Reprinted from: Purple Martin Update 3(2): 2-4
Dr. Bridget J. Stutchbury
Department of Biology
North York, Ontario
Summer in the lower Sonoran Desert: rainfall in June is virtually zero, and afternoon temperatures regularly exceed a blistering 105o F. This is an unlikely setting for martins; a rugged, rocky, landscape where the huge saguaro cacti (Carnegiea giganteus) dwarf all but the surrounding mountains. Many martin enthusiasts will be surprised to learn that the martins we attract to houses in eastern North America have "cousins" in the desert southwest. Purple Martins, belonging to a distinct subspecies (Progne subis hesperia), breed in natural woodpecker cavities in the giant saguaro cacti of the lower Sonoran Desert.
Martins in eastern North America have been dependent on man-made houses for hundreds of years, long before good ornithological records were kept, so very little is known about the breeding biology of martins in natural cavities. Montane [mountain-dwelling] western populations also use natural cavities, but are widely scattered and have seriously declined in numbers in recent years. In 1988, I began studying a natural desert population to help us understand how nesting in houses may have affected the behavior and breeding biology of Purple Martins. My study site is within the Saguaro National Monument and Tucson Mountain Park, just west of Tucson, Arizona.
Purple Martins are surprisingly abundant here; this is made quickly apparent during a pre-dawn trip to the desert. At 4:30 am, when the air is completely still, the only sounds one is likely to hear in the vast desert are the familiar dawn songs of male martins circling high above in the darkness, and perhaps the odd chorus of coyote howls.
Desert martins are dependent on woodpeckers, rather than people, for their nesting sites. The cavities used by martins are excavated by Gila Woodpeckers (Melanerpes uropygialis) and Northern (or "Gilded") Flickers (Colaptes auratus). When the tissue of the saguaro is damaged, it grows a woody scar tissue to seal the wound and prevent water loss. The woody tissue forms around the inside of a freshly excavated cavity and the skeletons of the cavities ("saguaro boots") can be found on the ground long after the saguaro itself has died and disintegrated. Purple Martins are not the only cavity-nesting birds to use saguaros; in addition to woodpeckers, Brown-crested Flycatchers, Elf Owls, and sometimes Cactus Wrens use the cavities. I even saw a small bat enter a cavity at dawn, presumably to roost for the day. Fortunately, European Starlings and House Sparrows are not a problem because they are so uncommon out in the desert, a rare treat for a martin biologist. However, in areas where the city is encroaching upon the desert, Starlings and House Sparrows have begun to use saguaro cavities and may represent a future threat to martins in more pristine areas.
Purple Martins in the Sonoran Desert are not colonial like their eastern counterparts; I never found more than one breeding pair per saguaro. In fact, no pairs were closer than 100 yards apart, and the average distance between breeding pairs was about 300 yards. Some saguaros contain many woodpecker cavities, but martins defend an area with a radius of about 40 yards around their nest cavity. When other martins come close to a breeding pair's cavity, the owners often perched in the entrance of their cavity or chased the intruders away. They defended extra cavities within their territory, just as martins in the east defend more than one nesting compartment in a house. I did find one male that appeared to be polygynous, as he had two different females building nests in their respective cavities only one yard apart in the same saguaro. My excitement at this discovery was abruptly dampened a few days later when the saguaro was blown over in a storm, destroying both nests.
Although the martins in the desert are spaced relatively far apart, they cannot be considered solitary nesters. Over the entire study area, nests usually occurred in clumps of 4-7 pairs, whereas other areas of similar appearance and densities of saguaros had no breeding martins. It appears that the martins are choosing to nest close to each other, but still defend an area around the nest. Indeed, there are many social interactions within these "colonies." This is demonstrated most clearly by their mobbing behavior. When a person approaches a nest that contains nestlings, the parents will circle, giving alarm calls and sometimes dive bomb. Within several minutes, a group of 8-10 martins can appear, forming a "mob" to harass the predator. I did an experiment to study this behavior, by erecting a plastic model of a crow on a pole near a nesting cavity, and found that the more solitary-nesting pairs attracted less help than pairs nesting in a "colony."
The desert martins do not arrive in the area until late May or early June, and do not begin egg-laying until early July. This is surprising, since martin populations in the southeast begin breeding in March and April. Most other desert birds breed early in the spring; when the martins first arrive the woodpeckers and flycatchers are already feeding young. Why the long delay in martins? During May and June, the extremely dry conditions limit the number of flying insects available for food. However, from mid-July to late August, afternoon thunderstorms bring much-needed rain to the region. Dried-up desert plants suddenly sprout fresh green leaves, and flying insects become abundant. The martins' eggs hatch in late July, which coincides with the start of the monsoon rainy season. Presumably, the martins would have a hard time finding enough food for their young during the dry months of May and June, and delay breeding so that the young hatch at a time of plentiful aerial insects. The dry climate also means that there is virtually no standing water, so martins have no opportunity to drink water and must get their water from the insects they eat. Martins do nest in the city, and one Tucson resident told me that a pair of martins nesting in a saguaro in his yard would regularly get drinks from his swimming pool.
