> Wild!  Does this sharing of nests by different species happen often?

  True nest sharing apparently has happened with cardinal and robin.

 The earlier posted picture showed the true situation - separate nests.
The later picture merely showed the acceptance by the robins
of a prematurely fledged/pushed/fallen swallow in the nest.
The swallow slept with the robin young overnight, left in the morning,
then returned to the robin nest, at noon, for a while, and accepted more
food from its own parents.

Another fledged swallow has been returning to an unused nest each day.
Some enter occupied nests and hog food meant for younger occupants.
So some sharing is somewhat normal WITHIN a swallow colony.
Cliff swallows can also carry their own eggs to other swallow nests,
and throw out eggs, a parasitism similar to cowbirds;
but I have not documented it occurring here.

Dave Hoag, Grand Isle, VT