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Friday, August 8, 2014

2014 Purple Martin Survey Report by Dr. Phoebe Prather, staff Biologist

Photo 1. Adult male purple martin in flight on Tejon Ranch.  Photo by Bob Steele.

Figure 1.  Range map of the purple martin (Tarof and Brown 2013).
The purple martin (Progne subis) is the largest swallow in North America and among the largest swallows in the world (Tarof and Brown 2013).  The species breeds in North America and winters in South America.  In eastern North America the species is broadly distributed but in western North America it occurs only locally in the Rocky Mountains, Sonoran Desert, Central Mexico, and Pacific Coast states and provinces (Shuford and Gardali 2008).  It can be found in North America as a summer resident from mid-March to late September, breeding between May and August, and gathering together in large flocks in September before they begin their southward migration.  The species is not well suited to the climatic regimes of middle and northern North America. 

Figure 2.  Distribution of the purple martin in North and Middle America (Tarof and Brown 2013).

Photo 2. Adult male on nesting cavity, Tejon Ranch.  Photo by Bob Steele.

The adult male purple martin is the only dark-bellied swallow in North America and is entirely glossy black, giving it the purplish sheen for which the species is named (Tarof and Brown 2013).  The diet of the purple martin consists exclusively of flying insects and while foraging is capable of flying higher than any other swallow.  The species evolved as a secondary-cavity nester, meaning it relies on natural cavities or holes already created by woodpeckers.  In the eastern United States, however,  by the year 1900 the species had completely switched from using abandoned woodpecker holes to human constructed martin houses due to the competition for natural tree cavities from the introduced European starling (Sturnus vulgaris) and house sparrow (Passer domesticus).  There are now only a few records of martins using natural nesting cavities east of the Rocky Mountains.  However, in western North America where the species is less common and breeds in localized populations in mountain forests, deserts, and coastal areas, it still nests almost exclusively in woodpecker holes or natural cavities.  There are very few other species that show such an abrupt geographic difference in the use of nest sites (Tarof and Brown 2013). 

Photo 3.  Example of adult female and second year purple martin plumage on Tejon Ranch.  Photo by Bob Steele.
Figure 3.  Range of purple martin in California (CABSSC).
The purple martin breeds widely across the state of California but is locally distributed in forest and woodland areas at low to intermediate elevations (Shuford and Gardali 2008).  It was once described as being “fairly common” and widely but irregularly scattered throughout the state (Grinnell and Miller 1944).  Currently it is designated as a Species of Special Concern by the California Department of Fish and Wildlife (Shuford and Gardali 2008).  Populations began to decline statewide in the 1970s (Airola and Williams 2008).  The decline is thought to be correlated with the increased number of European Starlings out competing martins for nesting cavities.  The starling arrived in California as a breeding species in the early 1960s and has affected all purple martin populations except for those in forested regions where starlings are not yet abundant (Shuford and Gardali 2008).  Several regional populations have shrunk substantially and purple martins are now extirpated from most interior and south coastal lowland areas.  

Photo 4.  Valley oak  nesting cavity on Tejon Ranch.  Photo by Robin Prather.
Purple martins utilize different nesting substrates in different parts of the state (Shuford and Gardali 2008).  In northwestern California, where the species appears to be more numerous and more uniformly distributed than anywhere else in the state, martins concentrate in redwood forests near the coast and local inland areas.  Here the martins utilize conifer snags, tall trees, and remnant redwoods that stand above regenerating forests.   In the Sierra Nevada mountain range martins have nested continuously in small numbers but current known nesting sites are widely scattered and small.  In the Cascade Range birds are concentrated around Shasta Lake where they nest in snags of trees created from the construction of Shasta Lake reservoir.  The persistence of populations in forested areas appears to depend on the presence of clusters of large snags or individual very large snags that can support multiple pairs of nesting birds.  In northeastern California the major nesting area is Lava Beds National Monument where birds nest in rock crevices in underground lava tubes.  In the Central Valley the species historically nested in buildings and riparian habitats from Stockton north through the Sacramento Valley until the 1970s.  But with the increase in starling numbers the purple martin has been extirpated in much of the region except for the city of Sacramento where it persists by nesting in bridges.  Along the central coast, martin populations are very local and confined to the conifer areas on coastal ridges.  A few sites in this region are the last places where the purple martin still nests in western sycamore (Platanus racemosa) riparian woodlands.  The Tehachapi Mountains, including Tejon Ranch, may represent the last place where martins regularly nest in oak woodlands.  However, a survey in 2000 found martins to be absent in low elevation oak woodlands where they once were present and where starlings are now abundant (Shuford and Gardali 2008).  

