Sacramento California Purple Martins

From: Stan Kostka Seattle
Date: 9/22/01
Time: 3:57:01 PM
Remote Name:


I read with keen interest Ray Feld’s observations of the Sacramento Purple Martin colonies (The Purple Martin Society NA Scout Report Vol 8 Issue 1). I first learned about them in Jesse Grantham’s account of his observations and nestbox experiments there in the Nature Society News Vol.29 no.8, 1994. Brian Williams discusses the Sacramento colonies at length in “Distribution, Habitat Associations, and Conservation of Purple Martins Breeding in California, 1998” . Eric Horvath’s “Distribution, Abundance, and Nest Site Characteristics of Purple Martins in Oregon 1999” also includes reference to bridge nesting martins, as does his follow up article in Oregon Birds, Vol 26, No. 1. I watched Jesse’s video this spring and took the opportunity to visit the Sacramento colonies in mid July of this year. Ray’s observations were much earlier in the season , and included European Starling breeding activity which had ceased by mid July when I was there. I was pleased to read that no Starlings were observed entering the weepholes. Starlings are known to use the bridge chambers adjacent to those clusters used by martins (Grantham). During my visits, I assumed that some significant portion of the remnant droppings on the roadway beneath the weepholes were from Starlings. If this is not the case in the immediate area where martins were observed nesting and returning to roost at the end of the day, the number of breeding pairs of martins there this year may exceed the unreliable impression I got during my visits.

I certainly appreciate Ray’s enthusiasm for Purple Martins. Believe me, I share it. But I need to voice an opposing viewpoint to the notion that “ they need to...try to get these birds into houses and increase the local population ”. I disagree. I was amazed and thrilled to see these birds nesting in this situation. This site, along with all others where Purple Martins nest in natural and manmade landscape features, should be considered unique biological phenomena that need to be preserved, promoted, supported, and researched. If the bridge nesting martins are switched over to nestboxes, the original colony will no longer exist, and the opportunity to further observe and gather data on western martins nesting in the superstructure of these concrete highway bridges would be lost.

The Sacramento colonies are the largest of all the known bridge nesting colonies in California and Oregon, and they may support one of the largest concentrations of unmanaged martins in the Western United States . Downtown Sacramento is the only location within the Central Valley where martins are known to nest, and of at least 20 urban sites active from the late 1800’s through the 1970’s, Sacramento is the only one remaining (Williams).

The working model, wherein our martins nest in human supplied housing, should not and does not translate into the axiom that all Purple Martins would necessarily be better off in artificial cavities specifically installed for them as nest sites.

This summer I had the unique opportunity to observe Purple Martins feeding their young “underground” in rock crevices in the lids of ancient lava tubes at Lava Beds National Monument in Northern California. That experience, perhaps the same experience humans had in those caves thousands of years ago, changed forever my impressions of these birds. Should we go back there next summer and erect some martin housing there in the high desert ? No. Should this nesting situation be preserved, not only for its intrinsic value, but also to be available in the future for research and documentation of all the intricacies of martin breeding biology that exist in this unique situation, that may be to some degree different from, or in addition to what we know now ? Certainly.

I have nothing against nestboxes or any other type of human supplied artificial cavities installed for the benefit of Western Purple Martins, where appropriate. I have, in fact, over the years constructed and installed many nestboxes at various locations within the Puget Sound Basin and have seen the known local population of martins increase dramatically as a result. However, known breeding Purple Martins were scarce to essentially absent in the areas where these colonies were established, areas where martins had nested in the past, now lacking only one vital element, a cluster of suitable cavities. The colonies that resulted from my nestbox installations should provide for further Martin recovery into other localities within the northern Puget Trough, where Purple Martins have nested historically but have since been extirpated.

Based on the model of martin recovery in the Pacific Northwest, there is very good potential for increasing Purple Martin populations in California by using starling resistant artificial cavities ( provided that mitigation of daily temperature extremes is taken into consideration when designing said cavities). Establishment of managed martin colonies could take place in areas where martins have historically nested , but have experienced population reduction, range contraction, or extirpation due to loss of natural nest sites and nest site competition from European Starlings.

