Brood Parasitism - Tumblr Posts

Footnote: Feel like seeing more rambles n' stuff? This is my current blog! Due to some technical issues with the old one, I will be rblging the original MMM and CFF posts on this account, as well as future write-ups!

Creacher Feature Friday 6: I Found a Tragedy and a Rarity in the Ferns 🪺

~Ah, brood parasitism, one of my favorite forms of symbiosis. Brilliant tactic, evolutionarily speaking. Why invest precious resources and time into raising your own offspring, when you can just entirely hijack the parental instincts of another species? The cuckoo bird is one of the most well known and famous examples of this reproductive strategy, never building their own nests, but happy to drop their eggs into the clutches of other birds to raise as their own.

*(fun fact: this is actually the origin of the slang words “cuckold” and “cuck”, from an old French term for the cuckoo bird.)

Today I would, however, love to talk to talk about the brown-headed cowbird, another parasitic avian species, common to the United States.

The reason why is because I found a lucky, incredible sight while watering my Aunt’s ferns once:

If you don’t immediately get why I was so excited when I realized what this was, this is a photo I took of an inactive house finch nest I had been keeping my eyes on last summer.

For some reason every year, they love to set up nests inside the hanging plants. A cowbird must have also decided it seemed like a lovely spot, and left behind one of its own eggs in the nest. Now I didn’t see the brood up close myself until the family had already moved on, and I snapped this picture when the time came to remove the nest, having no idea it had even been parasitized. And what was left behind is evidence of a little-known phenomenon called

Egg capping!

What makes parasitism like the This destructive to the original nest is that one cowbird hatchling usually dooms its adoptive siblings. They grow much faster than their host species and they immediately outcompete the other fledgelings in the nest due to their size for the host parents’ attention and feeding. I think you can see why they need all they can get. Exhibit A:

Another way they have of reducing competition in the nest, it’s been theorized, is to literally prevent their “clutchmates” from hatching at all- by “capping” one of the original eggs with its own eggshell. This is not something that the hatchling does on purpose, but with a little bit of luck, in the fact that they often hatch and develop faster than the hosts, this can happen by pure chance as the shell gets tossled around with the other eggs. And evidently, this is extremely rare to happen under normal circumstances, what with chicks synchronizing their hatching, the parents usually then removing the shells from the nest.

So, it’s a fortunate evolutionary bonus for these home-wrecker cowbirds that this is another way they manage to gum up the works.

An Extra note: though it is potentially upsetting news that Brown-headed cowbirds are detrimental to their host nests, they are still a native and federally protected species in their home range. It is illegal to remove or tamper with their eggs the same way it is illegal to disturb their hosts’, and upon finding a parasitized nest in your yard, remember that it is best to let nature take its course. Cowbirds are still part of this beautiful ecosystem, even if they are mooching free-loaders :)

But, there is a nasty little twist to the story in my nest photos. The cowbird chick probably didn’t find success either. You see… house finches were actually a really poor choice for the mother cowbird who decided to drop her offspring here.

While the majority of local songbirds here (brown headed cowbirds included) require insects in their diet to develop well and strong, house finches are a special exception- they are almost exclusively herbivores. The cowbird that capped this finch egg probably didn’t fare well under its unsuspecting parents, muscling out its competition in vein. I hope those finches have a better run next year, but I am very thankful I got to see some really neat ecology in action, and right outside the front door!

Nest parasitism

Bird meme by anonymous friend of mine; original comic by Nedroid