problems with cowbird control

December 5, 2005

in Birds, Science

In previous posts (Introduction, Cowbird 101, Response to Cowbird 101 comments; see also comments sections) I’ve tried to give an overview of cowbird ecology and dispel some common misconceptions on the level of threat that cowbirds pose to most songbirds.  Some people have taken that to mean that I don’t think cowbirds pose a threat to birds with small populations or restricted ranges, like Kirtland’s Warbler, or that I am perhaps proposing that reducing or eliminating cowbird control would not lead to fewer of these endangered species.

Hold on, let me finish.  And remember 1) I don’t work with Kirtland’s Warblers, and am relying on available published research and private comments from sources that do work with them, and 2) in doing so I’m trying to show that there is another side to this story.  That feelings have already run high in my e-mail and comments offer an example, I think, of how emotionally charged this issue is, and how difficult it is for some to change their minds about cowbirds.


Trapping and killing cowbirds has been used as a cornerstone of management with several endangered species: Southwestern Willow Flycatcher (Empidonax traillii extimus), Least Bell’s Vireo (Vireo bellii pusillus), Black-capped Vireo (Vireo atricapillus), and Kirtland’s Warbler (Dendroica kirtlandii).  The usual method is to lure cowbirds into large, screened traps with the use of decoys (live cowbirds kept in the trap).  Once inside the trap, the cowbirds do not know how to escape.  Cowbirds are removed and killed, usually once a day.

An unfortunate side-effect of cowbird trapping is the number of non-target species which are trapped.  These birds are released (although some die in the traps), but because the traps are only emptied once a day, they may have been away from their own nests long enough to cause failure.  In the cowbird traps in Kirtland’s Warblers areas alone during the 2001-2003 seasons, 1,826 birds of 29 non-target species were trapped, or over 12% of the total number of birds captured in the traps.

Cowbird control has been credited with saving some rare bird species.  But is it really the cure-all that the public thinks it is?

Experts maintain that trapping cowbirds is a stop-gap measure, and should only be used to help boost or stabilize populations of rare birds.  There are several important reasons why trapping cowbirds as a long-term management tool is not a great idea:

It prevents host species from developing defenses against parasitism.
The defensive behaviors of “rejecter” host species, outlined in a previous post, evolved due to selective pressure. Removing or relaxing that pressure – by trapping cowbirds and not exposing host species to nest parasitism – impacts the development of defensive behavior in these hosts.

It is not true that some species are always “acceptors”. For example, outside of the range of cowbirds in northern Manitoba, American Robins always accepted cowbird eggs experimentally placed in their nests.  In southern Manitoba, where cowbirds are found, robins only accepted the eggs about one-third of the time.

Nor is it true that defensive behavior does not evolve quickly.  In fact, once rejecter behavior appears in a host population, it rapidly spreads and becomes fixed.  However, defenses only evolve and spread when the rate of nest parasitism remains high.  (If you are skeptical that evolution can occur quickly, get a copy of the November 2005 special evolution issue of Natural History Magazine, and read the article “Evolution in Action”.)  It is the opinion of some leading cowbird researchers that  we are at a point where at least some cowbird control programs should be scaled back to determine whether hosts can defend themselves against parasitism.

Evolution works to thwart the usefulness of cowbird trapping in another way.  Trapping targets unwary birds, and long-term trapping may end up producing trap-resistant populations of cowbirds.

It shifts attention and resources from real problems and solutions.

Trapping cowbirds is a relatively easy, low-tech management tool that requires less effort and expense than doing what really needs to be done to save endangered species: acquiring and restoring habitat.  Habitat loss and degradation is the biggest threat to biodiversity.  Habitat loss is the primary reason for the decline of all the bird species mentioned in which cowbird control is used for management.  While short-term cowbird control helps increase rare host populations, it is effective only if there is enough suitable habitat available to support the surplus birds so that they can disperse and colonize historical areas.

