Should You Remove Cowbird Eggs from Host Nests?

Should you remove cowbird eggs from host nests?

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When bird nesting season comes around, we have a responsibility to make sure our feathered friends are protected and safe. This often comes in the form of offering the right types of bird food, like eggshells, and keeping pets out of the garden.

But what a lot of people aren’t aware of is the threat to certain species when other birds lay eggs in their nests.

The cowbird is one of the most common culprits, so if you find their eggs in another bird’s nest, you may face the moral dilemma of whether to remove them.

Why do Brown-Headed Cowbirds Lay Eggs in Other Bird’s Nests?

Why do brown-headed cowbirds lay eggs in other bird’s nests?
Under the same moon / Flickr / CC BY 2.0

Cowbirds are what is known as a brood parasite. These birds do not make their own nests for laying eggs and raising young but rather invade those of other birds. These birds will lay their eggs in the nest of another bird, tricking it into raising the young as its own. It saves the cowbird the hassle of having to raise its own chicks.

This behavior is not limited to birds but can also be seen in fish and insects.

The key to being a good brood parasite is to select nests where the eggs look like your own. This is called egg mimicry and fools the host into thinking that the eggs are their own. If they didn’t believe this, there would be a risk of them evicting the eggs or the chicks when they hatch.

These birds will either scare the hosts away in order to plant their eggs or wait until they naturally leave the nest. The appearance of the eggs, as we have mentioned, can be very deceptive but the problem is that they also usually hatch before the host’s eggs.

This often means that the invading bird will kill its ‘siblings’ in a very early game of survival of the fittest. The chicks are able to thrive and go on to reproduce while the parents get away with never having to do an ounce of work where raising young is concerned. Pretty clever, if not cruel!

What Bird Nests Do Cowbirds Mainly Lay Their Eggs In?

What bird nests do cowbirds mainly lay their eggs in?

One of the most fascinating things about the cowbird is that it isn’t fussy about where it puts its eggs. It’s thought that these birds will invade the nests of up to 140 different bird species.

The cowbird is the most common brood parasite in North America, and it’ll invade the nests of several species, including the sparrow, yellow warbler, and blackbird. The red-eyed vireo and the spotted and Eastern towhees are also at risk of being parasitized by the cowbird.

Both small and large birds could fall victim to the cowbird’s sneaking egg-planting techniques, with birds like the meadowlark and kinglet both being affected.

Can Birds Detect Parasitic Eggs?

Can birds detect parasitic eggs?

For the longest time, it was thought that host birds would detect a parasitic egg based on its appearance and how much the egg differed from its own. But amazingly, experiments have shown that host birds are more likely to discard eggs based on their color, even if it’s nothing like their own eggs.

During these tests, scientists noticed that birds would be more likely to discard brown-colored eggs from the nest and would favor green or blue eggs. This applied even to birds whose own eggs were not green or blue in color.

One of the ways that a bird will eject a host egg from its nest is by piercing the shell. The problem with cowbird eggs is that the shell is incredibly thick so the host birds are unable to penetrate it. This was noted by scientists due to the fact that some of the cowbird’s hosts lay eggs that are remarkably different from the parasite and would be immediately obvious. 

Some species, such as a yellow warbler are known to bury the parasitic egg underneath the nest material while other birds will completely abandon the nest, including their own eggs. The blue-gray gnatcatcher is a prime example of this behavior.

There are some birds that will literally pick the egg up and throw it out of the nest and surprisingly, as many as 95% of gray catbirds will detect and evict a cowbird egg. Moreover, even after the birds hatch, there are some species, like finches, that just cannot provide the right diet for the parasite so they never survive.

How to Spot if Cowbird Eggs are in a Host Nest

How to spot if cowbird eggs are in a host nest
Vélocia / Flickr / CC BY 2.0

If you’re keen to protect bird species in your backyard then knowing the signs that a cowbird has laid eggs in a nest is a great place to start. Here are some of the things you’ll need to look out for.

