Tuesday, June 25, 2013

The Evil of Cowbirds

A big parasite that persecutes a beautiful rare songbird is hard to take. Why torture delicate rare birds? For that matter, why burden people’s sensibilities by writing it up in a blog? And yet, sometimes by facing wickedness, we can understand it, and disarm it. 

Yesterday I saw something wonderful and (to me) scary. The inspirational part was the first orchard oriole nest I ever found. It was high in an isolated elm, surrounded by a meadow of rare flowers and grasses. The scary part was that the orioles seemed to be defending their nest from a female cowbird, even though there were babies crying from the nest.

The big fat, ugly baby on the right has likely killed two or three young orchard oriole 
siblings like the one on the left. Be still, my heart!

A while back I had arguments on this subject with a wise and wonderful bird conservationist, the late Jerry Sullivan. It was Jerry who developed the monitoring protocols now used by the Bird Conservation Network. Indeed, he developed them at Somme Prairie Grove, where in his years of work, while the restoration was in its early decades, he never saw a single orchard oriole. He would have loved to see them and their babies. He might not have liked this write-up.

I told Jerry that when I happened across a birds’ nest with a cowbird egg in it, I took out the cowbird egg. To me, it was like weeding a garden. Cowbirds are nest parasites that under modern conditions are pushing some bird species toward extinction – and vastly reducing the numbers of many others. “Why shouldn’t I help the species that need it?” I asked. As best I remember, Jerry felt strongly but was uncharacteristically inarticulate about why not. “They’re nature,” he said. “How do you know the result? Leave them alone.”

The nesting orchard orioles at Somme are a confirmation of a precious hope – that if you successfully restore a natural ecosystem, the now-rare animals of quality ecosystems will come back. (Well, perhaps I should write, “at least it may attract the rare animals that can fly and that still survive somewhere within flying distance.”) In other words, the orchard orioles that annually breed here confirm the quality and meaning of Somme Prairie Grove. In doing so, they affirm what some of us have been devoting much of our lives to for three plus decades. 

Unlike the much commoner Baltimore oriole, the orchard oriole breeds in savannas, not suburban neighborhoods or forests or prairies or anywhere else. (Actually, before the days of pesticides, they regularly bred in orchards, which are savanna-like in that they consist of scattered trees in grassland.) Since this species evolved to breed in a specialized habitat, without a place like the savanna at Somme, there would be no young orioles to carry on the species.

And it’s not just this species. Many shrubland and savanna birds (and butterflies, snakes, wildflowers, grasses, etc.) now breed merrily at Somme: among the birds - Woodcock. Black-billed cuckoo. Ruby-throated hummingbird. Eastern kingbird. Great crested flycatcher. Wood pewee. Northern flicker. Yellowthroat. Warbling vireo. Eastern bluebird. Rose-breasted grosbeak. Indigo bunting and many others. Some of these now or in the past have adapted to our culture’s farms, neighborhoods, or roadsides. But then come changes (mowing, pesticides, etc.), and the birds need to fall back on the nature they evolved in. Conservation land is increasingly critical to many of the species that now have babies or set seed each summer in Somme Prairie Grove.

When I texted Lisa with my discovery, she raced there with her camera. Most of the rest of this story is well illustrated by the photos that she took yesterday. 

This baby oriole 
on the branch 
hatched from an egg 
in the nest above. 
The hanging structure is beautifully woven of strong fresh grass and lined with fine grasses and plant down. The oriole mom lays four to six eggs and incubates them to keep them warm. Unfortunately, in this case, and many cases, the female cowbird sneaks in unnoticed and lays one big egg of its own. The female cowbird is too busy to help incubate or feed, because she on average lays 40 eggs per year in nests of other birds and races around protecting her eggs and young and chasing other female cowbirds 
from her territory. 
She's busy.

The female oriole has arrived
with a fat grasshopper. 

Who’ll get it? 

The cowbird is the one 
with the big mouth. 
Can you see the baby oriole? 
(Hint: the cowbird 
is standing on it.)

Now the mom is ready with the food. Babies need a lot to grow feathers and strength enough to fly and avoid predators. Guess which chick is going to get this meal. Hint: cowbirds are evolved to win this contest. Once “cow” birds followed herds of bison, which is why they couldn’t hang around to take care of junior. They are evolved to find their food in the short grass where the bison have been grazing. These days they find it in lawns, so they can stay put and defend territories.

Let’s see here. One of 
the best techniques is just to step on the head of the competition. 
The baby orchard oriole gets fed only when the parents bring food so fast that the cowbrat is too stuffed to choke down any more. 
Earlier, the cowbird parasite may use its big butt to push baby orioles to the side of the nest, and then up, and out, so that they fall to their death. Later some babies will just starve because Mr. or Miss Greedy-kins pigs up all the food.

Here’s the male 
bringing a juicy worm. 
When the orioles are successfully raising four or five (to as many as eight) of their own real chicks, sometimes the male and female each tend a sub-family of two to four chicks. When fledged, the chicks and their chosen parent can then wander to a part of the preserve where food is more plentiful.

Ahhhh! Finally, 
the baby oriole 
gets some grub. 
One surviving chick 
is better than none.

We wish this little one good luck. 
In time, the orchard orioles may evolve their own defenses against cowbird parasitism as some other species have done. Some species have learned to throw the intruder’s egg out of the nest. Some species recognize that the parasite’s egg is evil (from their perspective) and build a new bottom to the next on top of that egg, so that it doesn’t get incubated and never hatches. But evolution takes a long time. The orioles might need a bit of help (like habitat restoration to increase their numbers) if they are to accomplish such an evolution.   

Good luck, little oriole!


But below are a few more tidbits about orchard orioles and cowbirds, if you'd like.


I have to admit that Jerry Sullivan’s concern about removing eggs did trouble me. Am I wrong to “play God” with this egg removal? Am I counterproductively slowing the evolutionary process? Or perhaps am I causing some other kind of unexpected harm? How could I know? So I kept confiscating cowbirds eggs on the occasions when I found them – somewhat confident that I was on the side of the good.

One day, walking through Vestal Grove at Somme I noticed a female indigo bunting building a nest beside the footpath. “Nice,” I thought. “I can keep an eye on this one. Perhaps get some good photos.”

Next day on my way into the preserve, I noticed a pale and pretty little bunting egg in the nest. On my way out of the preserve two hours later I checked again, and the little egg had been joined by a big speckled cowbird egg. I removed the later.

Following morning I checked again, and the bunting’s egg was lying on the ground under the nest, punctured by a peck of some evil bill. Hmmmm. I suppose this is one option Jerry was thinking about. Cowbirds do guard nests where they’ve laid eggs and sometimes throw out the original egg if something happens to their own. Purpose? To get to owner to start laying a second time. The cowbird doesn’t want the other bird’s eggs to get ahead. If the parasite’s egg is laid at the same time as the host’s, then the fat cowbird egg (which hatches a bit quicker than other birds’ eggs) will be assured the head start that will make its chick big enough to be the bully.

I wondered what would happen next. I considered another approach. I could let the next cowbird egg stay in the nest until all are about ready to hatch (12-13 days for bunting eggs) and take out the cowbird egg at the last minute. Perhaps the cowthug wouldn’t still be watching?

But next day the whole nest had been torn and ravaged as if by a raccoon and both new eggs were gone. So I couldn’t find out.

Wikipedia says: “Cowbirds periodically check on their eggs and young after they have deposited them. Removal of the parasitic egg may trigger a retaliatory reaction termed "mafia behavior". According to a study by the Florida Museum of Natural History published in 1983, the cowbird returned to ransack the nests of a range of host species 56% of the time when their egg was removed. In addition, the cowbird also destroyed nests in a type of "farming behavior" to force the hosts to build new ones. The cowbirds then laid their eggs in the new nests 85% of the time.

But what if the egg is removed toward the end of incubation? And was that female cowbird I saw at the nest really interfering with the nestlings, as the commotion suggested when I first saw this nest? According to some studies and anecdotal observations, cowbirds do sometimes attack the nestlings that are competing with their own little freeloaders.


If you look up “orchard oriole” in the average bird guide, the male will be a handsome combination of black and deep brick red – not the mix of yellow and reddish that the male bringing food yesterday displayed. What gives? Atypically, in this species first year males have a distinctive coloration, much like the female, but with black markings on the head. Why would that be?

Here’s a theory: maybe females want a good way to distinguish older males, possibly because they’re better at the challenges of raising babies. Last year we had one mature male at Somme. This year we’ve just got two first year guys. That probably means that Somme is still somewhat of a marginal habitat. When I check out the orchard orioles and the impressive big habitat in the forest preserves along the DesPlaines River near the junction of Willow and Sanders Roads, I see more mature males. It would be good to compare reproduction rates there with those at Somme. Anybody up for some research?

All photos by Lisa Culp … except for the mature male oriole from ms.audubon.org by Bill Stripling.

Note on the language:
It has not escaped my attention that this blog uses emotional and slangy language. It seemed like a good idea in this case.