response to cowbird 101 comments

December 1, 2005

in Birds, Science

The last post, Cowbirds 101, received a few comments which I’d like to address.  I was going to do it in the comment section as well, but not everybody reads post comments, plus, I’m trying to improve “customer service,” so here goes.

Peter wondered about the reasons for cowbird declines.  I have not been able to locate any research speculating on the reasons for cowbird declines.  I am pretty certain it does not have to do with cowbird control for endangered species.  At least in the case of Kirtland’s Warbler, multiple sources have stated that the trapping has had absolutely no effect on cowbird populations. Culling of flocks of mixed blackbirds in agricultural areas, especially during winter, might be playing a role.  I believe the primary reason is habitat loss:  reforestation of agricultural lands in particular, consolidation of remaining small farms may into large monocultures, and urbanization.

Pamela, who participated in Ontario’s Breeding Bird Atlas, noted her surprise at public cowbird vilification, and also mentioned contemplating warbler behavior and parasitism rates.  This gives me an opportunity to expand a little on the “dire threat” misconception.

A minority of all host species make up the majority of the incidents of parasitism (almost 90%).  The top ten cowbird hosts are Yellow Warbler, Song Sparrow, Red-eyed Vireo, Chipping Sparrow, Eastern Phoebe, Eastern Towhee, Ovenbird, Common Yellowthroat, American Redstart, and Indigo Bunting (Lowther 1993).  I looked through the literature, and for each species I found documentation of rejector behavior, as well as the ability to raise their own young along with cowbirds. So even the most frequently parasitized species persevere — quite well, judging by how common these species are.

Kilgo and Moorman (2004) published a large study of nearly 1,400 nests of 24 cowbird hosts from the southeastern U.S., where cowbirds have only been present the last 50 or so years and presumably have found many new, vulnerable hosts. The parasitism rate was less than 10%.

Wisecrow thought we should err on the side of caution and not take a chance with endangered species by eliminating cowbird control. In a post which will arrive early next week, I will agree with this position, but conclude that selective removal of traps is probably worthwhile, if not necessary for ecological and political reasons.

To be continued…

Kilgo, J. C. and C. E. Moorman. 2004. Patterns of cowbird parasitism in the South Atlantic Coastal Plain and Piedmont. Wilson Bulletin 115:277-284.

Lowther, P.E. 1993.  Brown-headed Cowbird (Molothrus ater).  In the Birds of North America, No. 47. (A Poole and F. Gill, Eds.).  Philadelphia: Academy of Natural Sciences; Washington DC: American Ornithologists’ Union.

Peter Bryant December 1, 2005 at 6:08 pm

Thanks for the response. One of my other questions is this, based on an excerpt from your previous post: "In parasitized nests, the percent of host young fledged per egg is about 21%. For cowbirds, it is around 28%. In other words, cowbirds are not much more successful than their hosts."

Perhaps this is because of my ignorance of ornithology, but I don't understand the relevance of this comparison. Wouldn't a more important point be what the 'percent of host young fledged per egg' is in parasitized nests versus in non-parasitized nests?

I have yet more questions and comments, but perhaps I should save them up for the end of your series!

TroutGrrrl December 1, 2005 at 10:42 pm

Hey Nuthatch,

While I don't know enough about either side of this little controversy to impose my opinions very far, I do have one thought to share.

I don't find the blue paragraphs above to be especially compelling support for abandoning the cowbird trapping. Parasitism of just a few Kirtland's warbler nests may well have a much larger impact on that species than the thousands of cowbird nestlings hatched in thousands of song sparrow nests has on song sparrows, if you consider the relative total number of nest numbers for KW and SS. I suspect that just because Kirtland's warbler nests are not in the top ten cowbird choices doesn't mean there is no significant impact on Kirtland warblers. There aren't as many KW nests as other species' for cowbirds to parasitize, how could they possibly parasitize them as frequently as song sparrow or chipping sparrow nests? Am I reading this correctly?

Peter's comment makes sense to me too.

