|
As unbelievable as it may seem to a resident east of the continental divide, Purple Martins are downright rare out in California and the west, and if you didn't know exactly where to look you could easily journey throughout the state without ever encountering a single martin. But not too long ago martins were not as difficult to find out here. Grinnell & Miller, in their 1944 publication The Distribution of the Birds of California (still the state's best reference on bird distribution), rated martins as "fairly common where suitable habitat exists," and most seasoned birders and field ornithologists recall martins being historically more common. By 1974, however, the state's ornithological community generally perceived martins as a declining species, and this led to the Purple Martin's official state recognition as a Bird Species of Special Concern. Almost 20 years later the California Department of Fish & Game initiated a contract in 1993 to do a distributional review and some research on the species' basic biology. Without going into details here, some of the initial results were fascinating. That's why I've continued to do a little research each year even though funding dried up long ago.
Almost every western martin enthusiast with whom I've spoken has at his core an enthusiasm for martins born from memories and passions developed back east. In that sense I am an anomaly, as my fascination was born in the west. In fact, I can't even trace my enthusiasm to my childhood as I never even heard of a martin while growing up (very few in California have). Instead, my interest in martins is a pure outgrowth of my research on the species' remarkable habitat relationships and distributional history right here in California. One of the most intriguing areas of the state that I uncovered during my review was a secret spot located in the Tehachapi Range of the southern Sierra Nevada. I first heard about it from Jesse Grantham who had worked in the area for the California Condor program in the 1980's. He thought that there may be as many as 100 martins in that range. I subsequently heard from a few others who had found martins down there but I was never able to get access. Unfortunately, most of this range is a privately owned ranch and notoriously difficult to access. So I never did visit that area during the main portion of my study. However, one of the observers I spoke with was Clark Moore, a local Tehachapi resident and field ornithologist who lives in the northern portion of the Tehachapi Range off the main ranch. He mentioned fairly large numbers nesting near his home, and extended an invitation if I ever wanted to come. It remained on my wish list until last year. In February 2000 I applied for a PMCA research grant and was awarded $1000 that I used towards an intensive one-week study during the peak of the 2000 nesting season.
Without the PMCA I would not have been able to do the amount of work we did. But PMCA wasn't the only critical link. Clark Moore and his wife Jean helped arrange our stay in the private community of Bear Valley Springs, and gave us a thorough tour of all the known martin sites including private ranches that we would not have been able to access without his help. But what really made this research project successful were my hand-picked field assistants who basically volunteered for this project (aside from a modest per diem) despite long days in the field -- extended each day by about a half-hour of picking stickers out of our socks. Three of my assistants were former students: Maria Cisneros, a hard-working single mom and eternally pleasant field biologist; big Cliff Hawley, incurable birder who managed to measure a few trees while birding; and Phil Taylor, birder and tennis stud. Also volunteering everything, including his per diem and travel, was Ed Pandolfino, a retired Ph.D. biochemist who is now the Conservation Chair of Sierra Foothills Audubon Society and a relatively new resident of my hometown.
The five of us rendezvoused in Tehachapi on June 29th. Clark gave us a tour the next morning then we got in some training by watching martins and making habitat measurements at a few nests. The next day we split into teams and went out to find some nests. By noon on July 5"', five days later, we had taken data on 57 nest sites, way beyond the 20-30 I had hoped for. In fact we were still finding many nests on the day we had to leave for the annual Western Field Ornithologists meeting in nearby Kern River Valley (where I gave a presentation on some of my previous Purple Martin findings). In addition to measuring almost 40 characteristics for each of these 57 nests, we also measured characteristics of some randomly-chosen trees without martins. Why would we do such a thing? Well one of the things I wanted to study was habitat selection - do martins actually choose certain habitat features? A careful observer can often answer that question with analytical observation, but (publishable) science demands a higher standard, so we set up a design in which we measured the same characteristics for both a nest tree and a randomly chosen tree nearby. This was an important component of the study and we were determined to carry it out even if we had to venture along the slopes of near-cliffs to gather data.
