DISTRIBUTION, ABUNDANCE, AND NEST SITE CHARACTERISTICS OF PURPLE MARTINS IN OREGON Prepared for Oregon Department of Fish and Wildlife by Eric Horvath February 1999
ABSTRACT Nesting Purple Martins were surveyed throughout Oregon in 1998, and 784 pairs in 112 colonies were found. The breeding range has contracted since 1940, and martins are now lacking in Lake, Klamath, and Jackson counties where they formerly bred. 88% nested below 125m elevation. 75% nested in housing specifically put up for martins, while 25% nested in snags, pilings, and miscellaneous manmade structures. 73% nested over water, although others nested 5 km from open water. All pairs nested in open areas, >6m from live tree canopies. Purple Martins nested both solitarily and colonially. Management recommendations are provided.
INTRODUCTION The Purple Martin (Progne subis) is classified as Sensitive in the Critical category by the Oregon Department of Fish and Wildlife (Marshall 1992). Purple Martin populations have declined in Oregon (Lund 1977, Sharp 1986, Marshall 1992), yet much basic biological information is lacking. In this study, I sought to locate and inventory all breeding sites and gather information on the breeding biology of Purple Martins in Oregon. With this information biologists have the opportunity to manage Purple Martin colonies to increase population levels.
I compiled a list of all current and historical Purple Martin colony locations in Oregon from information provided by the Oregon Natural Heritage Program, Oregon Department of Fish and Wildlife, U.S. Forest Service, U.S. Bureau of Land Management, U.S. Army Corps of Engineers, Dave Fouts (a private citizen who manages many martin colonies along the Columbia River), and the approximately 400 members of Oregon Birders On Line (i.e. the Oregon birding community).
During June through 10 August 1998 I visited 111 of the 134 reported colonies. Of the 23 reported colonies where I made no visit, 6 were sites where other observers recently reported inactivity, and 10 were colonies that I judged likely inactive since the last observation was before 1979. Also, I found 45 previously unknown colonies by surveying many areas of suitable habitat .
I defined a colony as 1 or more pair of Purple Martins nesting more than 1 km from other martins. In some areas martin nest sites were scattered along waterways for 2-3 km, with many pairs, and the nearest neighbor less than 1 km away. In these cases the whole collection of nest sites was considered one colony. The 1 km figure is arbitrary, but was selected because male martin dawnsong can often be heard from more than 1 km, martins readily traverse 1 km, nest sites separated by less than 1 km were often in view of each other, and martins frequently associated with their neighbors from more than 100m away. Also, it was useful to have a uniform cutoff figure to document colony distribution.
At each colony I counted the number of pairs of Purple Martins by watching for birds entering nest cavities, especially with food or nesting material. Observations were made with 10X binoculars and a spotting scope. Most sites were surveyed by scanning from land, but a boat was used to access some areas. I observed colonies for several hours and on multiple days to ensure an accurate count, or relied on reports from other observers to augment the accuracy of my single visit. At some of the nestbox colonies, I opened each nestbox to determine the contents. In June and early July, I counted pairs only during the first 4 hours following sunrise, since during incubation many martins forage far from the nest, and large colonies can appear vacant at midday. After early July I censused throughout the day as most pairs were feeding young and were active at the nest even during midday.
Colony elevation was estimated from U.S. Geological Survey topographic maps. Colony location was recorded with a hand held Global Positioning System receiver accurate to approximately 100m. At most colonies I recorded location at a representative nest cavity but in some inaccessible locations I recorded the Universal Transverse Mercator (UTM) location from a convenient observation site close to the colony. Ownership was recorded when it was immediately apparent. Vegetation within 10m of the nest site was briefly described. I recorded distances to the nearest building (or nearest moored boat at busy docks), to the nearest large canopy tree, and to the nearest open water accessible to a flying martin.
For each pair I recorded the nest cavity type, whether single nestbox, apartment (multicompartment nestbox), gourd, under a cap on a piling, other artificial cavity, piling with an old woodpecker hole, piling with a natural rot pocket(a crevice remaining after decayed wood has fallen away), snag, or unknown nest site. I recorded clutch size at colonies where I opened nest boxes during the survey. For each snag I recorded diameter at breast height (dbh), height, cavity height, and height of the cavity above the brush and/or young trees. Heights were measured with a metric tape and clinometer. I also recorded snag tree species, degree of decay (on a subjective 1-5 scale with 1 being a fresh snag and 5 a quite rotten one), and whether the cavity was an abandoned woodpecker hole or a natural rot pocket.