Photo 5.  Ridge on Tejon Ranch supporting purple martin nesting trees.  Photo by Nicole Stephens.
Purple martins occupy some sites that are suitable only temporarily, such as recently burned or logged areas.  The species is also highly vulnerable to spells of cold and rainy weather during the spring and early summer that temporarily reduces insect food supplies and may eliminate or reduce regional populations (Tarof and Brown 2013).  These factors make the species too rare to be reliably surveyed during a general bird survey and result in extremely rough population estimates (Shuford and Gardali 2008, Airola and Grantham 2003, Airola 2009).  There are very few long-term surveys tailored specifically to purple martin populations and one-time studies are able to report where birds are at that time, but not the absences in areas previously occupied (Airola and Grantham 2003).  To better monitor the species statewide, small scale species specific surveys need to be conducted every year for each known population in the state.  The Tejon Ranch Conservancy has begun such an effort.

Photo 6.  Typical upper 1/3 of the slope nest tree position on a ridge on Tejon Ranch.  Photo by Nicole Stephens.

The first purple martin survey was conducted on Tejon Ranch by the Tejon Ranch Conservancy in June, 2010.  The survey was then conducted again in June of 2011 and June of 2014.  Conservancy staff and volunteers survey the ridges of the Ranch for a week during the month of June.  Each year previously known nesting trees are visited and new nesting trees are often found as well.  So far the Ranch has a total of 40 nesting trees.  We found 23 nests in 2010 and 21 nests in 2011.  The 2014 survey found a total of 12 nesting trees, with 8 of those being new trees .

Photo 7.  Typical nesting tree canopy cover on Tejon Ranch.  Photo by Nicole Stephens.

The major ridges on the Ranch include Cordon Ridge, Middle Ridge, Winter’s Ridge, and Tunis Ridge.  Cordon Ridge has a total of eighteen known nesting trees.  This year we found four active nesting trees, one previously known and three new nesting trees.  There appeared to be only one active cavity in each tree.  Middle Ridge has seven known nesting trees, two of those are new nesting trees found this year.  One of the new nesting trees appeared to have two active cavities.  Winter’s Ridge has six known nesting trees, three of those being new trees discovered during this year’s survey.  One of the trees had 2-3 active cavities, which might have just been different entrances to the same nest, illustrating another challenge with accurately surveying purple martins.  The three previously known nesting trees were not active this year.  Tunis Ridge has a total of nine known nesting trees.  This year only two nesting trees were found and they were both previously known.  

Figure 4. Location of purple martin nesting trees on Tejon Ranch.

Figure 5. Location of purple martin nesting trees and ridges on Tejon Ranch.
This year purple martin behavior was difficult to interpret.  Adults and second year birds did not seem as strongly attached to trees as they typically do.  There weren’t large numbers of birds flying around the tree together vocalizing and going in and out of cavities.  Nestlings were heard in a couple of nests, but even then there was not a large amount of activity around the tree.  Just as many things in the natural world are behaving strangely due to the prolonged drought, we hypothesized that the martins had started nesting earlier than usual and that we had missed the window.  However, the week after the survey staff was out on the Ranch and witnessed the typical behavior of large numbers of birds flying around and vocalizing at nesting trees found the week before and nestlings were heard in the nest.  We formed another hypothesis that the Ranch supported a late insect hatch that caused some of the martins to have a later than normal nesting period resulting in possibly two different nesting periods on the Ranch.  As a result of this complex nesting cycle, the Conservancy has decided to redesign their purple martin monitoring program for next year, but as of right now it is still in the tossing around of ideas stage.  The Tejon Ranch Conservancy acknowledges the importance of the Ranch for nesting purple martins and is committed to develop a strong long-term monitoring program that will track the populations over many years to come.

Photo 8.  Surveying for purple martins on Tejon Ranch.  Photo by Nicole Stephens.

Airola, D. A.,  2009.  Status of the purple martin in Northern California: Results of a pilot study to develop and apply a survey method.  US Fish and Wildlife Service.  Sacramento, California. 
Airola, D. A., and Grantham, J.  2003.  Purple martin population status, nesting habitat characteristics, and management in Sacramento, California.  Western Birds 34: 235-251.
Airola, D. A., and Kopp, D.  2009.  Recent purple martin declines in the Sacramento Region of California: recovery implications.  Western Birds 40: 254-259.
Airola, D. A., and Williams, B. D. C.  2008.  Purple Martin (Progne subis), in California Bird Species of Special Concern: A ranked assessment of species, subspecies, and distinct populations of birds of immediate conservation concern in California (W. D. Shuford and T. Gardali, eds.), pp. 293-299.  Studies of Western Birds 1. W. Field Ornithol., Camarillo, CA, and Calif. Dept. Fish and Game, Sacramento.
Grinnell, J., and Miller, A. H. 1944. The distribution of the birds of California. Pac. Coast Avifauna 27.
Shuford, W. D., and Gardali, T., editors. 2008. California Bird Species of Special Concern: A ranked assessment of species, subspecies, and distinct populations of birds of immediate conservation concern in California. Studies of Western Birds 1. Western Field Ornithologists, Camarillo, California, and California Department of Fish and Game, Sacramento.
Tarof, Scott and Charles R. Brown. 2013. Purple Martin (Progne subis), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: