Tiffany Thio & Indra Boving
BIO 342: Animal Behavior
Evolution of Tern Migration and Migration Routes
Questions of Migration
Why did terns evolve to follow the migratory paths they do?
Migration is thought to be a strategy which evolved for the purpose of maximizing an animal’s fitness in seasonal environments, and will change based on physical, ecological, environmental or geographical factors (Alerstam et. Al.). The more important of these factors are seasonality and the consequential spacial change in resource availability, and it is thought that birds and other long-distance migrators, like the whale, follow the patterns of resource availability to the limits of their capability for travel.
A study in 2012 used tiny geolocators (less than 2 grams in weight!) to track 11 Arctic terns in their migration from different breeding populations in Greenland and Iceland. By mapping the flight paths of the terns, scientists were able to observe that terns target areas where there is high marine productivity during stops in their flight (Fig. 2). This supports the hypothesis that migration routes evolved to follow resource availability.
The same study also showed that Arctic terns fly in order to maximize their use of global wind systems in order to reduce energy loss on their long flights.
How else did the migration routes of birds evolve?
Evolution of migratory birds and their migration routes is not constrained by distance, but is thought to be constrained by ecological barriers. Crossing an ecological barrier, like moving form an arid region to a more wet one, is shown to be minimized in bird migration routes. This is thought to be because it minimizes the more complicated adaptations needed to cross a barrier (just like we have to adapt our bodies when moving from a cold to a hot place by taking off layers of clothing). Arctic birds will effectively chose the most favorable exit and entry points in an ecological region as they pass through it, and will not necessarily chose the shortest distant between breeding and feeding grounds. This was done by digitally mapping information on bird migration from many sources using GIS technology, and then analyzed for connectivity with attention to ecological regions (Figure 1, right), (Henningson et. Al.).
Sources:
Henningsson, Sara S., and Thomas Alerstam. “Barriers and Distances as Determinants for the Evolution of Bird Migration Links: The Arctic Shorebird System.” Proceedings: Biological Sciences, vol. 272, no. 1578, 2005, pp. 2251–2258. www.jstor.org/stable/30047815.
Alerstam, Thomas et al. “Long-Distance Migration: Evolution and Determinants.” Oikos, vol. 103, no. 2, 2003, pp. 247–260. www.jstor.org/stable/3548160.
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Images:
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http://2.bp.blogspot.com/-5nKSCR9_XZc/UA-zKqIkukI/AAAAAAAAAI0/cIzb3u3V6FE/s1600/Arctic-Tern-04.jpg