May 11, 2005
Nest Predators
The osprey parents have displayed quite a turnaround in their egg monitoring skills, especially since those early days when they left the first egg uncovered at night and during the rainstorm. Now we hardly see them off the eggs, and even when they are apparently gone, it's highly likely that one of them is sitting on the camera arm, out of sight, but there to watch over the safety of the eggs.
But why is it so important that they stay with the eggs? First, there is incubation. The eggs must be kept at a certain temperature for the chick to develop properly. Also the eggs must be turned throughout the day (some sources say it's done once an hour) to keep the embryo from sticking to the inside wall of the eggshell.
Another major concern is predators. If the ospreys were to leave the eggs unattended for long periods, there are a host of animals that would be more than happy to relieve them of the burden of incubation. This is where the male osprey is especially important -- while both parents can incubate, the female does it the most, while the male mostly brings food and makes sure predators stay clear.
Common land predators that have been known to take or puncture osprey eggs include foxes, skunks, and raccoons. This is why ospreys often prefer water-based platforms -- because the threat of land predators is greatly reduced. Fortunately at our cam platform, land predators are not a major concern because the pole is so tall. But many folks who build their own osprey platforms often add sheet metal or a predator guard to the poles to keep animals off.
At our platform, the greatest threat is actually from the air. During the day, birds like crows, vultures, and even bald eagles would go after the eggs or chicks if they were left unattended. During the evening, nocturnal great horned owls pose a very real danger -- not only to the eggs and chicks, but in rare occasions, to the female herself. Osprey expert Alan Poole has discovered female osprey remains that possessed talon injuries, which he believed came from the fierce great horned owl -- perhaps the only animal that could pose a serious threat to a full-grown osprey.
Danger from these owls once extended to the eagle nest, too. Back in the winter and early spring, great horned owls were a threat to the eagle eggs and young chicks at the Eagle Cam nest; that was why you often saw one of the eagle parents standing watch over the sleeping eaglets when it was dark. Now the eaglets are big, so the owl is not a threat.
But there's no doubt that great horned owls are alive and well at Blackwater. Craig Koppie recently said that a few of the owls had even taken over old bald eagle nests at the Refuge, and the eagles didn't seem to challenge them. Owls have also been known to claim former osprey nests.
You can learn more about great horned owls here.
Until next time,
Lisa - webmaster