Immigration has been in the news a lot recently and I thought I’d talk, briefly, today about bird immigration. This is a different topic than migration, here we’re going to talk about permanent changes in where birds live as a result of their movements.
This mini-lecture introduces some important concepts.
The idea that the geographic ranges of animals were an interesting subject of study dates back to Alfred Russel Wallace, the co-discoverer of evolution by natural selection. Wallace founded the science of Zoogeography which today has been incorporated into the more inclusive field of Biogeography. Wallace’s extensive studies of the distributions of land animals caused him to divide the world up into six (technically five with one subdivided into two) biogeographic realms. They are: the Neotropics (southern Mexico, Central and South America, and the Caribbean), the Nearctic (the rest of Mexico, the coniguous US, Canada, Alaska, and Greenland), the Palearctic (Europe, most of Asia, and a northern strip of Africa on the Mediterranean coast), the Ethiopean region (since renamed the Afrotropics: the rest of Africa and the Arabian peninsula), the Oriental region (since renamed the Indotropics: the Indian subcontinent, Southeast Asia and the western part of Indonesia), and the Australasian region (eastern Indonesia, New Guinea, Australia, New Zealand and the oceanic islands of the Pacific). The Nearctic and the Palearctic are the most similar to one another in their flora and fauna and are combined into one larger region called the Holarctic.
These regions were defined by looking at where groups of animals are found and finding areas that had similar faunas (and floras). So what determines where a species is found. Simplistically you can boil it down to two elements: ecology and history. Can the species live there and did it ever have a chance to get there? The problems world wide with invasive species indicate that many species can live in a lot of places where they don’t (or didn’t until recently) occur. Organisms are limited in their ability to move around the globe and so their points of origin have a huge influence on where they occur now.
The term vicariance refers to geographical events (e.g. uplifting of mountain ranges, separation of continents, changes of river courses) that split species in two or otherwise change species distributions.
Birds are quite mobile organisms and thus are somewhat less constrained by geography than say salamanders or freshwater fishes. However birds still show geographic patterns of distribution. You don’t see birds of paradise in the Amazon or quetzals in Australia.
Here in the Nearctic we share a lot of bird families with the Palearctic such as nuthatches, chickadees, and finches. Other groups such as the corvids (jays, crows, etc) occur basically worldwide. However we have a lot of immigrants from the Neotropics that provide a lot of color to our more temperate habitats. Our warblers, hummingbirds, and tanagers are all basically tropical birds that are slumming it up north. Less colorful (mostly) but an important component of the our summer are the flycatchers. These are also neotropical birds.
We also have immigrants from other areas as well. The Horned Lark and the pipits are representative of Palearctic groups that ‘crossed the pond’ to make homes for themselves here. The norther Fulmar is even more impressive. This is a bird that resembles a gull and lives in the northern oceans. Its closest relatives all come from the far southern oceans.
However if we go even further back in time we find that the vast majority of our birds originally came from somewhere else. The study cited below by Per Ericson looked in detail at the distribution of the ‘land birds’. This group does not include all land birds (for example it does not include doves, hummingbirds, swifts, nighthawks, any of the chicken like birds, or ostriches and their relatives) but it includes the great majority of them that form a single evolutionary group (i.e. they can all be traced back to a common ancestor that isn’t the ancestor of any other birds). Ericson found that these birds could be divided into three groups based on their shared evolutionary history and biogeography. His conclusion was that one group (songbirds, and parrots) originated in Australasia, a second group (falcons and seriemas) originated in South America and the rest (owls, hawks and eagles, both groups of vultures, woodpeckers and their relatives, kingfishers, trogons, etc.) all originated in Africa. Thus although the Neotropics are today the area with the most bird species relatively few groups of birds originated in South America. And all of the ‘land birds’ came from the Southern Hemisphere!
Ericson, Per G. P. Evolution of terrestrial birds in threecontinents: biogeography and parallel radiations. Journal of Biogeography (J. Biogeogr.) (2012) 39, 813–824
So the take home message is that every organism on earth can trace their lineage back to somewhere other than where they live now if you go back far enough in time. We are all immigrants. Below are videos of birds from different parts of the world. Think about where their ancestors orginated.