Science Outside Interviews: Christie Riehl and the egg-tossing Anis

Greater Anis at their nest. Photo by Christie Riehl.

Our guest for this interview is Christie Riehl. Christie is an assistant professor of Ecology and Evolutionary Biology at Princeton University where she studies the intriguing cooperative breeding behavior of the Greater Ani (Crotophaga major). Greater Anis belong to the cuckoo family and yet breed cooperatively, which in their case means multiple pairs work together to raise offspring in one nest. Here is Christie’s story about observing the interesting behaviors that go along with this odd breeding system.

Science Outside: What is your favorite story from doing science outside?

Christie Riehl: One of the most intellectually exciting experiences for me was the time I managed to get a video of an Ani pushing an egg out of a nest. This behavior had been described in the 70s, but I had never seen it before. I knew that multiple female Anis lay eggs in the same nest, and if a female hasn’t laid her first egg, and there’s an egg in the nest, she’ll push that egg out. I had been studying these birds for six or seven years and had never actually watched them do this behavior.

SO: Before we go further, why would a female Ani push an egg out of a nest?

CR: If you imagine a group where multiple females are laying eggs in the same nest, the first female to lay an egg automatically has an advantage. Her chicks will hatch earlier and will grow up faster and likely out-compete any other female’s chicks. But if a second female removes the first female’s egg, then the second female’s egg is the first in the nest and she gets all those advantages. Any eggs that get laid late will be the runt in the nest.

SO: That’s so cool! Why is the egg-pushing so hard to see?

CR: Part of it is that their timing of laying is pretty consistent – female Anis lay in the late afternoon. You can go to a nest and watch a female visit the nest and check it afterwards and see when she laid the egg. But the timing of when another female might visit the nest and push the egg out is very unpredictable. Previous researchers would actually sit at the nest and wait and watch for females to come back. But because I was monitoring a large population, I never had the time to just sit down and wait at a nest. Nests are also not easy to observe because they are low and often over water, so you need to be looking through binoculars while standing in a boat to see what’s happening at the nest, so its often hard to see what’s going on.

So eventually I started using these little video cameras in nests to actually catch a female removing an egg from a nest on video. A previous researcher described it as egg-tossing, which to me implies that the bird picks the egg up in its beak and throws it out of the nest.

SO: So you find the eggs on the ground?

CR: Yeah so you see the eggs on the ground or in the water. I had DNA samples from the eggs and was able to find out which mothers were losing the eggs, but I had never seen it. And as a behavioral ecologist, the exciting thing about my work is seeing the behavior – you want to see what it looks like.

Ani Egg
Maternal DNA can be collected from dried blood on the surface of the egg to see which female laid it. Photo by Christie Riehl.

So one summer I was absolutely determined to get this on video, and you know as happens in the field, it never is as easy as you think it’s going to be. At first I thought the females might be “egg tossing” at dawn, when they first came back to the nest the next day. So I’d go out in the middle of the night with a flashlight in the boat and put these cameras up and nothing happened.

Sometimes the females would leave the eggs in the nest for days, so I’d have the camera up and the egg would just be sitting there day after day. Finally one day, a first egg had been laid in the nest, of a group with six birds (three pairs), so I knew that there were at least two other females in the group that hadn’t laid eggs yet. So I thought that this first egg was definitely toast, and it was hopefully going to get tossed in the next few days. So I put a camera up and went away and the next day I came back and the egg was gone. And I thought: “this is it! This is it!” So I took the video back to my office and put it in my computer and a juvenile bird-eating snake had come to the nest and eaten the egg!

I found another first egg, so I put out the camera again and again the same thing happened! I looked at the footage and this time a capuchin monkey had not only eaten the egg, but had also ripped the camera away from the pole and had dropped the battery pack into the water. So I thought, OK, third time is the charm.

The thing is, this is a behavior that happens at every single Ani nest, and I had monitored hundreds of nests, but still had never seen this behavior and was going crazy. I was obsessive. I had to see it happen. Finally I did get it the third time and I almost couldn’t believe it. I went to the nest and the egg was gone and thought: “Ok, let’s see what ate it this time”. So I got the video back and saw that a bird had come to the nest, looked at the egg, then she walked around the nest a few times. And her mate, the male, was there with her.

She gave a couple little agitated calls, she didn’t pick the egg up in her beak, but she bent over it and rolled it. She sort of scooped it in her bill and started rolling it towards the edge of the nest. And at that point the male, was sort of like “wait, what are you doing?”. They went back and forth across the nest a few times as the female was trying to roll the egg out. The female got the egg up onto the rim of the nest. Seeming annoyed by the male, she chased off the male, came back and pushed the egg off the rim of the nest into the water.

It was so amazing to see the behavior I knew was happening and I had written so much about and had based all of these conclusions on this behavior, but had never seen it in person. So that was really one of my favorite moments. It was really an exciting day to see that behavior.

Watch an Ani reject and egg for yourself:

Video by Christie Riehl

SO: That’s an awesome story, thank you for sharing! One last question: why do you do science outside?

CR: I grew up a bird watcher, when I was 10 I was an avid birder, and I still am. So I started out as a bird watcher and I never had an early passion for studying social behavior or cooperation, but I wanted to spend my life outside studying birds. And then once I got into it, the more I learned about birds, the more I loved the behavior and watching them. Being outside gives you a level of knowledge of the world that some people never even realize is there. Being a biologist doing fieldwork is an amazing privilege and something I’m struggling to keep doing as I progress in my career and it’s the reason I’m a biologist.

Christie Riehl is an assistant professor at Princeton University. For more on her research visit her website:

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