Working on the Sonoran Desert martins made me appreciate martin houses that could be lowered. The desert martins nest in cavities 25-30 feet above the ground, and checking nests becomes very complicated. Most martin nests are within 10 feet of the very top, and saguaros are very unstable, so using a ladder would risk toppling the saguaro. Not a healthy prospect for either the saguaro or the researcher! I opted for using mirrors to view the nest contents. My gadget consisted of a lighted 2-inch convex mirror (which fit inside the cavity) and a larger flat mirror placed at a 45-degree angle to reflect the image from the convex mirror toward the ground. This was attached to extendible lightweight poles, so one person on the ground could maneuver the mirrors, while a second person looked through a telescope at the large outer mirror to view the inside of the cavity.
Checking nests was very time-consuming, and could only be done before dawn, otherwise the bright sunlight would obscure the dim image in the mirrors. In many nests, I could not count eggs because the incubating female refused to leave her nest! Clutch sizes ranged from 3-5 eggs, considerably smaller than the typical 5-7 eggs laid by birds in martin houses. The martins in the desert are about 20% smaller in size and weight than the eastern birds, which may account in part for their smaller clutch sizes. However, the smaller size of the nesting cavities may also limit clutch sizes.
The nesting cavity is surrounded by several inches of moist saguaro tissue, which acts like a natural form of "air conditioning" by insulating the eggs and nestlings from the intense heat of summer days. Saguaros also afford martins excellent protection against most predators, because the outside of a saguaro is lined with columns of needle-sharp spines, each about 2-3 inches long. The saguaros are essential for sustaining this race of martin; there is no other source of cavities. Martin houses would not likely be successful here, because they are not sufficiently insulated.
The breeding behavior of the desert martins is very similar to house-nesting birds. Females do the bulk of nest building, and all the incubation duties. The parents share about equally in feeding the nestlings. Martins do add green leaves to their nests during incubation, but not as frequently as birds nesting in houses. This is not very surprising, since most desert plants do not have green leaves, especially in June and early July. Plants, such as the Ocotillo, do not leaf out unless there has been a recent heavy rainfall.
Since eastern martins no longer use natural cavities, it is impossible to know whether this natural population in the desert serves as a good model for what eastern populations would have been like before they began using martin houses. The availability and distribution of woodpecker cavities in the desert is not necessarily the same as that found in eastern beaver marshes or other areas with dead tree snags. However, the results from this study suggest that Purple Martins are historically colonial in the sense of choosing to nest in groups. I recently reviewed the literature of published reports of Purple Martins nesting in natural cavities in tree snags. Martins were known to breed in colonies of over 20 pairs in natural cavities, although reports of solitary pairs were also fairly common. The densities of martins in houses is undoubtedly much higher than what would occur naturally because natural cavities are more widely spaced out, but the total number of martins in a colony is fairly similar.
Working in the desert was quite a change from studying martins in people's yards. One is constantly on the lookout for rattlesnakes, though they are rarely encountered. The abundance of lizards darting over the gravel floor results in many false alarms and sudden jumps in heart rate. Tarantula spiders are common, but stay in their underground burrows during the hot days. Dawn was often accompanied by the eerie howls and yelps of coyotes. On lucky days, I would see javelina crossing the road in small packs. One learns quickly to keep clear of the "jumping" cholla, a cactus with very sharp spines that seem to leap out of nowhere to become embedded in your calf or arm. This area is a bird-watcher's paradise; many birds found only in the southwest are common here, including Harris' Hawks, Gambel's Quail, White-winged Doves, Elf Owls, Phainopeplas, Pyrrhuloxias, Black-tailed Gnatcatchers, Verdins, and all sorts of hummingbirds.
Becoming familiar with this intriguing desert community of animals and plants made it all the more depressing to see the ever-growing expansion of Tucson. Areas that were desert only a few years ago are now occupied by condominiums, convenience stores, and golf courses. The Saguaro National Monument and Tucson Mountain Park serve an essential role in preserving a large tract of desert from this destruction, but the size of these protected lands seems tiny compared to the areas occupied by agriculture and urban development. The Purple Martin, which is totally dependent on saguaros for nesting cavities in this area, is surely threatened by this increasing loss of desert habitat.
|Are the desert martins "real" Purple Martins? Most certainly. Often when a species is distributed over a large geographic area, local populations become adapted to their particular environment, in this case the desert. Because a local population is generally isolated to some extent, meaning it does not interbreed freely with the larger population, these adaptations are reflected in morphological and behavioral traits of the birds. The desert martins are smaller in body size, females and subadult males are paler in coloration, and their vocalizations are noticeably different from the typical house-nesting martin in the east. The term "subspecies" is used to indicate that a distinct race exists, although the differences are not great enough to consider desert martins as a separate species altogether. The Latin name is used to designate subspecies; the first two words are the same and indicate genus and species (e.g., Progne subis), while the third is different and indicates the subspecies (e.g., Progne subis subis for the eastern house-nesting race, and Progne subis hesperia for the Sonoran Desert race).|
Dr. Stutchbury received her Ph.D. from Yale University in 1991 comparing the breeding ecology of Progne subis subis with that of Progne subis hesperia. The title of her dissertation was: "Plumage color and reproductive tactics in male Purple Martins." She is the newest member of the PMCA's scientific advisory board. In August (1991), she led a PMCA research trip to Arizona's Sonoran Desert.
Copyright 1991 by Purple Martin Conservation Association. All Rights Reserved.
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