More research is needed in determining and managing other population-limiting factors, and some of it is likely not being done, with funding being directed instead at cowbird control.  For example, we know that the location and availability of feeding areas strongly influences cowbird numbers and distribution, yet there is insufficient data on exactly how landscape-level habitat management can reduce nest parasitism.  Specific impacts of cowbird control on parasitism levels are under-studied. In Texas, for instance, cowbird trapping is suspected to have actually increased parasitism by attracting cowbirds to local populations of Black-capped Vireos.

It does not contribute to the objective of self-sustaining host populations.
The goal of endangered species recovery is self-sustaining populations. Cowbird trapping plans for endangered species generally do not have pre-determined criteria which would reduce or end the trapping. The dependence of host species on human intervention, by means of cowbird control, is thus open-ended. Endangered species management programs without exit strategies that will result in self-sustaining populations are exactly the type of policies that opponents of the Endangered Species Act can cite as failures. (That these opponents are wrong-headed is beside the point — they wield considerable legislative influence.)

When the number of dead cowbirds stands in for measures of real progress in endangered species recovery, we are failing in our obligation to protect rare birds.

Next, Kirtland’s Warblers and cowbirds.  Is cowbird control really the reason warbler populations rebounded?  Is it time to see if the warblers can make it on their own?

Pamela December 5, 2005 at 10:45 am

I think your last two points are crucial. There is a strong tendency to direct conservation/restoration efforts towards individual species. I think this is at least partly because it is easier to sell people on the idea that money and effort should be directed to save a particular, beautiful member of the biosphere. The problem is that we can get into a bind, as you suggest, where we are stuck with an ongoing intensive management effort with no end in sight. And we don't end up addressing the real problem, almost always habitat.

I did a search on the Kirtland's warbler and the first thing that came up was this page ( at I was immediately struck by the fact that this bird needs forest fires to maintain its habitat, and part of the problem cited in one of the comments on the series was the existence of fire roads through the forest where the bird nests. The page describes a program of controlled burns in the area to create the right conditions, but it seems likely that the area is too small to maintain a real natural population for long (described as 4,400 hectares divided into 4 areas). Huge effort, one very fussy species. If they only breed in artificially maintained reserves and are also threatened by cowbirds, then sure, trap the cowbirds to protect the maintenance effort. But this doesn't sound like the road to species recovery to me.

But maybe the next instalment will describe cheerier prospects for this species. I'm looking forward to it.

Mike December 5, 2005 at 11:24 am

I love how rationally you are challenging certain deeply-held misconceptions. These are tough questions that you ask. Your decision to address them over time, to give them the careful deliberation they deserve makes your argument very powerful.

Nuthatch December 5, 2005 at 11:46 am

The issue with fire is indeed a difficult one. The huge block of habitat that really made the difference in Kirtland's Warbler recovery was a presecribed burn that got out of control in 1980, destroying 40 homes and killing one person. Since that time, the use of fire to create warbler habitat has been pretty much of a non-starter. Habitat is now created by the much more expensive and laborious method of plantations.

Ultimately, we may be faced with a really hard choice: whether or not we should devote so many resources to saving a single species that was, to the best of our knowledge, always rare to begin with. I think endangered species management will undergo radical changes in the next couple of decades; these are topics a little beyond the scope of the current issue, but which I can bring up somewhere down the road.

Peter Bryant December 5, 2005 at 12:59 pm

Thanks for clearing up the confusion from the earlier posts. I was not previously aware of the (misguided) concern about the effects of cowbirds on songbirds as a whole, as all my experience with the issue has been related to saving endangered species.

You have some excellent points here that I haven't heard before, in particular the point about letting natural selection help protect threatened birds against nest parasitism.

My next question concerns this: "In Texas, for instance, cowbird trapping is suspected to have actually increased parasitism by attracting cowbirds to local populations of Black-capped Vireos."

This is surprising to me since what I've read and heard (albeit I've not had the chance to read very much) indicates the opposite. For example, the Fort Hood study from the 1990s. Can you point me to the studies suggesting the increased parasitism?

