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April 17, 2010: Feeding, Diet and Nestling Rivalry

by Bob Sallinger

kgw.com

Posted on April 17, 2010 at 8:18 AM

Updated Saturday, Apr 17 at 8:29 AM

The chicks are now 2-3 days old. Already they have grown significantly and their downy coat it much thicker. They are also much stronger--now able to hold their heads up for extended periods to beg for food.

Some people have wondered whether they are already fighting with one another. They tend to whack into one another pretty often and also appear to be pecking at one another. The answer is probably not---they don't see very well yet. They are probably just responding to one anther's movement and that is causing them to try and nose about for food. Noise and movement will trigger a feeding response. Their necks are not very strong yet so they tend to flop all over the place. As they get older however, they will become more aggressive in trying to take the food that the parents bring into the nest. The fact that all three were hatched within 24 hours of one another at least means that no chick is likely to have a huge advantage over the others. Also there is plenty of food about and the adults are experienced parents so the competition will probably have more to do with who goes first rather than who gets fed at all.

Some people have written in concerned that the parents are not feeding often enough. Actually if you watch continuously they feed pretty darn often! The hawks have something called a "crop." The crop is a food storage pouch that is located near the top of the esophagus. The parents actually cram quite a bit of food into the young each time there feed them---check out the size of the food bits relative to the size of the nestlings. The crop expands as the birds eat and the food is stored there and then slowly moves through the digestive system over the next couple of hours. (Think of it as a slow release system). With the young it means that the adults can feed a lot at once and rather than small bits non-stop. For adult birds, it allows them to eat wherever they find prey and then move on.

So far almost all the prey that has come into the nest has been city pigeons (Rock Doves). However, Red-tails actually eat a wide array of prey species. A compilation of 11 Red-tailed Hawk studies in the book, Hawks, Eagles and Owls of North America by Johnsgard showed that mammals average about 68% of their diet, birds averaged 17.5% of their diet and reptiles and amphibians (mostly snakes) averaged about 7% of their diet and invertebrates averaged about 3.2% (the remaining points are "other"). There is however significant variation from bird to bird and site to site. Based on what we are seeing with this pair it looks like the vast majority of their diet is city pigeon (rock dove) which which makes sense since they are super common in downtown Portland.

Finally again for those who cannot get enough of birds of prey, Audubon will be releasing an adult bald eagle in Lake Oswego this Sunday at 1:00 pm at George Rogers Park. The eagle was injured in a fight with another eagle a couple of months back and has been recovering at our Wildlife Care Center. We will do a quick presentation on eagles and then set the bird free. All are welcome to attend. For more information check out our eagle release webpage.

 

Bob Sallinger

Audubon Society of Portland

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