Black-throated Gray Warbler is a dapper black, white, and gray warbler with a wheezy song. It breeds in western evergreen forests, including pinyon-pine/juniper habitat in Colorado, Utah, New Mexico, and Arizona, and taller pines, hemlocks, and Douglas Fir in the Sierra Nevada and Pacific Northwest.
Comparing the spring and fall migration patterns is always interesting, and often reveals some interesting patterns. In spring, migrants tend to rush northward to the breeding grounds. Not only does spring tend to be a quicker and more directed pulse (less protracted), it is also a time that fewer birds occur away from breeding areas. Many Black-throated Grays seem to suddenly appear on breeding areas, essentially skipping over the lowland river valleys, towns, and migrant traps that are often so good for migrating warblers (certainly, one can find some Black-throated Grays in these locations, but relatively fewer when compared with fall). There is a brief window in the spring when they are found in the Central Valley of California, but the better bet for birders in spring is to seek them out on the breeding grounds.
In fall, we see a somewhat different pattern. The migration is longer and there is less structure to the map, indicating that the species is a bit more catholic in its habitat preferences. In September, one could find a Black-throated Gray in the mountains, desert, river valleys, or along the coast. Or at least, the probability of finding one at each of those areas is closer to equal in fall than in spring, where the mountains and evergreen breeding areas are by far the best areas to seek them. The Central Valley and southeastern California are good places to watch for this pattern.
This pattern is replicated by a number of species. One factor is surely the overall larger population of birds moving south in fall (assuming each Black-throated Gray pair produces an average of two young, then the population should at least double). Also, the younger birds are less experienced and more likely to stray toward the coast or into the deserts. Watch for these differences between fall and spring on all the STEM maps, and you will notice that the model is understanding this basic truth of bird migration.