Finally, I evaluated the possibility of adding nestboxes for each colony. This usually included a description of the condition and availability of pilings or poles to put nestboxes on, and the practical chance of martins taking to the housing put up there.
RESULTS AND DISCUSSION
I observed 784 pairs of Purple Martins at 112 colonies in Oregon in 1998. Prior to this study, no systematic inventory existed for martins in Oregon (Marshall 1992). Since virtually all known colonies were inventoried and many suitable areas were surveyed, this study represents the first minimum population estimate of nesting martins in Oregon. While I surveyed all managed colonies and searched most lowland rivers and estuaries where old pilings are common, there probably remain martins that escaped my detection. However, most of these were likely nesting in snags in remote uplands, either solitarily or in small colonies. Because my sampling of upland snag nesting martins found few pairs relative to the managed colonies, I estimate the 1998 statewide population was likely between 784-1000 nesting pairs.
Purple Martins were uncommon and local in most of western Oregon in 1998 . Some counties lacked Purple Martin colonies (Washington, Yamhill, Clackamas, Marion, Josephine, and Jackson counties). Martins were locally common along the Columbia River, in some coastal estuaries, and at Fern Ridge Reservoir.
Martins once nested more widely in Oregon including colonies in the Upper Rogue River Basin (Browning 1975), and in the Klamath Basin (Lund 1978, Gabrielson and Jewett 1940). I found no martins at these former sites. While martins still nested in 1998 at Lava Beds National Monument, just south of the Oregon border (Brian Williams, pers. comm.), their absence from Lake, Klamath, and Jackson counties represents a contraction in range since 1940.
Gabrielson and Jewett (1940) described the Purple Martin as a "rather uncommon summer resident and breeding species of western Oregon, most common in coastal counties; decidedly rare east of the Cascades”. Gullion (1951) reported Purple Martins as common in the southern Willamette Valley in the 1940's. Anecdotal reports indicate a drastic population decline from the late 1940's to the late 1970's (Sharp 1986, Fouts 1988).
In 1977, Lund (1977) censused 168 pair of Purple Martins in Oregon. From 1976-1978 he found 31 active colonies (Lund 1978). This total was the result of field checks of historical nesting sites ("about a hundred"), and was a decline from 60 active sites ten years previously. Unfortunately, in neither paper did he detail his methods or list the areas surveyed. Nevertheless, Lund's data likely reflect a reasonably thorough attempt at documenting the known statewide population. The increase in Purple Martin pairs from 168 in 1977 to 784 in 1998, while not directly comparable, most likely represents a significant increase in the Purple Martin population. This population rebound is probably due to nestbox installation by Lund and others, notably Dave Fouts and Oregon Department of Fish and Wildlife.
In managed colonies (defined as colonies where 50% or more of the pairs nested in housing specifically made for martins, i.e. nestboxes, gourds, and apartments), mean colony size was 11.2 pairs (n=55, S.D.=12.0, range 1-55 pairs). In contrast, unmanaged colonies (where more than half of the pairs used snags, pilings, etc;) were much smaller on average, with mean colony size of 3.2 pairs (n=49, S.D.=3.2, range 1-16). Unmanaged colonies included martins nesting in holes in pilings, under caps on pilings, in snags, and in a miscellaneous collection of odd sites including a broken out streetlight, in pulleys at the top of a boom of a crane, in holes under a dock made of old railway cars, in holes under a concrete highway bridge, in crevices between beams on navigational markers, and in a horizontal metal pipe.
In Maryland, Morton et al. (1990) observed that new colonies began with at least two nesting pairs, and that martins only nested colonially. They (ibid.) hypothesized that martin females avoid noncolonial breeding and constrain male yearlings to join colonies.
