Conservation of Purple Martins in California by Brian Williams

From: posted by Stan Kostka, Seattle
Date: 11/14/01
Time: 9:20:27 PM


Conservation of Purple Martins in California by Brian Williams

Although Purple Martins have been associated with nestboxes in California in a few rare instances, they nest almost exclusively in "wild" unmanaged colonies, mostly in natural snag cavities. Here is some current information on California’s snag nesting martins.

Comments: Forest Management for Southern California Purple Martins 24 October 2001

Dear Southern California Forest Biologists:

I recently learned of your intention to update forest plans for the four Southern California National Forests. I value the public review process and would like to provide you with recent information on Purple Martins (Progne subis arboricola). I have been studying the distributional ecology of this species since 1993 (with initial funding from the California Department of Fish & Game) and have learned a great deal about Purple Martin ecology, much of it with direct applicability to contemporary issues of forest management.

As you may be aware, martins are presently designated as a California Species of Special Concern and are now quite rare in southern California. My extrapolated population estimates give a probable range of 800-1200 breeding pairs in the state with approximately 100 pairs in Southern California exclusive of the Monterey County & San Luis Obispo County portions of the Los Padres NF. Those estimates make this species one of the rarest regularly breeding passerines in California. This was not always the case, as martins were formerly more common and widespread, ranging from lowland towns, oak woodlands, and sycamore floodplains to forested regions over 6,000 feet. Presently, with the exception of a scattered population along the Santa Ynez River watershed in Santa Barbara County, all remaining martins have been reported from conifer regions of the National Forests. That is why management on your forests is so critical for this species.

Habitat selection by martins is particularly interesting. While I have not personally studied martin habitat in the mountains of southern California, I have characterized and measured many nest sites in northern and central California and I am confident (based on basic biology, the literature, recent reports, discussions, and photographs) that the same relationships apply to the forests of Southern California. Briefly, martins respond to clusters of very large snags in open areas in prominent and often remote positions in the landscape. Typical nest tree diameter (average > 40" dbh) is one of the largest for any bird, and that relationship is supported by observations from the historical record from Southern California. Perhaps even more importantly, nest sites are usually (roughly 80-90%) located in prominent positions, often on ridges, hilltops, and other commanding sites. In modern times (and almost certainly historically, though probably to a slightly lesser extent), martin habitat is almost exclusively caused by fire -- typically "stand-replacing" fire. A familiarity with fire suppression history in California combined with widespread salvage logging practices should make it clear why this semi-colonial species has declined.

I hope in your plan that you will address the habitat needs of this rare species. Specifically, it should be explicitly recognized that 1) the natural fire regime, especially stand-replacement fires (yes, stand-replacement fires did occur historically despite some of the common but uncritical beliefs to the contrary), is clearly the primary process that creates martin habitat in coniferous forests; 2) other sources of tree mortality (disease, insects) may create martin nest sites but only in very open habitat conditions; 3) martins select very open habitats, typically <10-30% (and often near zero) large tree canopy cover within 100m of the nest as well as on a larger landscape scale; 4) martins select nest trees on ridgelines and other topographically prominent positions; 5) martins preferentially select large snags (average dbh > 40", excluding coast redwoods which are not present in your area except for a tiny portion of Los Padres), especially in clusters; 6) salvage logging of large trees (any tree >24" dbh but especially >36") removes potential habitat, and if done extensively and/or on ridgelines is the most severe threat to eliminate all potential martin habitat; 7) trees must be given enough time to grow to large size which will require longer rotations in some areas; 8) logging can coexist and may even help create habitat as long as the above conditions are met; 9) martins are unlikely to colonize or persist wherever European Starlings (Sturnus vulgaris) are numerous (i.e. lowlands, agricultural valleys, urban areas, etc.); 10) Acorn Woodpeckers (Melanerpes formicivorus) and Northern Flickers (Colaptes auratus) are the primary excavators of nest cavities and therefore suitable cavities are most likely to accumulate where there are nearby or scattered living trees; and 11) recent colonization of newly created habitat (i.e. caused by wildfire) in previously unoccupied regions (30-80 years since last sightings) proves that martins can respond positively if management allows suitable nesting habitat to remain.

Thank you for the opportunity to provide input for your forest plans. The intrinsic value of Purple Martins is no more or less than any other species on your forests, but the martin is a truly rare species in the state (especially in Southern California), and you now have the knowledge to positively affect martin populations on a large scale by following a relatively simple management process. If you would like a copy of any draft reports or publications or my thesis (170 pages), I can provide them to you if reimbursed for photocopying and postage. If you have any questions, including my recommendations for locating the best habitat on your forest, please feel free to contact me.


Brian D. C. Williams Wildlife & Conservation Ecologist

Granite Bay, CA

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