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Before I start, know I’m going to be citing lots of academic articles that are likely behind a paywall. If you want to know more about the articles and can’t access them please DM me! Science should be available to everyone, but that is an argument for another time. 2/37
The Brown-headed Cowbird! A sharp-looking blackbird from North and Central America. Males have black bodies and chocolate-brown heads. Females are sleek and brown. They make fun bubbly noises. 3/37 allaboutbirds.org/guide/Brown-he…
Brown-headed Cowbirds are obligate brood parasites, meaning that they lay their eggs in the nests of other birds and let those hosts raise their young. Why? So they can dedicate more time and energy to laying more eggs—maybe up to 40 in a year! Source: [Holford & Roby 1993] 4/37 
Is this brood parasitism evil? No! Birds are birds and we can’t hold them to our moral constructs. Many of your faves (sparrows, ducks, etc) actually parasitize nests of others in their species—this “conspecific brood parasitism” means more eggs in more baskets! [Poysa 1999] 5/37
Obligate brood parasitism (only laying in others’ nests, like cowbirds do) likely evolved from this strategy and has evolved multiple times in many different species! [Source: Davies 2000] 6/37
Cowbirds were originally called bisonbirds by colonizers because they follow bison and eat the insects on them. It used to be assumed that mobile bison led to this brood parasitism strategy (cowbirds need to keep moving to follow bison, therefore couldn’t settle on nests). 7/37
However, evidence suggests brood parasitism in the cowbird genus (group) evolved in South or Central America (without bison). 8/37 birdnote.org/blog/2015/05/c…
Perhaps the best example of obligate brood parasitism exists in the Eastern Hemisphere cuckoos! But, fun fact, North American cuckoos (like this Yellow-billed Cuckoo) aren’t brood parasites at all! 9/37
Old-world cuckoos lay their eggs in host nests, removing all other eggs (or letting their chicks do it!). Cowbirds do not—they often remove one host egg but leave the rest. Cowbird baby (white mouth) and Dickcissel host (yellow mouth) 10/37
So how does this work out for host birds and nestlings? It really depends. First of all, cowbirds parasitize a lot of species! Our Brown-headed Cowbirds parasitize over 200! [Source: Ortega 1998] 11/37
Sometimes they really screw up, such as parasitizing duck nests! [Source: Koons 2000.] Ducks don’t feed their nestlings like songbirds do, so the cowbird will never make it. 12/37
Cowbirds evolved on the Great Plains, so many species there are equipped to handle cowbirds. They may have evolved defense strategies like fighting cowbirds, burying their eggs, abandoning their nests, preferentially feeding their own babies, etc. [Source: Aviles 2018] 13/37
Some hosts don’t defend at all. Why? Well, cowbirds may retaliate if hosts hurt their young, attacking the host eggs or nestlings like a “mafia”! 14/37 sciencemag.org/news/2016/12/t…
Other hosts may be more likely to damage their own eggs while trying to remove cowbird eggs, especially since cowbird eggs may be thicker than host eggs. [Source: Peer et al. 2018] 15/37
Sometimes hosts don’t need to worry about cowbirds in the nest. Finches feed their nestlings too many plant parts for cowbirds to survive—they need insects and will almost always starve to death in finch nests. [Source: Middleton 1991] 16/37
In other cases, the host nestlings grow up just fine despite the cowbird competition. In the #BoyleLab at @kstatebio we are studying how this works with project #PrairieBabies (my Master’s thesis). 17/37
Cowbirds can also be nest predators (like Blue Jays or meadowlarks, don’t judge) and may preferentially destroy nests without cowbirds! Here is camera footage we have of cowbirds killing nestlings (sorry it is sideways) 18/37
But here is more camera footage of a cowbird coming to a nest and only killing a single cowbird baby! Maybe the host parents scared it off? Remember, from a natural selection perspective, if these cowbirds aren’t related they are competitors! 19/37
So despite some ridiculously high parasitism rates in the Plains (up to 90% of nests parasitized, as many as 9 cowbird eggs in a single nest!) the hosts here might be doing okay. Source: this photo I took at @KonzaLTER and @Bram_Verheijen 20/37
