I am, at long last, concluding the cowbird/Kirtland’s Warbler series which began with a brief Introduction, went on to provide an overview of cowbird ecology in Cowbirds 101 and Responses to comments on Cowbirds 101, moved to discuss some Problems with Cowbird Control in the management of endangered species, and most recently introduced the poster-bird of cowbird control in rare bird management in Kirtland’s Warbler 101. I will try here to bring it all together and put a bow on it.
Experts agree that habitat availability is the primary limiting factor of Kirtland’s Warbler populations. The surge in warbler numbers when the Mack Lake burn habitat “came online” is strong evidence of this. It is assumed by many that the warblers would not have survived as a species to occupy this chunk of habitat had cowbird control not been implemented. Rothstein and Peer (2005, references below) note, “…though it has been suggested that cowbird control saved the Kirtland’s Warbler from extinction, there is not conclusive evidence confirming this suggestion.” They go on to add:
…it requires an odd coincidence to suggest that the Kirtland’s Warbler was headed towards extinction only to be saved by cowbird control just when its population happened to be at the carrying capacity it would have for the next 18 years.
There are many factors limiting populations of the warbler on the breeding grounds. Even long-time warbler researcher Harold Mayfield acknowledged this, and noted in his book The Kirtland’s Warbler, “Therefore we must not suppose that eliminating the cowbird would bring to fledging all the young whose loss is statistically attributable to cowbirds.”
The truth is, we don’t exactly know what would have happened in the absence of cowbird control or, more importantly, exactly what would happen if we reduced it now.
The historical data on cowbird parasitism, which led up to cowbird control, relied heavily on information summarized by Mayfield in his book, which was published in 1960. All the data is based on studies done “at various times” between 1932-1957. No scientific methodology was presented in the book. While the average rate was given as 55%, the annual rates varied from 18 to 88%. Sample sizes were small. There is really no telling how accurate the conclusions reached from these studies really are. I was told by a member of the recovery team that we don’t know how much parasitism the warbler can withstand and still maintain stable numbers. For some other endangered species, this threshold is between 20 and 40%. It would be less expensive to manage for, say, a 30% parasitism rate than for a 5% rate, but we don’t know what rate we should be aiming for. If the exact current parasitism rate is known now, I have not found it in the literature.
This type of research is apparently simply not being done, and hasn’t been done for years. I could only find 26 papers on Kirtland’s Warblers published since 1970 in major peer-reviewed journals, of which only two or three addressed parasitism and reproductive success specifically. These appeared prior to 1990. This type of detailed research on reproductive success (which would also help us identify predation rates and other causes of reproductive failure) requires labor intensive field work spanning multiple breeding seasons, for which the funding and resources have not been available. One government biologist, a participant in Kirtland’s Warbler recovery efforts, told me that this research really needed to be done, and he would choose to do it if money were available — including conducting it on subsets of warblers in which cowbird control had been withdrawn. In 2005, there was less funding for cowbird control, and fewer traps were deployed. The impact on the warblers? We don’t know, because there were no researchers studying what happened.
That is the most surprising and sobering part of the Kirtland’s Warbler recovery story. It is not that funding may be cut to the cowbird control program, it’s that there are not funds to adequately understand all of the current ecological dynamics.
I don’t think cowbird control funding for Kirtland’s Warblers should be cut, although I don’t think a reduction would be a disaster. In addition to the fact that our paranoia may not be based on reality, there are a couple of other reasons for optimism. First, we know that the impact of cowbirds on their hosts declines as host populations increase. Not only has the warbler population shot up since cowbird control began, according to Breeding Bird Survey data, cowbird populations have been declining in Michigan since the 1960s.
Second, according to one of my sources that works with Kirtland’s Warblers, the birds are also beginning to occupy areas with less agriculture in the surrounding landscape. This is good, because cowbirds prefer to feed in open areas and are attracted to agricultural settings. A study done in northern Michigan found that the probability that cowbirds would occur at a site was over 3 times greater when agricultural lands were present within 3 kilometers. Indeed, this person rarely saw cowbirds in these plots. Warblers nesting in areas away from agriculture are probably less vulnerable to parasitization.
