Brood parasitism -- one species laying its eggs in another birds' nest for the "host" parents to raise -- is in an interesting behavior that a few species have adopted. The Brown-headed Cowbird is our local example. Old World cuckoo species also do this, and have been the subject of many a BBC nature program. (As an aside, our cuckoos don't do this, although they're known to lay their eggs in each other's nests.)

Many potential host species (or populations) have developed responses to brood parasitism, such as abandoning a nest with a foreign egg in it and renesting.

One aspect to our situation in California is that cowbirds historically didn't occur here, at least not west of the mountains. With the advent of ranching, agriculture, and urbanization, people's activities allowed cowbirds to expand their range westward into Southern California and the Central Valley (and beyond).

In short, the local species/populations had never before in recent evolutionary history had to contend with brood parasitism. Our local host species don't recognize the interloper's egg and continue to brood the clutch. For many small species (in particular), the cowbird chick out-competes its "adopted" siblings -- or even ejects them from the nest. In the end, the host parents spend all their time and energy to raise a single (often much larger) cowbird chick.

For a species like the Bell's Vireo, the entire reproductive effort of whole local populations can go into raising cowbirds. That, combined with the historical loss of their riparian vegetation habitat, led to a "one-two punch" that brought the California subspecies (the Least Bell's Vireo) close to extinction, going from one of the more common riparian species in the Central Valley and Southern California historically to a few hundred individuals at few specific SoCal localities in the early 1980s.

Since then, efforts to protect and restore areas of riparian vegetation, combined with targeted cowbird trapping has allowed the population of Least Bell's Vireos to increase at least 10-fold. After retreating to extreme Southern California, they are now once again starting to expand northward. Cowbird management has been a key aspect of this reversal.

All that said, cowbirds are not evil (even if you might be inclined to think so as you sympathetically watch the host-species nestling slowly starve to death because the bigger, more aggressive cowbird chick is taking all the food the host parents bring). They're actually quite interesting. If you think about it, how do cowbirds know they're cowbirds At the end of the season, a group of cowbirds congregate together at your local food court, one may have been raised by Song Sparrows, another by Yellow Warblers, and another by Bell's Vireos -- and yet, in the end, each one knows it's a cowbird and that, being of a feather, they should flock together. Fascinating!

-Gjon

-GjonOn May 7, 2019, at 5:57 PM, Joe Morlan wrote:

Alex,

My photo of a similar trap and an explanation is at

https://flic.kr/p/rJCV2f

On Tue, 07 May 2019 16:26:06 -0700, "Alex Tolkachev"
wrote:
There is a large bird trap operated, I assume, by OC Parks not far from the creek crossing where the dirt trail and paved trail originating at Awma Road and running on opposite sides of the creek meet. Today, it was inhabited by about six Brewer's Blackbirds (unless I am mistaken; I did take photos just in case). There is food and water inside. I started reading the sign, but the birds were getting agitated by my proximity, so I only gave it a very cursory reading before stepping back. I gathered from the sign that the purpose of the trap is to help endangered species breed more successfully, that the trap is checked daily, and that the public is asked in
the strongest terms not to mess with it.

Does anyone know the details of this operation and can anyone explain to me what this trap is all about I don't believe it is meant for blackbirds, they must have just chanced in. In fact, it is probably not targeted at any one bird The rangers/ornithologists probably want to see who all get in and take it from there Or maybeit IS intended for blackbirds in the sense that they are the ones bothering the endangered species, and therefore need to be removed They will be treated humanely, the sign says, but I don't know what that means. Euthanized humanely Relocated Banded, checked and released

Next time I am there, if the trap is empty, I will read the sign fully. Until then, I am really curious about what others know and think about this trap. Are there many of these around --
Joseph Morlan, Pacifica, CA