One of the most striking aspects of bird biology is migration. While birds are certainly not the only animals to migrate they are, by far, the most obvious to us. Given the current furor about human movement around the world I thought a discussion of bird migration would be timely. Also I found an interesting, if frustrating paper on the evolution of migration in raptors.
First a bit of introduction. It’s important to distinguish between migration and dispersal, terms that are not always used consistently even within biology. In an ecological sense migration refers to a regular movement between areas (i.e. move from A to B and then back to A). Dispersal refers to one time movement from one place to another.
There are two main types of migration in birds: latitudinal migration and altitudinal migration. Altitudinal migration is a common phenomenon in tropical mountains where birds breed at one elevation and spend the non-breeding season at another elevation. We are more familiar with latitudinal migration where birds breed at high latitudes (closer to the poles) and winter at low latitudes (closer to the equator). Latitudinal migration allows birds to use resources for breeding in a habitat in which they could not survive year round.
You can further break down latitudinal migration into short and long distance migration. Robins are a good example of a short distance migrant. Their range shifts south in the winter but there is substantial overlap between their breeding range and their wintering range. A barn swallow is a good example of a long distance migrant where the birds leave North America completely and there is a substantial geographical gap between the breeding range and the wintering range. Another way to think about this is that short distance migrants are often wintering in similar but warmer habitats than those in which they breed whereas long distance migrants are wintering in completely different habitats.
Of course like many dichotomies the distinction between long and short distance migrants doesn’t fit all cases, particularly those involving water birds. Birds like the arctic tern and the sooty shearwater winter and breed in similar habitats but in opposite hemispheres. In contrast the breeding habitat (lake) and wintering habitat (ocean) of a loon might only be separated by a short distance in some cases.
Some northern birds are irruptive, meaning that they migrate south in the winter to different degrees in different years. Snowy owls are among the more spectacular of the northern irruptive species.
Migration is an extremely stressful time for birds as they often have to fly long distances without feeding. Migratory flyways follow geographic features such as coastlines. Migratory birds can become confused and end up far out at sea. A more modern hazard is that humans tend to build large cities on coasts. Many birds fly at night while migrating and collide with buildings.
Which leads us to the question: what factors have favored the evolution of migration. I’m going to talk about the paper:
Life history traits, bioclimate, and migratory systems ofaccipitrid birds of prey (Aves: Accipitriformes), which was published in the Biological Journal of the Linnean Society* last year. The authors are: JENŐ NAGY1, ZSOLT VÉGVÁRI and ZOLTÁN VARGA.
*The Linnean Society was where Darwin and Wallace’s idea of evolution by natural selection was first publicly presented, in 1858, the year before The Origin of Species was published. Although Darwin and Wallace were credited as the authors of the paper neither was at the meeting where it was presented. Wallace was in Southeast Asia and unaware of the event until after the fact.
The authors state that there are two different hypotheses for the origin of migration. The ‘tropical origin’ hypothesis states that sedentary tropical birds evolved migratory behavior to exploit temperate habitats. The ‘northern origin’ hypothesis states that migration is driven by changes to breeding ranges in temperate birds. This study is a large scale comparison across members of the Accipitriformes, an order of birds that includes the hawks, kites, eagles, and old world vultures across three different areas that include a major migratory route: North and South America, Europe and Africa, and eastern Asia.
They used the scientific literature to gather data on almost two hundred species in these areas including whether they were migratory or not, their clutch size (number of eggs), wing and tail length (indication of flying abilities), foraging mode, diet, habitat in which they were found, and so on. They then performed a big and very complex statistical analysis to see which factors were associated with being migratory or not.
I described this paper as being frustrating above because it is an interesting topic and approach but I felt the author’s presentation of their results left much to be desired. In some cases this is a language issue. The authors are Hungarian but in this case it wasn’t that the language was hard to follow but rather that it was that it was very vague and didn’t really give the reader the meaty details. I’ll try to make my presentation more satisfying.
The factors that were most strongly associated with a particular species being migratory or not were distribution (tropical or temperate breeding range) and continent (in which of the three biogeographic regions mentioned above they were found). The former is not particularly surprising — temperate birds are more likely to be migratory. The latter reflects differences in hawk migration across the three areas which will be explained below.
The study then analyzed each of the three regions separately to see which factors were most closely associated with migratory behavior. In both the new world and in Europe/Africa the two factors that were most important were clutch size and hunting strategy (hunting for prey in the air, on the ground, or both). Clutch size for migratory birds in known to be generally larger than for non-migratory birds. This is thought to be an adaptation to the lower survival probability for migratory birds. In other words you should have more offspring if you migrate because fewer will survive.
Frustratingly they never say what the pattern is for hunting strategy but I am assuming that birds that hunt flying prey are more likely to migrate.
They also mention (but don’t provide data) that wing and tail length proportions differ for migratory species between the two regions which they relate to differences in the wintering habitat (more grassland in Africa and more forest in South America).
In east Asia clutch size is also strongly associated with migratory behavior as is bioclimate (which they never define properly). The major difference between this region and the other two is the absence of major barriers (like the Gulf of Mexico, the Mediterranean, and deserts) leading to more short distance migration.
Despite its flaws I think this is a valuable paper in that it points out how the same phenomenon can be driven by different factors in different places. In the new world and in Europe/Africa land-based migrants face major barrier to getting into the tropics. So you may see a split between species that just migrate within the temperate zone and species that undertake really long migrations. In Eastern Asia this spit into two groups may not exist and bird species may undertake a much more continuous range of migratory distances.
Migration is an issue of conservation concern. To preserve species habitat must be protected in multiple locations. Migration is a source of high mortality and increasing the size of migratory barriers by removing habitat on migratory routes may increase that mortality. So this kind of work is important in understanding how we can protect birds over the long term.