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Photo 1. Adult male purple martin in flight on Tejon
Ranch. Photo by Bob Steele.
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Figure 1. Range map
of the purple martin (Tarof and Brown 2013).
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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.
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| Figure 2.
Distribution of the purple martin in North and Middle America (Tarof and
Brown 2013). |
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Photo 2. Adult male on nesting cavity, Tejon Ranch. Photo by Bob
Steele.
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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).
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Photo 3. Example of
adult female and second year purple martin plumage on Tejon Ranch. Photo by Bob Steele.
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Figure 3. Range of
purple martin in California (CABSSC).
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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.
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Photo 4. Valley oak nesting cavity on Tejon Ranch.
Photo by Robin Prather.
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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).
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| 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.
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| 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 .
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Photo
7. Typical nesting tree canopy cover on
Tejon Ranch. Photo by Nicole Stephens.
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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.
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| Figure 4.
Location of purple martin nesting trees on Tejon Ranch. |
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Figure 5.
Location of purple martin nesting trees and ridges on Tejon Ranch.
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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.
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| Photo
8. Surveying for purple martins on Tejon
Ranch. Photo by Nicole Stephens. |
References:
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.
