|Do Green-tailed Towhees (Pipilo chlorurus) and Spotted Towhees (Pipilo maculates) use habitat in the same ways?
July 7, 2003
Birds are some of the best adapted creatures on this planet. They live on land, in the water, and in the air. They eat seeds, fish, and other animals. There must be a reason for these differences. One reason is competition.
If all birds ate only insects, they would deplete the resources very rapidly and begin to die off because the insects would not have a chance to reproduce. This, however, does not always happen. Birds have found ways to reduce competition by evolving different feeding habits. They take advantage of all the resources and, therefore, make it possible for the food sources to recuperate.
At Black Canyon of the Gunnison National Park, we noticed that there were not only the widespread Spotted Towhees, but also the more habitat-specific Green-tailed Towhees. Because such closely related species would compete heavily with each other, we inferred that they must have slightly different feeding habits. We were unable to find any other studies on these birds, but we did find an article on Brewer’s Sparrows (Spizella breweri) and Sage Sparrows (Amphispiza belli) including much of the same type of data. From what we found, we made a hypothesis that the Green-tailed Towhee stays closer to the ground and in more open habitat than the Spotted Towhee, which tends to stay higher in the shrubbery and in denser brush.
Materials and Methods
To test our hypothesis, we spent two mornings observing Spotted and Green-tailed Towhees in the scrub around our campsite. The dominant woody plants in the area are gamble oak (Quercus gambelii), serviceberry (Amelanchier alnifolia) and mountain sagebrush (Artemisia tridentata vaseyana), and the dominant non-woody plants are needle-and-thread grass (Stipa comata), cheat grass (Bromus tectorum L.), and lupine (Lupinus spp.). Birdlife in the area includes Plumbeous Vireo (Vireo plumbeus), Warbling Vireo (Vireo gilvus), American Robin (Turdus migratorius), Dusky Flycatcher (Empidonax oberholseri), and Yellow Warbler (Dendroica petechia), as well as the two Towhee species we researched. Other dominant wildlife in the area includes Golden-mantled Ground Squirrel (Spermophilus lateralis), Mule Deer (Odocoileus hemionus), and Least Chipmunk (Eutamias minimus).
When a towhee was spotted, we marked the exact location with flagging (red for Spotted Towhee, pink for Green-tailed Towhee). Sightings of the same bird after it had flown to another location were not included. After the bird had been scared away, another bird in the same area could be counted. However, when we scared it to record its location, its next resting place was not recorded. Next, we recorded the plant species that the towhee was spotted in. Then, the height of the plant and the distance of the bird from the edge of the shrub and the center of the shrub was measured with a 30dM pole. After that, we measured the width of the plant, and the height of the bird in the plant. Finally, we measured plant density by making lines to indicate North, South, East, and West. By walking straight out from each of the four angles formed, we were able to find the number of paces to the nearest shrub on each side. Since we knew how many paces translate into a meter, we were then able to change the number of paces into a more usable format.
We were able to record the locations of 38 Green-tailed Towhees and 18 Spotted Towhees. Green-tailed Towhees averaged 12dM above the ground, and Spotted Towhees averaged 27dM (Figure 1). Green-tailed Towhees were found on average 11 dM from the trunk or center of the shrubs in which they were found, while Spotted Towhees were found on average 13dM from the trunk (Figure 1). Green-tailed Towhees averaged closer to the edge of the trees at 12dM, than Spotted Towhees, which averaged 16dM from the edge (Figure 1).
Figure 1. Average dimensions of vegetation where Green-tailed Towhees and Spotted Towhees were located.
The Green-tailed Towhees were found in smaller shrubs, with an average of 27dM height and 36dM width, than were Spotted Towhees, which averaged 35dM in height and 42dM in width (Figure 2). Both towhee species averaged the same in plant density (Figure 2). Also, both towhees were found mainly in serviceberry at 70% and 61% for Green-tailed and Spotted Towhees respectively (Figures 3 & 4). The towhees not found in serviceberry were found in Gambel oak, at 25% for Green-tailed Towhee, 39% for Spotted Towhee, and sagebrush, at 5% for Green-tailed Towhee, and 0% for Spotted Towhee (Figures 3 & 4).
Figure 2. Distance from Green-tailed Towhees and Spotted Towhees to various areas of vegetation.
Figure 3. Shrubs used by Green-tailed Towhees. SEB = Figure 4. Shrubs used by Spotted Towhees. SEB =
Serviceberry; GO=Gamble Oak; SA=Sagebrush. Serviceberry; GO=Gamble Oak; SA=Sagebrush.
From our findings, we discovered that Green-tailed Towhees prefer to stay closer to the ground than do Spotted Towhees. The coloration of the towhees seems to agree with our findings. Green-tailed Towhees are colored pale green, brown, and tan - perfect colors to blend in with the dusty terrain that they inhabit. Spotted Towhees have darker, white-spattered plumage which helps them to blend in with the dappled sunlight reaching the inside of the shrubs. This also makes it more likely for Spotted Towhees to perch in large shrubs, where it is darker and better suited for their dark plumage.
Unfortunately, due to lack of time and the small area we surveyed, the information we can get from this study is limited, and generalizations cannot be made simply from what we found in the time we had. If we had been able to increase the length of time during which we studied these birds and the size of the survey area, we would have been able to obtain more accurate results.
The camp members of On the Wing would like to thank the following people and places:
Glenn Giroir – For his strong group leadership
Jennie Rectenwald – For her unsurpassed guidance and planning as director of OTW
Dinosaur National Monument and campground
Black Canyon of the Gunnison National Park and South Rim campground
Montroses’ Burger King for the best burgers in town
Montrose Public Library for letting us access much needed resources
And well deserved thanks to the guest speakers who further expanded our knowledge: Stan Johnson for teaching us the art of fly fishing, the Astronomy Club for showing us the Universe, Dick Shultz for showing us the geological aspect of Black Canyon, and to Bill Alther for showing us all of the birds at the Denver Museum of Nature and Science. Rich Levad showed us the birds of the Uncompahgre Plateau.
Special thanks to Mr. & Mrs. Cooper for letting us use their pool free of charge, and to all of our parents who made this camp possible.
Rocky Mountain Bird Observatory. 2002 On the Wing Research Papers. Unpublished document.
Terres, John T. The Audubon Society Encyclopedia of North American Birds. New Jersey, Wing’s books.