Based on the information available, this is not the case in Sacramento. “Although no one ever made a thorough census of the martin population in Sacramento prior to 1991, it is apparent that they have increased in numbers after their transition to the bridges in the 1960’s.” (Williams) On my way south, I visited the Sacramento National Wildlife Refuge, about eighty or so miles northwest of Sacramento. The refuge hosts literally clouds of dragonflies and not one breeding Purple Martin pair. Why ? No suitable cavity clusters. Here is an appropriate site for a nestbox program, since distant Sacramento is the only location within the Central Valley where Martins are presently known to nest.

Some bridge chambers at Sacramento have been described as 3 to 4 foot square. While observing martins, in smaller numbers, breeding in a similar situation along the California coast , I took the opportunity to enter a chamber in a bridge after I discovered someone had vandalized the access cover and it was unsecured. The chamber I entered had no martins, but it was very large, two meters high, four to five meters wide, and perhaps 8 meters long. Are the adjacent chambers where martins nest as large ? I was unable to access any other chambers to determine their size. Consider the possibility that these bridge chambers represent the largest cavities martins have ever nested in, a place where nestlings have extensive space to exercise their wings, to move out of and well away from parasite infested nests, and although this is unlikely, perhaps fly somewhat before they fledge. Martin behavior inside the bridge chambers needs to be documented . With all that room to exercise, might not stronger fledglings develop that are more able to negotiate reentry into a vertical entrance than we may realize ? Are the nestlings wandering between chambers? How far from the weephole entrance are they building nests ? It is generally accepted that martins always build as far from an entrance as possible. Is this true in the bridges ? Are martins in fact, as Ray speculates, going in one weep hole and coming out another ? This year I observed a martin nesting in a natural cavity in a piling at one of my sites do just that . In the side and out the top, and vice versa. If the martins are doing this within the bridge structures , this needs to be documented as well. Do these birds have elevated levels of carbon monoxide in their blood, or toxins in their tissue as a result of breeding in an urban area so prone to contamination by a concentration of vehicle emissions ? You've heard the old saying, “Don't throw the baby out with the bath water.” In my opinion, any attempt to help these martins by moving them out of the bridges and into martin houses is doing just that.

Regarding nestling mortality, Jesse Gantham (Nature Society News Vol.29 no.8, 1994) writes that following his installation of the wire mesh sleeves to prevent nestlings from falling out of the chambers and being killed by cars “no young were lost”. Therefore nestling mortality in traffic should no longer be used as a reason to change the situation. The wire mesh sleeves also seemed to greatly aid the return of fledglings. I saw none fall into traffic on the two evenings I watched them return to roost at this site, and I found no road kills. The wire mesh sleeves seem to have solved two major problems encountered by these bridge nesting birds. That’s an excellent accomplishment. A minor adjustment, not a major change.

I agree with Ray that someone there, anyone, publicly or privately, needs to work for the benefit of these birds. The greatest threat to these colonies may in fact come from Cal Trans, the railroad, or whoever has recently blocked some of the weepholes. At a site about fifteen blocks to the west of the one Ray describes, the holes were retrofitted with inserts that block entry to the Purple Martins and White-Throated Swifts that nested there. Was this done deliberately to block access to birds as a maintenance consideration, or without any knowledge of the birds ? Are there plans to block more weepholes ?

Current estimates put the West Coast population at 2300-2700 pairs. A very few of Washington’s, many of Oregon’s, and virtually all of California’s martins breed in unmanaged (wild) colonies. If I had to choose between two future scenarios , wherein one held 1 million martins on the West Coast all in human supplied housing, no snags, no bridges, no lava tubes, and another wherein the population level and diverse nesting characteristics were the same as today, I may well be inclined to choose the latter.

In closing, let me restate my support for artificial cavities where appropriate. The best long term strategy to enhance recovery of Purple Martin populations on the West Coast is not to manage all martins into nestboxes indefinitely, but rather to manage the landscape for creation and retention of natural martin nest sites, such as preserving large emergent snags across a forested landscape, especially in areas where starlings are not present (Horvath). However, in the short term, the time it takes for some land use practices to change and large snags to be replaced, martins most certainly will benefit from the installation of suitable clusters of nestboxes or gourds to increase abundance and expand distribution into new colonies. We also need to learn all we can about them in areas where they seem to be doing OK without human support, such as Sacramento.


Stan Kostka Seattle