A rise in practical jokes and tricks happens in homes, schools and workplaces thanks to April Fool’s Day. Do animals have any tricks up their sleeves? Some do, but not to get a laugh like with humans. Animal “tricks” have adapted over time to help them survive.
The survival of brown-headed cowbirds depends on their ability to trick other birds to raise their young. Brown-headed cowbirds don’t build nests. They don’t take over abandoned nests, like owls do, either. But they do lay eggs. Where do they lay their eggs if they don’t have nests?
Observe, plan, then strike
Instead of building her own nest, a cowbird lays her eggs in another bird’s nest, tucked in with the eggs already there. This sounds neglectful — laying an egg and then leaving — but it takes planning to get it right.
The cowbird will watch an occupied nest to learn the habits of the bird — when it sits on the nest and when it flies away. While the other bird is away, the cowbird must sneak in, quickly lay her own egg and leave without being spotted.
Make room for one more
The cowbird must choose a nest that already has eggs in it, or it would be too obvious that her egg does not belong. A bird flying away from an empty nest and returning to a full one would notice the change. If your sibling snuck their unwanted vegetables on your plate while you looked away, you would definitely notice if your empty plate became suddenly full!
But eggs have already been laid in the nest! How will the cowbird fit hers in it? Sometimes there’s room for one more egg, but not usually. How do you make room in a crowded space?
Words to know
Instinct: An innate pattern of behavior in animals that occurs in response to certain stimuli.
Neglectful: Not giving proper care or attention to something.
Have you ever wanted to avoid taking out a full garbage can, so you crush everything down to make room? Sometimes cowbirds will destroy an egg or two to make room for their own.
Has your desk ever been so crowded that something falls off when you push things out of the way to make room to do your homework? Sometimes cowbirds push eggs out of the nest for extra space.
Has your freezer ever been too full after grocery shopping, so you finish a carton of ice cream to make more room? Sometimes cowbirds do that too. But instead of ice cream, they eat an egg. Not only does that make room for their own egg, it also provides calcium the cowbird needs to stay healthy and be able to lay even more eggs in even more nests.
Another trick
Cowbird eggs hatch sooner than other bird species. So they hatch first, before any of the other eggs in the nest. Most species of foster parent birds will feed the cowbird baby the same as their own. Maybe this is because they don’t notice the egg is different. Or maybe because instinct takes over and they would rather raise the cowbird baby than no babies at all.
Since it hatches first, and gets fed first, it also grows bigger faster than the other baby birds that hatch later. The cowbird takes advantage of being the biggest by insisting on getting most of the food. Sometimes the other baby birds don’t get enough to eat because the baby cowbird hungrily eats its share.
Any nest will do
Cowbirds can lay more than three dozen eggs throughout the nesting season, which lasts from May through July. That means finding 36 different nests to lay their eggs in. Do they choose the same species of birds to foster their babies? Nope. Cowbird eggs have been found in the nests of more than 300 different species of birds!
Identity crisis
Do cowbirds raised by cardinals identify as cardinals? No. Once the cowbird reaches maturity and leaves the nest, it finds its way to a cowbird flock. Scientists think cowbirds use clues from their own bodies. They may recognize their own call, much like hearing someone speaking your own language. They may notice ways they look alike. Maybe you could study this science mystery and make a discovery of your own!
Can I help?
If you find a nest with eggs and one looks very different, you may have found a cowbird egg. Knowing that the cowbird will hatch first and likely take resources from the other babies may make you want to remove the egg. Do not remove it. Federal law protects wild birds’ eggs, nests, feathers and young. Besides, by removing cowbird eggs, they won’t survive either. Baby cowbirds rely completely on foster birds for their own survival. You need to let nature take its course.
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