I have not studied Black-backed feeding behavior at the nest, but perhaps
they are similar to White-headed Woodpeckers. I have taken detailed feeding
observations on two pairs of WHWO at their nests, and at both nests, the
birds partition feeding visits where the male makes about 6-7 trips in a row
over about a 30 minute period, then the female takes over and does the same.
I had assumed that feedings were done by both parents in an alternating
fashion, male brings food, then female, etc, etc. But, that doesn't appear
to be the case, at least during late morning and afternoon. Now, how they
communicate this and how they determine when to take over duties is beyond
me. But, it may be a way for one partner to go and forage, preen, etc,
while the other is making the feeding trips to the young. This may be the
same for BBWO.

Rereading your post, perhaps the adults take turns in the morning when young
are the hungriest and then go into the feeding behavior I described in the
afternoon when demands on them are lower. I have not had the chance to view
WHWO feeding young early in the morning, but they very well may exhibit the
same behavior, alternating feeding when young are the hungriest.

Jeff Kozma
Yakima, WA