My goals for this series of posts was to explain cowbird ecology, help people see that they are not a dire threat to most of their hosts, and that cowbird control as a management tool for endangered species is not as straightforward as it might appear on the surface. This is perhaps especially true for Kirtland’s Warbler, although each species has its own unique issues.
So the messages that went out from conservation organizations, which prompted this series, were correct to sound an alarm. The alarm was a little off-mark — it’s not the cutting of funds for cowbird control that is the real threat to Kirtland’s Warblers, it is the lack of adequate funding for the full complement of research and management that is required for us to effectively help this emblematic and charismatic little bird.
They were correct, too, in that we need to let those that hold the purse strings — Congress — know that we want endangered species programs to be strengthened, not weakened, and we want both research and management fully funded. Budget cuts are not all borne out of a lack of tax dollars, they are choices which reflect our values as a nation. It is up to us to let the people we have elected to Congress know where we want our money spent, that we value birds and biodiversity, not bacon and bombs.
The data in this series of posts was found in the following references:
- Mayfield, H. F. 1960. The Kirtland’s Warbler. Bloomfield Hills, MI: Cranbrook Institute of Science.
- Mayfield, H. F. 1992. Kirtland’s Warbler. In The Birds of North America, No. 19. (A. Poole, P. Stettenheim, and F. Gill, eds.). Academy of Natural Sciences and the American Ornithologists’ Union.
- Mensing, C. J. 2004. 2003 Brown-headed Cowbird control and Kirtland’s Warbler tour report. U.S. Fish and Wildlife Service, East Lansing Field Office.
- Morrison, M. L., L. S. Hall, S. K. Robinson, S. I. Rothstein, C. D. Hahn, and T. D. Rich, editors. 1997. Research and Management of the Brown-headed Cowbird in Western Landscapes. Studies in Avian Biology No. 18. Cooper Ornithological Society. And this paper in particular:
- Stribley, J.M. and J. B. Haufler. 1999. Landscape effects on cowbird occurrences in Michigan: implications to research needs in forests of the inland west. Pp. 68-72.
- Olson, J. A. 2002. Special animal abstract for Dendroica kirtlandii (Kirtland’s warbler). Michigan Natural Features Inventory, Lansing, MI. 5 pp.
- Ortega, C. P., J. F. Chace, and B. D. Peer. Eds. 2005. Management of cowbirds and their hosts: Balancing science, ethics, and mandates Ornithological Monographs No. 57. American Ornithologists’ Union. These papers were of particular interest:
- Ortega, C. P., A. Cruz, and M. E. Mermoz. 2005. Issues and controversies of cowbird (Molothrus spp.) management. Pp. 6-15.
- Kus, B. E. and M. J. Whitfield. 2005. Parasitism, productivity, and population growth: Response of Least Bell’s Vireos (Vireo bellii pusillus) and Southwestern Willow Flycatchers (Empidonax traillii extimus) to cowbird (Molothrus spp.) control. Pp. 16-27.
- Chace, J. F., C. Farmer, R. Winfree, D. R. Curson, W. E. Jensen, C. B. Goguen, and S. K. Robinson. 2005. Cowbird (Molothrus spp.) ecology: A review of factors influencing distribution and abundance of cowbirds across spatial scales. Pp. 45-70.
- Peer, B. D., S. I. Rothstein, M. J. Kuehn, and R. C. Fleischer. 2005. Host defenses against cowbird (Molothrus spp.) parasitism: implications for cowbird management. Pp. 84-97.
- Rothstein, S. I. and B. D. Peer. 2005. Conservation solutions for threatened and endangered cowbird (Molothrus spp.) hosts: Separating fact from fiction. Pp. 98-114.
- Ortega, C. 1998. Cowbirds and Other Brood Parasites. University of Arizona Press.
- Siegle, R. and D. Ahlers. 2004. Brown-headed Cowbird Management Techniques Manual. U.S. Dept. Interior, Bureau of Reclamation Tech. Serv. Center, Ecological Planning and Assessment Group, Denver.