However, as colonizers opened up the dense forests in eastern N America, cowbirds moved in, encountering species that haven’t evolved these defenses. Cowbirds in some parts of the US (like the SE) have only been there for ~50 years! [Source: Peer and Sealy 2004] 21/37
Birds that have never encountered cowbirds were suddenly parasitized. Species that were already in decline due to habitat loss/degradation were devastated by this new nest threat. The Kirtland’s Warbler (allaboutbirds.org/guide/Kirtland…) is one of those species 22/37
Scientists made a last-ditch effort to save the endangered warbler in the 1970s by removing and killing cowbird adults in the warblers’ range. It was quite successful—there are very few cowbirds there and the warblers are starting to recover. 23/37 fws.gov/midwest/endang…
Removing cowbirds may not always be a good idea, however. In KS, removing cowbirds led to more successful nests, but that benefited the few cowbirds that remained—in the end, not helping the hosts at all. [Source: Kosciuch and Sandercock 2008] Ecosystems are complicated! 24/37
Apparently there is a max number of tweets per thread? But I have so much more to say! I will post a second half of the thread next.
How legal is cowbird removal? Well, cowbirds are protected under the Migratory Bird Treaty Act (fws.gov/birds/policies…). For the average citizen, killing adults, eggs, or nestlings is illegal. Researchers can do it with federal permits, but they really need to justify why. 25/37
Cowbirds may hurt some of these eastern forest birds, but remember that this wouldn’t be a problem without human-caused habitat destruction. And we don’t harm other species because of range shifts (like predatory hawks expanding northward), so why target cowbirds? 26/37
Plus, cowbirds are really really cool! What hormones lead adults to provide parental care? How do birds know how to find nests? How do cowbirds know they are cowbirds and not their host species? 27/37
Cool scientists like @cowbirdlab and @lowellmills2000 study these questions with brood parasites to know more about birds in general. 28/37
I want to know how food availability/quality and fear of predators affects nestling growth. But there may not be a lot of differences between members of the same species—sparrow babies may grow about the same, get about the same food, have the same risk, etc. #PrairieBabies 29/37
But cowbirds live in the sparrow nests and also the nests of hundreds of species, all of which have different growth rates, food availability, predator risk, etc. Cowbirds allow me to ask these questions with much better data! #PrairieBabies 30/37
Plus, cowbirds are precious, ugly babies. Baby sparrows and Dickcissels scuttle away when I try to measure their growth, but cowbirds sit on my finger and beg for food. They make me feel like a Disney princess. #PrairieBabies 31/37
I clearly like cowbirds (I have been accused of being a “cowbird apologist” before), but you don’t have to take my word for it! Check out this cool Audubon article: audubon.org/news/is-it-oka… 32/37
.@JasonWardNY , #birding celebrity, reminds us that: “Cowbirds are a native species w/ a very cool adaptation. Just b/c it makes you feel uncomfortable isn’t grounds to remove the egg.” 33/37
Plus, bird parents keep track of the number of eggs in their nests. If you remove the cowbird egg, you are making it look as if a predator stopped by and grabbed that egg. Parent birds will often abandon nests if they think a predator knows where it is. 34/37
After all, why bother to invest time and energy into incubating eggs and feeding nestlings if they will ultimately end up as a snake snack? Especially since many birds will simply start another nest. Predation video showing the cool predators that eat our #PrairieBabies 35/37
So, long thread short: you may not like that your backyard bird nests have cowbird eggs. But removing the cowbirds is illegal and can ultimately lead cowbirds to retaliate or host parents to think their nest has been compromised by a predator. 36/37
Let nature take her course, and in the meantime appreciate the amazing development of cowbird and host nestlings! [This very long thread brought to you by my five years of growing to love ugly cowbird nestlings] 37/end