In Oregon, most martins also nested colonially, but 11% of the pairs at unmanaged sites nested solitarily
Habitat within 10m of the nest site varied substantially between all 112 colonies, and included open water, grassy fields, and recent clearcuts and burns with brush and young trees. All colonies were in openings, with mean distance from the nest cavity to large canopy trees 145m (n=75, S.D.=179, range 6-500+m). It is significant that martins were not found nesting closer than 6m from the edge of the canopy of large trees; they apparently require exposed locations away from trees.
Martins nested from 0 to 5,000m from buildings, and there was no pattern of nesting close to, or away from, buildings (mean distance 424m, S.D.=636, n=81 colonies). While martins in the eastern United States prefer locations 10 to 40m from buildings (Hill and Chambers 1998), this is not the case in Oregon.
In general, a wide range of habitats were accepted. Features common to all sites included distance greater than 6m to large live trees, presence of an unoccupied cavity, and for sites over land, height of more than 5m above the brush and young trees.
Most martins nested over water, although a few nested >5,000m from the nearest river, lake, or estuary. This indicates that martins may prefer sites over water, but this is not an obligate requirement. I hypothesize that nests over water may be safer from mammalian predators. At sites over water, height above water level appeared to be less important than nest height at sites over land, since many occupied nest cavities were within 2m of water level.
Nest Cavity Types Purple Martins nested in a variety of cavity types . 74.5% of all martin pairs in Oregon nested in housing specifically put up for martins (i.e. nestboxes, gourds, and apartments). Other nest sites (i.e. snags, pilings, and crevices in manmade structures) accounted for 25.5% of the population. While Brown (1997) characterized western Purple Martins as nesting almost exclusively in woodpecker holes or natural cavities, martins in Oregon currently nest mostly in artificial cavities. However, martins are not totally dependent upon artificial nest boxes in Oregon today, as proposed by Marshall (1992). No martins were found using commercial aluminum martin apartments of the style commonly put up in the eastern United States, however only two of these were found, both close to trees. Martins did use homemade wooden apartments at 9 colonies in Oregon. While old woodpecker cavities accounted for most of the nest sites in snags and pilings, martins also readily used natural rot pockets as nest sites.
Snags were the only natural substrate where I found martins nesting. Data was collected on 35 pairs nesting in snags . Thirty-two pairs used old woodpecker holes, and 3 used natural rot pockets in the snags. Most frequently only 1 pair nested per snag (n=14), but "natural apartments" of 2 pair (n=4), 3 pair (n=3), and 4 pair (n=1) in a single snag were also found. These cases of multiple pairs nesting in one snag indicate that high density colonies can form in natural as well as artificial colonies.
Mean colony size where more than 50% of all pairs nested in snags was 3.6 pairs (n=11, S.D.=2.5, range 1-8).
Martin nest snag trees were Douglas-fir (Pseudotsuga menziesii), Cottonwood (Populus trichocarpa), Red Alder (Alnus rubra), Willow (Salix), and Oregon White Oak (Quercus garryanna). Douglas-fir snags were most commonly used, and mean dbh of these snags was 1.2m (n=17, S.D.=0.39, range 0.51-2.27m).
For the overall sample of all snags, mean snag height was 19m (n=22, S.D.=9.9, range 6-44m). Mean cavity height was 15m (n=35, S.D.=8.4, range 5-44m). Mean cavity height above the brush and young trees was 12.7m (n=35, S.D.=7.4, range 5-38m). This last measurement may be an important variable in nest site selection by martins. The effective height of cavities in snags surrounded by brush and young trees is not the distance to the ground, but instead the distance above the brush. Martins chase off small mammalian predators by diving at them (pers. obs.), and are only able to defend cavities that are well above the brush. Hence martins may be selecting higher cavities to avoid predation. Compass direction that the nest faced was recorded for 27 cavities in snags used by martins. No pattern was found; cavity orientation was randomly distributed in all directions.
Martins used snags in a wide range of decay, from relatively young (10 years since burned) snags with bark and fine twigs still present to old snags with no limbs or bark and red cuboidal rot invading the center of the tree. Martins most often occupied snags with substantial decay. These snags frequently had many old woodpecker holes .
Mean distance from the snags to tall, live canopy trees was 195m (n=19, S.D.=140 m, range 10-500m). Martins selected snags which were isolated and/or apart from live canopy trees. This is likely an important factor in nest site selection. In one former (inactive) colony, martins had reportedly been nesting in a live green tree. This tree, however, had an emergent dead top which functionally resembled a snag.
