What It Is, Exactly, That I’m Doing Out Here
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.
Hi Jay,
Thanks for the explanation. I now have a much better idea of what you’re up to out there.
When you run a transect, do you always visit the points in the same order? If so, do you make sure that other transects visit the habitat types in a different order? Otherwise I could imagine some biases in the data due to the exact time of day you visit a particular habitat type.
Also, if studies of this type are done by single observers, there would seem to be no way to control for the skill of the observer. You might notice everything, while another observer might be so bad that he’d miss a turkey vulture unless it landed on his head. (I suspect I’m more of the latter than the former.) Is there a prescribed method for how you scan each point on your transect? Is it just assumed that every field biologist is competent enough as an observer to notice pretty much everything within 50 meters of the transect point?
Lastly, why on earth would you pick a profession that requires you to be up before sunrise every day?
Regards,
Keith S.
Good questions Keith.
In general, I do visit the points in the same order and roughly the same time of day. This really isn’t a problem for bias because I’m not trying to glean information from that point but rather that habitat type. It’s amazing how variable bird activity is. You can visit the same point at the same time of day and one day it can be hopping and the next week completely dead. While things are generally more active earlier in the morning, even that rule isn’t universal.
That’s what the large number of points (40 in each habitat type) and 4 repetitions is for — to account for any bias that comes from minor variations in weather conditions and differences in bird activity.
You’ve hit the nail on the head — the single most important variable of this study is the skill of the observer. Without an observer that really knows what they’re doing, the value of a study like this is nill.
As far as a prescribed method, I generally stand still at the point itself. If I see a bird fly in and need to move a few steps to get a look at it, I’ll do that. But I do not walk around actively looking for birds, nor should I. (As you walk away from the point, you get much further away from the 50 meter boundary behind you, and closer to the 50 meter boundary in front of you, thus making it very difficult to judge if a bird’s within your circle).
I would not say that it’s assumed that every competent observer will notice everything within 50 meters as much as it is determined by comparing studies with differing distances that 50 meters works best. (This methodology is largely one that’s been refined over serveral decades of trial, error, and comparison). I should also point out that I see very few birds compared to hearing them. Thus 50m is a distance at which I can hear everything. Get much further than that, and the observer is going to detect species with “loud” songs or calls more frequently than those with “quiet” songs or calls.
Finally, in regards to your last question — I always equate this to God and the problem of evil.
He gives me the gift of birding while neglecting the gift of being a morning person. (I’m completely joking of course).
I could always have picked herpatology. I would have been working every afternoon in the desert. Some call it Hell. Mammalogy would be a good fit for the person that likes to stay up late. All the little furry critters are active at night, but then again, they’re all brown. I like brown, but a little variety is nice too. Botony can be very unfairly made fun of as food science (everything eats plants after all) and while entomology is truly fascinating, I perfer a group that I can actualy wrap my mind around. There’s 10,000 species of birds and arond 420,000 species of beetles alone. Too much.