So what did we learn? Well, so much I almost get goose-bumps sitting here thinking about it. One of the things that I was most pleased with was that my predictions were true. As a scientist this is a real indication that your understanding is correct. Granted, studying birds is not as challenging as hypothesizing about the formation of the universe, but most of my other relations with tree-nesting martins were in coniferous trees in very different habitats. It was reassuring to see that martins seemed to be responding to the same factors whether they were using a coast range Douglas fir (Tseudotsuga douglasii), a Modoc Plateau (NE corner of California) ponderosa pine (Pinus ponderosa), or a Tehachapi oak. Ah, yes - oaks! I have not yet mentioned that the Tehachapi Range martins nest in oaks. That's another big factor that drew me here, as the historical literature shows that martins and oak woodlands have a long-time relationship in California. Unfortunately, martin declines have hit oak woodland populations particularly hard and the Tehachapi Range apparently represents the last place where martins still use oak woodlands.
Consistent with their preferences elsewhere in California, martins tended to nest in the largest valley (Quercus lobata) & blue oaks (Q. douglasii), averaging 41 inches in diameter (sd 9 inches; range = 22-69; n = 42) and about 50 feet tall. No martin nests were located in black oaks (Q. kelloggii) even though they represented 15 % of random trees. Random trees (n = 46) were smaller and shorter on average and only 11 (24%) had apparently suitable cavities. Another consistent finding was that martins preferred trees on the upper portion of a slope. Martin nests were most likely to be located on the upper third of the slope within 100m (Kolmolgorov- Smirnov goodness of fit; P<<<0.001)- not a single nest was found on the lower 1/3 of the landscape. These prominent positions also differed from random trees showing that martins did indeed choose such trees. These topographic positions may allow better predator detection, but more importantly I think that this allows martins to be closer to the winds that both aid in flight and carry insects from the valleys below. A great side benefit of following Purple Martins in California is that you are often led to very commanding views.
As far as the nests themselves, most were in hollow portions of living branches (93%) that were abandoned woodpecker cavities. Our famous Acorn Woodpeckers (Melanerpes formicivorus) are abundant here and are the primary excavators, but flickers (Colaptes auratus) and other woodpeckers may also provide a few nest sites. Other nest sites included broken branches and dead limbs, but we could confirm only two nests that had not been excavated by woodpeckers; these included a split in a trunk and a rotted branch. Martin nests were not equally distributed among the branching of the trees (Chi-Square; P<0.001), as most nests were in primary or secondary limbs with few nests located in tertiary branches (n = 7). This is what you would expect, since although martins like to be as high as they can, there are few nesting cavities in the smaller limbs near the top or outer edge of the tree. Martins did not seem to respond negatively to the presence of vegetation per se (most nests had vegetation within 1-2 feet) but they did seem to select cavities with an unobstructed aerial pathway. A few nests had vegetation within 3 feet of the entrance on a horizontal plane with the nest, but this vegetation was usually sparse and/or well-spaced from other nearby vegetation. Several nests had completely unobstructed flight lines that looked like they would extend to infinity. Overall, we did not see martins using holes that had lots of vegetation around them. I believe this is a significant factor in nest selection.
The relationship with European Starlings (Sturnus vulgaris) was also interesting. There is considerable circumstantial evidence that starlings have been a primary cause of martin declines in lowland California, where martins once nested both in oak woodlands and in large sycamores of riparian floodplains but are now very rare in both situations. But starlings were uncommon where we found martins, and at one area with at least 18 martin nests we did not see a single starling in three days. This is exceptionally rare in any oak woodland in California. Why were they so rare here? Well, a few hypotheses. First, this was the only area over 1 km from any development. Down in the urbanized valleys starlings were common and we saw no martins. But sites with martins were also near the upper limits of the oak woodland belt which can get pretty cold at over 5000 feet. Perhaps the cold of late winter and early spring (when starlings usually begin to nest) make poor conditions for starlings. Another possibility, more likely than the cold, may be that the relatively high winds of the ridgelines make those places energetically demanding and less desirable for starlings. Regardless, it is true anywhere in California that you do not find martins where starlings are abundant. With starlings now common in California, martins end up almost exclusively in fairly remote or rugged country.
After plugging away a little bit year-by-year, I've managed to collect some very interesting data on martins which I'm currently working on publishing. My current plan leaves me with only two short field projects (2001 & 2002), with the next hopefully in the rugged country of California's redwood coast. I am also planning a one-week California Purple Martin Birding Tour for June 2002, so you are welcome to come if you are interested in birding California while visiting some of the most fascinating places where martins still nest in the west.
Trackback(0)
 |
TrackBack URI for this entry
Subscribe to this comment's feed
Show/hide comment form