To add to Peter Whitlock's good comments on predator mobbing, there=
is no
reason to consider mobbing by neighbors as "nonsense" behavior. If=
mobbing
serves to acquaint individuals with potential predators in the area,
neighbors should do it. If it serves to inform the predator that=
these
birds know of its presence and are therefore poor risks for predation
attempts, neighbors should do it. If it serves to chase the predator=
away
from the area, neighbors should do it. Probably most of the advantages=
that
accrue to the nesting pair (protecting their nestful of eggs or young)=
also
accrue to nearby unrelated individuals. I don't think this happens=
from
"time to time," I think it is the rule. Whenever I imitate pygmy-owl=
calls
to attract passerine birds just to see what's in the area, the sound
attracts birds from all territories within hearing distance, as far=
as I
can tell. The fun starts when a pygmy-owl comes in too!

An important theoretical paper on the subject (E. Curio, U. Ernst,=
and W.
Veith, The adaptive significance of avian mobbing I & II, Zeitschrift=
f=FCr
Tierpsychologie 48: 178-202, 1978) listed quite a few potential advantages
of mobbing. Some of them seemed to be rejected by available evidence,=
but
there were a surprising number of hypotheses that weren't rejected.=
A
multifunctional adaptation.

Dennis Paulson