A flock of Rio Grande turkeys

Just west of this precipitation line is where Rio Grande populations begin to thrive.  These turkeys occupy the portions of the state that receive from 16 – 33 in. of annual rainfall, thus inhabiting central Texas.  While Rio Grande wild turkeys exist exclusively in areas with less annual rain fall than do their Eastern counterparts, they show tremendous population growth during years of above average precipitation, and why shouldn’t they?  Wetter conditions result in increased forage supply, better nesting cover, and reduce the negative effects that hot dry ground can have on egg temperature (Rio Grande eggs often over heat during drought years).  Why then can only Eastern wild turkeys survive in East Texas while Rio Grande distribution remains limited to the much drier conditions of central and west Texas?  What is so different about these birds?  At first glance they almost appear to be the same bird and their behavior patterns in many respects mirror each other.  They both forage in the same way, eat the same types of foods, breed at the same time, have similar clutch sizes, prefer to run during daily locomotion instead of fly, roost in the tallest tree they can find, nest in cover that is tall but not too tall to see predators, etc.  You get my point; they’re both turkeys.

While the two subspecies share too many similarities to list, lets analyze some of the subtle contrasts that exist between the two turkeys in search of an answer to this query, starting with general appearance.  To the casual observer a Rio Grande and an Eastern turkey may look like the same kind of bird, but visual differences do exist.  Generally speaking, Rio Grande wild turkeys are comparatively pale and copper-colored, exhibiting slightly more elaborate coloration than the Eastern wild turkey.  While the coverts (short feathers that cover the longer feathers below them) of the Eastern wild turkey are a chesnut brown the coverts of a Rio display more of a tan or yellowish buff coloration.  Rios show off a pinkish iridescence on the tips of their breast feathers contrasting the black tipped breast feathers of the Eastern turkeys.  While these coloration differences most likely developed as a result of the separate regions the two subspecies inhabit, it would be a stretch to hypothesize that this variation is a major cause of the Rio Grande wild turkey’s inability to establish a viable population in regions historically inhabited by Eastern wild turkeys and vice-versa.  I find it much more plausible (with respect to physical variations) that the body shape and size differences between the two subspecies may help explain why the two subspecies are geographically separated into such abrupt regions of specific average precipitations.  An adult Eastern wild turkey is typically larger, weighing 3 to 4 pounds more than an adult Rio Grande. While the Eastern wild turkey supports up to 22% more overall body weight than the Rio Grande, the Rios have longer legs.  The disproportionately long legs of the Rio Grande is one of the more distinguishable characteristics of the  subspecies that no doubt relates to several of the behavioral contrasts between Eastern wild turkeys and Rio Grandes.

The Rio Grande wild turkey is known for traveling distances of up to 60 miles from winter roosting grounds to nesting areas.  Often described as nomadic, Rios have very distinct winter and summer home ranges.  They travel long distances in search of very specific roosting sites generally comprised of large branching trees along river bottoms which can be few and far between in some western portions of their range.  As a bird with an affinity for traveling extensive distances via terrestrial locomotion, its elongated legs and light frame must come in handy, whereas the Eastern wild turkey has little need for such adaptations.  Easterns typically utilize the same area for several years, possibly traveling only a few miles between roosting, nesting, and brood-rearing sites.  Rio Grandes further exhibit behavioral contrast from their Eastern counterparts by forming large groups of several hundred birds during the winter roosting period, while Eastern wild turkeys form much smaller flocks to settle in on roost sites that are much less specific than the roosting sites of Rio Grandes.  While Eastern turkeys do prefer to roost in trees near water, these trees can be near a swamp, pond, lake, bayou, or just a muddy area prone to flooding. They show little preference to trees adjacent to one type of water body over those adjacent to another.  Conversely, Rios show a strong affinity for large roost trees with sprawling branches located along river bottoms.  These behavioral differences suggest adaptations to the separate habitats they live in.