that explore new regions, reducing range expansion.”

To test their hypotheses, the researchers analyzed the effect of seasonal migration, including the distance of migration, on breeding range size and on how much of each species’ climatically suitable habitat was covered by its range. With the help of U-M Undergraduate Research Opportunity Program (UROP) students Vera Ting and Charles Kotila, they used the collection of North American birds at the U-M Museum of Zoology to measure wing shape, a proxy of flight ability, on over 1000 specimens to assess whether the high flight ability of migratory birds allows them to expand their ranges more than other species. 

Their results did not support either hypothesis: within North American songbirds, migratory behavior is providing neither an opportunity nor a limit when it comes to range size. Other factors, such as the availability of each species’ preferred habitat, may be responsible for variation in range size. North American songbird ranges are also not limited by flight ability: migratory birds that can fly efficiently are not necessarily better at expanding their ranges than non-migratory birds in this region.

“Even though my results did not support either hypothesis for an effect of migration on range size, the opportunities and limits of seasonal migration affect other important evolutionary processes in birds, which I am exploring in my current research,”  said Pegan.

Compiled by Gail Kuhnlein