It begins the last week of February and peaks during the last two weeks of April and first week of May. During this time, diversity is at its height and Rio Grande Village produces the greatest variety of sightings. Many resident species nest as early as April or May even while the neotropical migrants are just arriving or passing through. While surprises are few in summer, many southwestern United States birds, some tropical species, and a couple of Big Bend specialties can be seen with ease. Although fall migration is less spectacular than spring, it lasts much longer, from August through early December, and peaks twice during that time.
Winter is generally mild and species are quite predictable.
Look for greater diversity in ducks, shorebirds, and sparrows at this time. Spend several hours to several days at the Rio Grande Village Campground checking the river, cottonwood trees, and desert habitats. Cottonwood Campground, near Castolon, offers similar habitats, but often yields different species. Ranch are both desert springs that act as hubs of activity for some desert birds. The Chisos Mountains rise in the center of the park to nearly 8,000 feet providing a cooler, moister and more wooded habitat for birds.