Many tropical species of birds have expanded their ranges northward in the past 40 years. GCBO Research Coordinator John Arvin, who has spent many years in northeastern Mexico and the Rio Grande Valley of south Texas, recently summarized some of these range extensions and speculated on possible causes.
The basic distributions of northeastern Mexican birds, and the birds of south Texas, were established in the early 20th century, with little change between the late 1800's and the 1950's. As a reference point in this discussion, we'll use the El Cielo Biosphere Reserve located at approximately 23° north latitude in northeastern Mexico. Examples of species that have very recently extended their ranges northward into this region include Double-striped Thick-knee, Montezuma Oropendola, Sungrebe, Black-crowned Tityra, and Solitary Eagle. Other species such as Keel-billed Toucan and Emerald Toucanet may be vagrants or pioneering colonizers. At the same time, some montane species like Spot-crowned Woodcreeper and Tufted Flycatcher seem to retreating upward in the Sierra Madre Oriental. They were once common breeders at 1300 m. and cannot be found below 1700 m. presently. Species which formerly reached their northern limits in the El Cielo region such as Boat-billed Flycatcher, Ivory-billed and Olivaceous Woodcreepers, and Melodious Blackbird are now found northward to the headwaters of the Rio Corona at 24°. The Rio Grande Valley, which forms the Texas-Mexico border, is at 26°. Some major extensions that have carried species beyond the bounds of the region in question began early in the 20th century with species like Great-tailed Grackle, Inca and White-winged Doves, and Cave Swallows, which are now found northward into southwest Louisiana. By the 1970s and 80s species like Clay-colored Robin, Hook-billed Kite, and Altamira Oriole, once considered rare vagrants, had become regular breeding residents in the Rio Grande Valley. In the last several years, more and more Mexican species have been recorded in the Valley, including Social Flycatcher, Crimson-collared Grosbeak, Blue Mockingbird, and Golden-crowned Warbler. It is worth noting that colonization can take place very rapidly as in the case of the Tropical Kingbird and Mangrove (Yellow) Warbler which have built substantial populations and, in the case of the kingbird, has spread widely through the area in less than a decade. Finally, many species which were considered at the northern limits of their ranges in the LRGV around 1960 now occur commonly as far north at the edge of the Edward’s Plateau at about 29° N. Numerous examples include White-tipped Dove, Green Jay, Great Kiskadee, Buff-bellied Hummingbird, Couch’s Kingbird, and Ringed Kingfisher.
A number of hypotheses have been proposed to explain this phenomenon, but all have major flaws except the strong positive correlation with increasing global temperatures. While it is not currently possible to state with certainty that these changes in bird distribution are a result of Global Warming (or a combination of warming and habitat destruction in Mexico), the timing and correlation are interesting. In the mean time, head to south Texas for your next birding trip - you never know what might show up....
Bill & John
(Ringed Kingfisher photo by Greg Lavaty)