Kentucky Warbler is a secretive Neotropical migrant, breeding throughout the Southeastern forests and migrating to Central America for the winter. It returns northward in April and is highly detectable when singing its loud ringing song in May and June.
As a secretive bird, its migration pattern very much recalls another secretive species, the Wood Thrush. Although they are not closely related (they are in different families), their behavioral similarities drive some similar patterns on these occurrence maps. Both are ground-dwelling birds that are very vocal when singing and territorial (April to July, roughly), but become very hard for birders to find when they stop singing. Finding a migrant Kentucky Warbler or Wood Thrush is a real challenge–they stay in deep woods, don’t call much, and are generally inconspicuous. Conversely, in spring the migrants are often singing and very easy to record on an eBird list if you know their song.
These patterns are obvious in the occurrence maps for both Kentucky Warbler and Wood Thrush.Absent in winter, the spring migration brings a wave of birds northward, with each ensuing week showing the birds farther and farther north. In fall, however, both species just seem to fade away, with no obvious southward progression. The reason for this are the bird’s behavioral changes, which occur when territorial singing stops and they go quiet. Molt occurs at this time too, and the birds change their behavior to be extra-secretive during this vulnerable time. As a result, the occurrence maps show them seeming to fade away, when in fact, they are just getting hard to find. When they actually migrate, they are no easier to find. They do not sing, remain in thickets, and do most of their migrating at night. For birders that live in areas where these species don’t nest, finding one in fall is a real triumph. The result is that while the fall migration involves 2-3x more birds (since it includes recently fledged young), eBirders find many fewer.
The eBird line graphs for Kentucky Warbler give a great representation of detectability changes like this. Open the graph of “total reports” and look at how the 2500-3000 found per week from 1-15 May drops to about 1300 per week in mid-June, and drops lower still to 100-150 in the first half of August. Most are not migrating on this date, this is just their quiet time.
This pattern is mirrored by other secretive birds (other Catharus thrushes, Ovenbird, cuckoos, some Empidonax flycatchers), but notice how different it is for conspicuous fall migrants like Blackpoll Warbler and Western Tanager.
As is the case with some of these maps, there are some anomalies that appear to be caused by the model making incorrect assumptions about where the species occurs. A notable one on this map is the bright spots in west-central Texas that appear in April, with a large one just east of Big Bend appearing in June and July. This area does not get Kentucky Warblers with regularity, and this is certainly a case where a finer-scale model would compensate for these erroneous predictions–but that requires ever more data (the smaller the scale, the more data needed). A similar process may be driving the occurrence shown for the Adirondack Mountains of northern New York–Kentucky Warblers do not occur here either. As the eBird data set continues to grow, we are confident that these anomalies will become rarer and rarer.