Not long after the start of this summer field season, keiths asked if I could explain what I was doing for my thesis in a little more detail. I said I would, I meant to, but somehow I got lazy when it came around to actually doing it. Well, I aim to be a man of my word, and with this summer’s field season coming to a close, I figured I’d better elaborate soon. In terms of humor, interesting writing, and insightful commentary, I promise nothing. This is simply a brief description of what my thesis involves and what my days are like out here in the Davis Mountains.
My goal is to quantitatively describe the bird communities of the Davis Mountains. What I’m wanting to do is sample the main habitat types out here to learn about the movers and shakers, so-to-speak.
In order to do this, I must collect data in a statistically useful way. I place transects (a route made up of 20 points, give or take) through the habitat types I wish to sample. Each morning starting at sunrise, I walk a transect stopping at each point, spending 5 minutes recording every bird (both species and number of individuals) that I see or hear within a 50 meter radius of that point. When the 5 minutes is up, I walk to the next point and repeat until I finish all the given points on that morning’s transect. Each transect must be run within four hours after sunrise to ensure decent rates of bird activity and song. If the weather or the wind is bad, no data is collected. Finally, each transect is run four times. Thus, data gets recorded at each point four times. This should account for things like luck or differing conditions between days. It helps ensure an evenness to the data collected.
I’m sampling three main habitat types — ponderosa pine, “open” pinyon / juniper / oak (PJO for short), and “closed” PJO. The habitat types one samples are arbitrary, but the idea is to sample the main habitat types for the location of interest. Ponderosa was easy. It’s easily sampled, occurring in stands in the canyons where additional moisture sustains its growth. The other habitat types took a little thinking about when setting up my study. Because different species of oak, juniper, and pinyon occur mixed together, it would be impossible to sample just oak or just juniper habitats. Thus, it was decided to divide them into two types depending on the distance between trees. Open PJO, as it’s name implies, is largely arid grassland with scattered trees. It’s found mainly on south facing slopes, which receive less moisture. One could easily walk through this habitat in more or less a straight line. Closed PJO on the other hand is dense. The branches of neighboring trees interlock such that you have to force your way through vegetation or go around it as you walk through.
While there are a number of variations on how one sets up transects, there are well established guidelines for ensuring that the data one collects is statistically useful. I won’t go into that methodology, but I will describe how I’ve set my study up.
I’ve spaced each point at least 250 meters from the last point. This is done to ensure that birds you record at one point are highly unlikely to have been recorded at a previous point. I have at least 40 points in each habitat type. These points do not have to occur sequentially. For example, one transect (a 20 point route) might have 8 points of Ponderosa, 5 points of open PJO, and 7 points of closed PJO. The goal is simply to get a total of 40 points in each habitat type, once all the transects are setup. Since I’m surveying 3 habitat types, I’ve got a total of 120 points on 6 transects. (To be precise, I actually have 121 points, and some transects have a little more or a little less than 20 points).
In summary — 3 habitat types, 40 points in each habitat type for a total of about 120 points, approximately 20 points on a transect for a total of 6 transects, each visited 4 times. That gives me about 480 snapshots of bird life in the Davis Mountains. And from this, repeated over 2 seasons — this summer and next, I can quantitatively describe the bird communities in these mountains.
This type of work has been done for many of the other southwest mountain ranges but never the Davis Mountains. Thus my study will be useful as a baseline (or starting point) for these bird communities. The Nature Conservancy can repeat the same study (even sampling the same points) every five or ten years to see how things change. Are management policies having the desired results. Are the ecosystems healthy? Are bird populations stable? That kind of thing.
In addition, my study should be able to make a nice comparison between the Davis Mountains and the other southwest mountain ranges — the so called “sky islands” — such as the Guadalupe and Chisos Mountains in west Texas, the Chiricahuas and Huachucas of Arizona, and the Sierra del Carmens of northeast Mexico. It will be interesting to see the similarities, but even more so the differences, that come to light from such comparisons.