Nuthatch December 2, 2005 at 10:30 am

Peter: There are a lot of ways of looking at how parasitism impacts reproductive success. I used percent of host young fledged per egg because, well, that's the data that was available!

TroutGrrl: You are correct in that the blue paragraphs are not an argument for stopping cowbird control for Kirtland's Warblers, or endangered species. It was just meant to emphasize that cowbirds are not a dire threat to songbirds in general, in response to Pamela's observation on how many people just hate cowbirds.

I have had many well-meaning people tell me that they kill cowbirds on sight and/or remove cowbird eggs and chicks from nests (direct approach to any nest frequently leads other predators to it later, by the way) in order to "help" other birds. These people are convinced they are doing the right thing and that cowbirds pose a huge threat to the birds they are helping — which are nearly always in that top ten list.

My main intention with the posts so far was to point out that most people don't really understand cowbirds or parasitism in general, and hopefully offer some enlightenment. Wait til I get the the next two posts, where I toss out some ideas specific to endangered species. I'm getting a little apprehensive…

Wisecrow December 2, 2005 at 11:32 am

Surprising that most all of the top ten parasitized birds are eastern birds. I wonder why that is? Yellow Warbler is the only one that is fairly common in the west.

Nuthatch December 2, 2005 at 12:06 pm

Two reasons come to mind. First, since they have a longer history with cowbirds, many western birds may be primarily rejectors. Of course, cowbirds would not have survived if all the hosts in their historical range were complete rejectors — while I don't have the exact criteria used to categorize the top ten, I presume it is the species which are the most frequent hosts that accept cowbirds most or much of the time. Second, I'd guess there might be some sort of "observer bias" in this list. In other words, perhaps more research has done on cowbird parasitism has been done in the east, hence the bias towards eastern species.

Anonymous December 2, 2005 at 12:14 pm

as someone who understands cowbirds and Kirtlands breeding grounds well, I hope you take it into account that their breeding grounds are riddled with roads, that are fire trails essentially. To reduce the trapping there would have devistating effects Nutty. The Kirtland's is definitely in the 'top ten list' of nests to be parasited in their breeding range.

Two gentlemen who know much more about the raw data from the recovery program can share their opinion/expertise with you and maybe convey in words what I find difficult to do here-

Phil Huber, Wildlife Biologist, USFS Mio
(don't have his number handy, but will find it)

Mike DeCappita, with the USFWS department in Lansing: 517 351 6274

It *has* to be kept in perspective here that the breeding territory of the Kirtland's warbler is unique. It's unlike any habitat found in Michigan. For your next intallment I'll share an airial map of the area, so you can see how very attractive it is to b.h. cowbirds.. it's one huge edge environs. Fire trail upon fire trail weave their way through the jackpines and both gentlemen above who I spoke to personally, expressed the fact that Kirtland's populations WILL go down if the trapping is reduced. There is no getting around that fact.
It's too big of a risk to take, because alot of people have worked damn hard out in the field to get those numbers up.

Nuthatch December 2, 2005 at 12:33 pm

To reiterate, I've been trying to explain, thus far, cowbird ecology and songbirds in general — and have not really touched upon endangered species yet.

I would love — after the last post next week — to have responses from Mike and Phil, since much (most?) of the current research and data on cowbirds and Kirtland's Warblers specifically is apparently unpublished and I have been unable to access it. I am going on other published research on cowbirds, including a recently published monograph by the American Ornithologists' Union.

I agree that much of the range of Kirtland's Warbler is very appealing to cowbirds. Forest roads are a huge problem. I'd like to hear about the efforts, practicalities, etc. of doing something to further restore and enhance these habitats so that they ARE NOT as appealing to cowbirds. That's part of my point.

TroutGrrrl December 2, 2005 at 5:18 pm

Gotcha. I'm looking forward to following the path this discussion takes…

Cindy M. December 3, 2005 at 5:57 pm

nutty, drop me an email when you get a chance.. I've lost yours (another crash) and I have a special print I want to send you for the holidays.

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