  • The appearance of the eggs may not be the most reliable way to tell if they are parasites since the bird will try to choose nests where the eggs look the same. Cowbird eggs will usually be white/gray and have streaks or spots in either gray or brown.
  • Looking at the size of the eggs is a good way to tell whether they are parasites since cowbirds sometimes lay larger eggs than their host birds. However, this is not always the case. For example, the Northern Cardinal is often parasitized by the cowbird, and their eggs are around the same size.
  • In some cases, the cowbird will remove host eggs in order to make room for her own. Check the area for discarded eggs.
  • Since cowbirds tend to hatch before the host eggs, the chicks will look larger than the rest of those in the nest. As the birds get bigger, they may even outgrow the host parent, so if you see a bird feeding one that’s larger than itself, it’s probably a parasite.
  • Cowbird chicks will also have different colored gapes than their hosts being redder as opposed to yellow.

Can Cowbird Nestlings Harm the Hosts Nestlings?

Can cowbird nestlings harm the hosts nestlings?
Kelly Colgan Azar / Flickr / CC BY-ND 2.0

You might think that cowbirds simply lay their eggs in the host nest, the chicks are taken care of, and everyone is happy. But this sadly isn’t the case.

When the female first chooses her host nest, she’ll often evict one of the host eggs, effectively wiping out any chance of the chick surviving.

If that wasn’t bad enough, when the chicks hatch, they’ll often kill the other nestlings in a fight for the most food and in order to survive. They’re typically much bigger and so the other birds don’t stand a chance.

Even if it doesn’t kill the siblings, the cowbird is bigger and so the parents will pay it more attention in the belief that it has a greater chance of survival. Not knowing that they’re doing nothing to aid the survival of their own species.

Should you Remove Cowbird Eggs from Host Nests

Should you remove cowbird eggs from host nests

Understanding the negative effect that cowbirds can have on other species is naturally concerning. However, we simply have to accept it as part of nature and should not interfere.

While you’re probably tempted to remove cowbird eggs when you find them in other bird’s nests, this could actually do more harm than good. In many cases, host birds will abandon their nest if they suspect it has been tampered with. When you remove the cowbird egg, there will be an unexpected number which tells the bird that something is wrong. If they ditch their eggs, there’s no chance that any will survive.

Even more shocking is that the female cowbird is still keeping an eye on things and if she sees you removing her egg, there’s a good chance she will return to destroy the nest. There are a couple of theories on why this is. Primarily, it’s thought that she will do this as a way of forcing the hosts to make a new nest with new eggs and the cowbird can try again. Alternatively, she may be trying to sabotage the survival of the host species.

But even so, removing cowbird eggs from the nest could land you in trouble with the law. It’s illegal under the Migratory Bird Treaty Act to remove protected bird’s eggs from the nest under any circumstances. While the cowbird may be a nuisance, it’s still protected under this act.

Distribution of Cowbirds in North America

Distribution of cowbirds in North America

The brown-headed cowbird is common across North America and can be found year-round throughout the mid-United States. The birds tend to spread further North, as far as Canada during the breeding season, and may migrate south outside of this, moving as far down as Mexico and Central America.

In terms of habitat, the cowbird is typically found in fields, pastures, and other open spaces. They’re very common on garden lawns, and if you look up into the trees, they’ll usually be spotted on high-up, prominent branches.

How to Deter Cowbirds from Bird Feeders?

How to deter cowbirds from bird feeders?

While we cannot stop cowbirds from planting their eggs in other bird’s nests completely, there are things we can all do to deter them from our backyards. When we are successful in this, we are better able to protect our backyard birds from these parasites. Here are some tips on how to deter cowbirds from feeders in your garden.

  • Avoid offering foods that the cowbird likes, such as sunflower seeds and millet. They’re less likely to show an interest in your feeders if you use foods such as nyjer seed, nectar, peanuts, or suet.
  • Cowbirds are larger than many of the birds they target, so keeping them away from bird feeders involves thinking about size. Don’t use feeders that will make it easy for the cowbirds. Instead opt for those that are designed for smaller birds. Look for feeders without a bottom catcher and that have small perches.
  • Don’t put food on the ground or on a platform feeder as this will attract cowbirds. Any food that spills from feeders should be regularly cleaned up.
  • One suggestion is to hang some strawberry basket from a tree and include a fake nest which can often fool cowbirds into laying where the eggs won’t survive.
  • If you’re generally interested in looking for nests, make sure there are no cowbirds around when you do this as it’ll make the nests easier for them to find. There have been reports of nature lovers being followed by cowbirds as they make their way through an area.

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