Eastern Bluebird is a vibrant and much-adored thrush that has now recovered from a low population dip of the mid-1900s. Nest box programs throughout the species’ range have driven much of its rebound, and it is now common in most areas east of the Rockies. Partially migratory, it breeds into Maine and southern Canada, but withdraws southward in October and November, returning north in March and April.
There are some similarities between this map and the Red-headed Woodpecker map. These are fun to notice, and to connect with your experience in the field. Quite often it is true that areas with Red-headed Woodpeckers–both in southeastern pine woods and prairie watercourses–also have Eastern Bluebirds.
Missing from this occurrence map is the small population of west Mexican Eastern Bluebirds that sneaks into southeast Arizona. We know these Arizona bluebirds are being reported to eBird, since they show up on our grid maps. In this case, one of two things could be going on. First, it may be that the model is simply failing to understand the biological factors that predict Eastern Bluebird occurrence in southeast Arizona. This could be due to our habitat layers (which are rather coarse, but will get better in coming years) and also may relate to the scale at which predictions are made. Currently, predictions cover a relatively large area (9 x 12 degrees) in order to have enough data for the model to perform well. These blocks overlap (to mitigate the effect of the large scale), but in some cases the occurrence patterns in one part of the species’ range may incorrectly drive the occurrence pattern in other parts of the species’ range. As we get more data and begin looking at finer and finer scales, we expect the models to perform better with small isolated populations like this one in southeast Arizona. But for now, this is a known issue with some outlying populations.
Second, it may be that the model does predict Eastern Bluebird occurrence in the Sky Island mountains of southeastern Arizona, but predicts the occurrence to be so low (2.5% or less) that it does not show up on this map. In general, these maps are scaled to show us where most of the higher occurrence areas are. If it were scaled more liberally, to show areas of <1% as orange, it might give the impression that the species occurs there when it really doesn’t. The scale we decided upon is set automatically, but shows the species at an occurrence scale that fairly closely matches what field guide range maps show. This is not necessarily the “ideal” scale to show, since it certainly depends on what question you are asking. For example, to show the full extent of the range, we might want to set a more liberal scale (which may reveal the southeast Arizona population, but may also reveal some erroneous extrapolation into other areas where bluebirds do not occur). On the other hand, a more conservative scale would better show the core areas where bluebirds are truly common, but would lose the edges of the range where the species is rare but regular. Anytime we consider bird ranges, it is worth considering that there is no right or wrong answer to where the boundaries are drawn: bird ranges are dynamic and variable. Even your field guide is inconsistent in where and when small outlier populations are shown or ignored.
Please remember some of these caveats when looking at these maps and spend some time thinking about what it actually means to define a species range. It is not as easy as it seems! What is the regular range of Clay-colored Sparrow, for example?