I’ve been seeing Brown-headed Cowbirds (BHCO is their four letter code) in our yard. I have mixed thinking about Brown-headed Cowbirds. I’m dismayed on behalf of their victims, but I have to say intrigued by their successful evolutionary survival strategy.
Male Brown-headed Cowbird Scouting House Finch Nests 
Female Brown-headed Cowbird Waiting For The All Clear Sign
BHCOs are brood parasites. They do not build nests. The female lays her eggs, up to 40 per season, in involuntary hosts’ nests. BHCOs are not picky about the foster parents. They have been observed parasitizing over 220 different species. Hosts do all the parenting. Since the BHCO’s eggs typically incubate earlier than the host’s chicks and grow faster and bigger, the host’s chicks don’t fare well in growth rates or even making it out alive.
This raises the question of why don’t the hosts reject the obvious ‘bad egg’ or later the obvious impostor. Well some do and this is where it gets interesting. Interesting in a macabre kind of way, unless you’re a Godfather fan. Recent research is suggesting that the BHCOs make a deal the hosts can’t refuse – Raise our chicks or we’ll destroy your nests and chicks. This theory comes from observations that BHCOs do in fact destroy egg-rejecter nests immediately after the BHCO egg(s) are destroyed. This annihilation forces the host to start again. Apparently, and it is not known exactly how, the hosts come to the evolutionary conclusion its better for the survival of the species to raise a few chicks plus the BHCO’s than no chicks. There is also recent evidence that BHCOs will ‘farm’ nests after the host’s eggs are hatched. They destroy the chicks to force another nesting and brood that they then parasitize.
And, as often happens in bad environmental stories, humans have had a hand in this arms race. BHCOs were native to the plains and followed the bison herds. Laying their eggs in other grassland species nests allowed them to pick up an move when the herds moved. Their damage to other species was dispersed over large land tracts and large populations of species native to the plains. Well we know the fate of the bison herds. BHCOs responded in a practical, if not admirable, fashion, they moved right into large-scale commercial beef producing and dairy farm eco-systems. It was even easier than their former lives in some ways, not as much nomadic living. And, they had a whole host (pun intended) of new bird species in adjacent habitats and eco-systems to parasitize and because this was new encounters for the new hosts the BHCOs had (have) an significant evolutionary gap to exploit. The migratory songbirds living on the edges of large agricultural tracts and open de-forested urban and suburban fragments had not, evolved a defense strategy against the BHCO parasitic and bullying behavior. And, when they have tried the obvious, they trigger the BHCO’s retaliation only making things worse. This is an evolution arms race. Some species, the Kirtland Warblers being one, are not faring well and BHCO parasitization is considered a major contributor to their population decline.
Male Brown-headed Cowbirds On Dairy Farm 
Female Brown-headed Cowbirds On Dairy Farm
Like I said, mixed thinking. I hate to see the sparrows and finches lose chicks, but the evolutionary battle playing out in the yard is fascinating. Kind of like Godfather Part MMXX.