Hermit Warbler is one of several warblers that breeds in the western mountains and winters in west Mexico. Like several western species, there is a pretty short gap between the spring and fall migrations.
This map is not perfect. One might look at the mid-June frames and think that Hermit Warblers breed in the Arizona mountains; they do not. In fact, their breeding-season range should be limited to the SierraNevada, Cascade, Coast Range, and Olympic mountains, with not much away from those areas. Migrants occur throughout the West (especially in mountains) in April and May and then again in August and September. The general movement through the West is shown well, but the mid-summer distribution pattern is missed.
Two explanations may help to understand why this may is not as good as some others. First, the predicted occurrence for the species is very low. Whenever these maps are dealing with occurrence metrics of 0.004, the results should be viewed with some caution. Hermit Warbler tends to be reported rather infrequently, perhaps due to its preference for underbirded montane areas. Second, its spring migration extends relatively late and its fall migration begins relatively early. The model uses observations from nearby areas and also nearby dates to understand where and when birds are occurring. At such low occurrence metrics, the model is probably drawing on far less data than say, the
model for Scarlet Tanager or Black-throated Sparrow (which have occurrence metrics 10x and 5x greater, respectively). This means that positive observations from several weeks before and several weeks after are factoring in to these predictions for dates in mid-June. As we often say, more data will help to solve this problem (more summer mountain data in particular). As the eBird dataset grows, we’ll be able to take smaller slices in space and time and reduce these sorts of errors. Modeling very common species alongside very rare species with the same model causes some of these
challenges, as well, but this is really the ultimate goal of these maps and not something we plan to change.