Eeeek – mice!
We’re getting asked whether mice could or did destroy eggs in our eagle nests. Our answer? A guarded “No”. In North America, researchers have found limited predation by mice in the nests of smaller birds, and none in the nests of birds of prey or larger birds.
Researchers have been looking at nest predation for a long time. How do nest location, weather, landscape, habitat fragmentation, and predator-prey/predator-predator relationships impact predation rates? Can nest predation be reduced? How threatening is nest predation to species survival? To get answers to these questions, we need to better understand what animals predate nests and how birds guard against them.
In general, this has been done in one of two ways: by setting up controlled research ‘sets’ or by surveying wild nests. In the case of research sets, a variety of eggs (including plastic dummy eggs) are put into artificial nests. Researchers can introduce specific predators to see which eggs they eat, or watch nests to see what comes to eat the eggs. In the case of wild nests, camera traps and field surveys collect data to determine which animals are predating nests. These observations can be combined with other data to yield a more complete picture of nest predation. What seems like a simple question (“Are white-footed mice significant nest predators?”) can have a surprisingly complex answer.
Let’s start with research sets. In a 1996 study, researchers introduced wild-caught white-footed mice to nests containing eighteen Zebra Finch eggs and eighteen Japanese Quail eggs. After 24 hours, sixteen of the Zebra Finch eggs had been depredated, but none of the Japanese Quail eggs had been touched. A follow-up study looking at the efficacy of using finch eggs and plastic eggs found that captively-bred white-footed mice were much more likely to destroy finch and dummy plastic eggs than the eggs of House Sparrows, which are larger and have thicker shells. While this is good news for us, it was bad news for researchers, since it meant that mouse predation of wild bird eggs couldn’t be accurately modeled using zebra finch or plastic eggs.
Fortunately, camera traps were coming into their own. Instead of spending hours in a blind – not much help when studying tiny nocturnal predators! – researchers could deploy camera traps to study wild nests. While mice sometimes destroyed the eggs and nestlings of small birds, they were nowhere near the most significant nest predator. Depending on nest location, temperature, and many other factors, that might be other birds, members of the squirrel family (Sciuridae), short-tailed weasels, chipmunks, or raccoon. When mice did destroy eggs, they tended to be very small. Mice were recorded destroying the eggs of song sparrows, house sparrows, and spotted sandpipers. While these studies are far from conclusive, none of them have found that mice predate the eggs of large birds. They may just be too big for mice to bite into and, as at least one mouse found out, intruding on an eagle’s nest can have fatal consequences.
So how do the eggs of eagles compare with the eggs of birds that mice predate? Let’s find out!
Eggs predated by mice
|Eggs not predated by mice|
Egg Length 0.63 in (1.6 cm)
Egg Width 0.39 in 1.2 cm)
Egg Length: 1.2 in (3.3 cm)
Egg Width: 0.9 in (2.5 cm)
Egg Length: 0.8-0.9 in (2-2.2 cm)
Egg Width: 0.6-0.6 in (1.4-1.6 cm)
Egg Length: 2.3-3.3 in (5.8-8.4 cm)
Egg Width: 1.9-2.5 in (4.7-6.3 cm)
Egg Length: 0.7-0.9 in (1.7-2.3 cm)
Egg Width: 0.6-0.7 in (1.4-1.7 cm)
Egg Length: 1.1-1.4 in (2.9-3.5 cm)
Egg Width: 0.9-1.0 in (2.2-2.6 cm)
As you can see, bald eagle eggs are a lot larger than any of the eggs that we know mice will predate, even though egg predation isn’t especially common. In fact, bald eagle eggs are longer than mice (not counting the tail), taller than mice, and weigh more than mice do, which makes it even less likely that mice would predate them. It would be hard to bite into something round that was larger not only than your mouth, but than your entire body! The mice we are seeing are nest scavengers and pests (poor Mom and Ma FSV!), but they are not bald eagle nest predators or egg destroyers.
I was unable to find any records of mice destroying bird of prey eggs in the Journal of Raptor Research or BioOne. The information in this article was taken from the following papers.