Willow Flycatchers are late spring migrants, and the animation shows their return in very late April and subsequent surge into the country in May and early June. Departure for the wintering grounds is similarly early, with the first southbound migrants in early August.
Empidonax flycatchers, like the Willow Flycatcher, are among the greatest identification challenges in the United States. Although we strive for high data quality at eBird, it is impossible to weed out all identification mistakes. Fortunately, when we see very accurate range, patterns of arrival, and departure data, we can be confident that even for species with comparatively high error rates, eBird is valuable and shows real biological patterns.
On the animation, notice the earlier departure from the East. Comparing the eastern and western patterns of migration, Willow Flycatchers have overall much higher occurrence away from breeding areas in the West. (Watch southern California for example.) Arrival on the breeding areas in the East is rather sudden, with little signal of passage migrants south of breeding areas. Fall migration in the East is even more invisible, and the breeders seem to gradually disappear from breeding areas by late August without a noticeable signal south of breeding areas. Migration patterns would be more clear with more eBirders looking for landbirds in July and August. In the West, by contrast, Willow Flycatcher is one of the more common migrants in both spring and fall and the September and October passage is obvious. Whereas eastern records are almost nonexistent in October, the species is still regular in California during the first half of the month. To confirm that this pattern is not a modeling artifact, see the October grid map. Even in September, Willow Flycatcher is fairly rarely encountered in the East, while it is much more common in the West. Again, you can confirm this with an eBird grid map for September.
These animated occurrence maps are great for highlighting these regional differences, since there are few other places where you can directly compare eastern and western migration patterns on the same map. In the case of Willow Flycatcher, these patterns of occurrence probably correlate well with the different subspecies that have different migration schedules. The eBird grid maps for July provide a good sense for where the breeding season observations of Willow Flycatcher are coming from, and it can be seen that there are population centers in the Northeast and in the Pacific Northwest states. Empidonax traillii traillii of the Eastern US and Great Plains appears to arrive suddenly in May and depart suddenly in late August. The Northwestern forms E. t. brewsteri and E. t. adastus probably account for the bulk of the passage along the West Coast, which is substantially later in fall. In all likelihood, these differences in timing also relate to birds heading for different wintering grounds.