This week eBird released its all new "Occurrence Maps". These maps, which are called STEM (Spatio-Temporal Exploratory Model) maps, use eBird checklists that report all species and include effort. The location of each checklist is associated with remotely-sensed information on habitat, climate, human population, and demographics. Fine-scale patterns of species occurrence relative to these variables (over 1000) are generated both regionally and temporally, to produce predicted occurrence at some 30,000 locations for every day of a single year (2008 in this case). This massive volume of information is then summarized on maps, which in many cases provide unprecedented information about the annual cycles of North American birds. These maps showcase the power of eBird – year-round, continental-scale monitoring of all species. Each species map is displayed with a text overview of the broad-scale migration patterns, along with an interesting biological story to consider.

Below is the map for Western Tanager:

Western Tanager Occurrence Map. Couresty of eBird

According to eBird:

The model performs exceptionally well for this species, and we believe these maps are accurate representations of where and when Western Tanagers occur in the United States. While some species (e.g., Mourning Dove or Red-tailed Hawk) do not have specific habitat associations, Western Tanager generally prefers pine forests for breeding. They do not breed in desert habitats, so their breeding range in the West is strongly defined by elevation and rainfall, and the tanagers are found in mid- to upper elevations in the Rocky Mountains and Sierra Nevada ranges. The fine-scale accuracy of the model is probably largely due to the fact that the habitat layers used for the models closely match the actual habitat preferences of the species. In general STEM maps do very well with evergreen specialists, like Western Tanager and (in the East) Brown-headed Nuthatch, since one of the key layers defines this forest type quite well. During migration (April, and September - October in particular) Western Tanagers occur in a variety of western habitats, including urban and suburban parks, deserts, and even patches of trees in grasslands.

During the breeding season, the map appears to focus on the montane forests, when the migrants arrive at their breeding grounds. The pressure to arrive at breeding areas and claim territories causes the spring migration to occur relatively rapidly, as compared with the more protracted fall migration. Fall migration in Western Tanager actually begins in early to mid-July, when the first adults return to Mexico. From August to October, migration brings Western Tanagers back to the lowland areas that they use in spring, but notice how much more slowly their overall departure progresses.

Pretty cool stuff! Although this model worked out really well there seemed to be some talk about some species models weren't working out that well. Currently there are only a handful of maps available. They can be seen here.