The snags used by martins were often far from water, with mean distance 2,500m to open water (n=13, S.D.=2,119, range 0-5,000m). Also, the nesting colonies in snags tended to be at higher elevations than the general population, with mean elevation 364m (n=13 colonies, S.D.=220m, range 5-732m). These locations were often remote areas in forested uplands that lacked European Starlings (Sturnus vulgaris). Starlings appear to occupy most suitable cavities in lowland snags, competitively excluding martins from many potential and former areas. In colonies where European Starlings were not controlled in Texas, Brown (1981) found that Purple Martin populations were significantly reduced. Purple Martins would likely have a much wider use of snags in Oregon without European Starling competition.
Most active colonies were less than 60m elevation , and 88% of the statewide population nested below 125m. Although the highest elevation I recorded nesting martins was 800m, Lund (1978) reported martins nesting at 2,100m in the Klamath Basin.
I recorded clutch size at 9 colonies during colony counts in July 1998. Mean clutch size was 4.6 eggs (n=126, S.D.=1.04, range 1-7). This value closely matches the clutch sizes described for the eastern subspecies of purple martin, P. s. subis (Brown 1997). Few other records of clutch size exist for the western race P. s. arboricola, although in British Columbia clutch size also averaged 4.6 eggs (n=7, Fraser et al. 1997).
The Purple Martin population of Oregon is limited by lack of available nesting cavities (Lund 1978). Formerly snags were more abundant in the landscape, and historical accounts indicate martins formerly nested often in snags (Lund 1978, Gabrielson and Jewett 1940). Now snags are limited and only 5% of the statewide population nests in snags .
Nest site competition by the introduced European Starling has been a big factor in reducing nest site availability for martins in Oregon. For example, in Coos Bay where large areas of foraging habitat for starlings exists, starlings are now common, nesting in the many old pilings around the bay. However, in the 1960's and 1970's, starlings were less abundant than they are today, since they only began nesting in western Oregon during the 1960's (Jabonek 1993). During this time, records in the Oregon Natural Heritage Program database indicate that there were martins nesting at Coos Bay at several localities. In 1998 only 1 martin colony of 2 pair, in starling resistant nestboxes, could be found on Coos Bay. In contrast, nearby Tenmile Lake has very little starling foraging habitat because it is surrounded by forested uplands, and lacks mudflats. I detected very few European Starlings there during my 1998 surveys. Purple Martins, however, remain common on Tenmile Lake, with 40 pair in 12 colonies.
The best long term strategy to increase the Oregon martin population will be to manage for long term maintenance of snags across the forested landscape. These should be large dbh (greater than 1m), tall (greater than 20m), and standing more than 10m away from large live trees. In addition, old pilings which currently account for 5% of the nest sites in Oregon should be retained.
In the short term, martins can be managed by putting up nestboxes to increase abundance at existing colonies and expand distribution to new colonies.
Substantial effort will be required to make, install, and monitor the new nest boxes. Outside of efforts by government agencies, currently only approximately 20 people manage martin nest box projects in Oregon. More people could be involved in martin colony management, since there are many colonies that could use additional nestboxes.
During the 1998 survey, I saw many different nestbox designs and styles of placement. Some styles are more successful at attracting and retaining martins than others. A few nestbox recommendations follow:
--In areas subject to Starling competition, the nest boxes can be made Starling resistant by restricting the entrance opening to 1 1/4 inch high by 2 3/4 inch wide. --If a roof of aluminum or galvanized sheet metal is put on the nestboxes, they last much longer than nestboxes with wooden roofs. There are at least 8 colonies that still have nestboxes put up by Tom Lund in the 1970's. He put metal roofs on the boxes, and they remain unrotted and occupied by martins.
--At locations subject to House Sparrow competition, installation of the boxes far from shore is preferable. 50m or more of open water deters House Sparrows, and the martins readily accept boxes far out over water.
--Martins will not nest in boxes close to (less than 6m) large live trees.
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. 1981. The impact of Starlings on Purple Martin poulations in unmanaged colonies. Amer. Birds 35 (3): 266-268.
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