When did incubation begin? December 4 or 13th?

[NewMexiKen]

Unlike the past two breeding seasons, this year we cannot be sure of the date when the first egg was laid. A complicating factor has been the nest structure, as the incubating adult is usually completely hidden from view. In past years we could see the adult between the branches, even if the top of its head was not visible. Now the front rim of the nest seems to entirely obscure the view of the incubating adult. Perhaps the eggs are positioned more to the rear of the nest, which could contribute to the difficulty in seeing whether a bird is on the nest.

Further, four cold fronts have passed through since early December. Swaying trees have made it very hard to get a good view of the nest as well as the comings and goings of the adults. Inclement weather also discouraged observers from spending as much time at the nest site.

On November 29 I photographed the pair mating on the nest and they were definitely not incubating.

The next report, by Trisha, was on December 4, when one adult was seen to assume a position deep in the nest for about 3 1/2 hours, repositioning itself at least once. Then it briefly flew off without another adult picking up incubation duties. The adult then returned to the nest and sat very low again. This was highly suggestive of incubation behavior.

Two days later, the nest appeared to be vacant. An adult flew in with a fish and I distinctly heard the calls of two eagles. I never saw two birds at once, as there were high winds and it was very difficult to see what was happening at the nest. The next view I obtained was of an adult eating a prey item on the nest. After 15 minutes or so it peered into the nest and sat rather high, not the typical low incubation position.

Later that day (December 6), Jill saw no eagles in the nest area, and she was quite sure that the nest was empty.  Usually the incubating bird will stand up briefly at intervals to reposition itself, thus revealing its presence. Often it looks down into the nest and seems to be rearranging the eggs or nesting materials before settling down again, but this behavior was not observed.  

The next observation was by Jill, on December 11, when she heard an eagle calling, but did not see any in the area. Mary Lou and I made two visits on December 12 and also saw no eagles and believed that the nest was empty.

On December 13, Jill observed the top of an adult's head above the nest rim, an almost certain sign that incubation was underway. Since then, either incubation behavior has been confirmed or observers failed to see any bird on the nest.

This means that the eggs may have been laid as early as (on or before) December 4, or as late as December 13.  Eggs were laid on December 13 and 18 in 2008 and 2009 respectively. Since incubation takes 5 weeks, the eggs may hatch as early as January 17 or as late as January 22.