Thanks again for this interesting and informative series of posts.

Peter Bryant December 5, 2005 at 1:09 pm

One more thing. "It shifts attention and resources from real problems and solutions.
Trapping cowbirds is a relatively easy, low-tech management tool that requires less effort and expense than doing what really needs to be done to save endangered species: acquiring and restoring habitat."

I'm an outsider to this effort, but it seems to me that cowbird control is not a competing solution to preserving habitat, but a complementary one — a tactic that is used to help stop the decline while the long-term, politically-difficult, solution of preserving habitat is dealt with. Is there an example of cowbird control being used to maintain a threatened population of birds without a serious attempt at habitat-preservation/restoration alongside it?

Cindy M. December 5, 2005 at 8:21 pm

I concur with many of your observations Nutty- but I'm standing my ground on this issue.. maybe because I have a somewhat emotive attachment to this species, maybe because I've seen the traps personally (usually 3-4 times per month during breeding season) and have never seen a non-cowbird species in the traps. Ever. But I *have* seen parent Kirtlands feeding fledgling cowbirds as fast as they can. Those estimates can be manipulated by those who wish to pull the rug out of the recovery project.

"If they only breed in artificially maintained reserves and are also threatened by cowbirds, then sure, trap the cowbirds to protect the maintenance effort. But this doesn't sound like the road to species recovery to me."

Huh?? These methods have WORKED and have literally brought the Kirtlands back from the brink of extinction. That is not an exaggeration, that is a fact.
I find this ludicrous that birders everywhere are cheering on the IBW project, funneling millions into making sure they're even there to begin with.. but birders are questioning sound methods that have kept another species from dissapearing from North America. I just don't get it.

Nuthatch December 5, 2005 at 9:19 pm

Peter —

I can't put my hands on the reference to cowbird trapping increasing parasitism at the moment (although it was in one of the papers I'll reference at the end of the last post). It had to do with traps in more isolated areas, where there were fewer cowbirds to begin with. Apparently, the decoy cowbirds in the traps attracted other birds, perhaps more than would have been in the breeding area than if no traps were in place.

The situation at Fort Hood with cowbirds, Black-capped Vireos, cows, and entitled ranchers is a great example of a big fat management mess. A non-scientific overview is here:

There is also a good paper on the Fort Hood situation is in the AOU Monograph titled "Management of cowbirds and their hosts: Balancing science, ethics, and mandates."

Peter Bryant December 5, 2005 at 11:38 pm

Thanks for the link to the very interesting story on the politics surrounding the Fort Hood environment. It certainly does sound like a mess there.

But even that story doesn't challenge the results of the cowbird control study that I've also seen reported elsewhere: "in spite of its flawed implementation the results still showed that cowbird parasitism of vireo nests declined from 34.8% to 1.6% in the first year following the partial removal of cattle and increased trapping of cowbirds. Parasitism levels increased from 2.4% to 9.5% after reintroduction of cattle in 1999 even though trapping continued."

Well, the part about the reintroduction of cattle was new to me, but still, if these numbers are correct, they show a dramatic reduction in the parasitism rate with cowbird trapping, even with cattle remaining.

As far as the study you mention showing increased parasitism when cowbird traps were put in areas with few cowbirds, this sounds like more of an implementation mistake, rather than a fundamental problem with cowbird trapping when done more carefully.

I guess I'm ending up playing devil's advocate to the devil's advocate. But I'm learning a lot in the process. Looking forward to the next installment!

TroutGrrrl December 6, 2005 at 10:17 pm

Great post Nutty. I, like previous commenters appreciate your rational discussion of this type of issue. You do such a good job of it.

Do you also feel qualified to argue the arguments FOR cowbird trapping? I'd like to read more about that too in case it's more complex than the obvious. I suspect you would also handle that side of the argument in a non-emotional way.

Thanks for another nugget of education!

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