March 04, 2006
Feeding and Crops
Nest Update: Based on the scenes we've seen over the last few days, it appears that the father eagle enjoys hunting for food early in the day. This morning we saw two fish and part of a bird in the nest waiting for the chicks. It was interesting that when given the choice, the mother fed the chicks part of the bird first.
Also on the nest we've been seeing some interesting behaviors regarding temperature regulation. In the wonderful book, The Bald Eagle: Haunts and Habits of a Wilderness Monarch, the two authors share the knowledge they gained from observing eagle nests in Saskatchewan, Canada, and they mention how an eagle parent regulates a chick's temperature: "Quite commonly, a chick will rest at its parent's breast, and in the first ten days at least, the adult may pull dried grass on top of the little bird." Thursday we saw the parents put so much grass on the chicks that at one point they briefly disappeared. And then later we saw the eaglets peeking out from under mom's warm breast. The chicks are about eight and ten days old today, so the timing for these behaviors is right on target.
Several cam watchers have asked about the feeding habits of the eagle family. Eagle biologists report that in nests that they've observed, the chicks are fed every 2-3 hours. On our nest we've seen that to be true, but we've also occasionally seen the feedings done in shorter periods as well as over longer periods. There has been plenty of fish in the nest, so the mealtime variances may be affected by how much the chicks are expressing their hunger.
At this stage the chicks are small, so they fill-up faster. Also the chicks have their crops to aid them. A crop is an organ located near the base of the bird's neck, and it stores food for later consumption. Our chicks are still small, so it's hard to see their crops right now, but we have a pair of photos from last year where you can see our youngest eaglet's crop growing in size as he gets fed.
For the parents, the advantage of a crop is that it means the chicks don't have to be fed as often, since the young have food stored in their pouches. Adult eagles and most other birds have crops too, but they're often harder to see under all the feathers. It's been reported that an eagle with a full crop can usually survive at least five to seven days without eating.
An interesting raptor note: Owls do not have crops. They swallow their prey whole and it immediately goes into their gizzard -- fur, bones, beaks and all. Both owls and eagles regurgitate pellets (small projectiles of indigestible food). The eagle does not eat its prey whole and does not eat as many mammals and birds as the owl, but the eagle's crop collects fur and bones it can't digest. Also the eagle's strong stomach acids dissolve most smaller fish bones, so the eagle's pellets do not have as many bones. But the owl's pellets often contain much of the skeletons of the animals it ate.
This Owl Pages website gallery has an interesting photo of a barn owl's pellet showing not only skeletal remains but also pieces of eggshell.
If you'd like to see how an owl expels a pellet, the Owl Pages website has a short video of an immature great horned owl discharging his pellet.
Until next time,
Lisa - webmaster
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