Each spring, millions of North American birds return from southern wintering quarters in South and Central America, heralding the arrival of spring. Many transient species pass through Alabama on their way to nesting grounds farther north, and they can often be seen in fruiting trees, especially after inclement weather or heavy spring rain, when migrant birds may gather densely in the trees. Warblers are among Alabama's most colorful birds, a diverse group of small insect-eaters ranging from the Yellow-breasted Chat to the Yellow Warbler. Vireos are similar in size but not closely related, with subtler gray, green, and yellow coloring, larger hooked bills, bright eye-rings or eye-stripes, and distinctive calls. Alabama also has two species of tanager, whose males are bright red and females yellowish green, with Scarlet Tanagers identified by jet-black wings and Summer Tanagers lacking that feature. Thrushes are less colorful but are known for their beautiful voices, and five species pass through Alabama during migration, distinguished by subtle differences in breast spotting and facial patterns. These North American songbirds that winter south of the border, known as Neotropical migrants, have declined for several reasons, including habitat loss and fragmentation across their wintering grounds, migratory routes, and northern breeding grounds, the hazards of long-distance migration, and predation by free-ranging house cats.