When I made preparations to put up a large number of bird nest boxes for the
season on our 45 acres of pasture and fence rows, I made an assumption that
predation would not be a major factor. This assumption goes against the advice
of various groups that deal with blue bird boxes but their anti-predator
designs require a considerable extra effort and expense compared to simply
buying a box and screwing it to a fence post, tree or garage. Also I had never
actually observed any nest predation. So I put up 40 boxes without predator
baffles or guards. The results in to date indicate that predation is in fact a
major obstacle to bird reproduction in boxes. At least that is my conclusion
from the results below:
(note that documentation of the entire phases of nesting is not completely
adequate so there is uncertainty about many details)
1. 40 boxes were installed without baffles or guards
2. 33 boxes were used to the extent that nests were built by tree swallows,
bluebirds or house wrens
3. 18 nests had documented eggs and/or nestlings
4. 7 nests probably fledged young (2 bluebirds, 5 tree swallows)
5. Predation was directly observed in one TS nest by a black rat snake and
indirectly surmised in many others by the presence of nesting material pulled
out of the entrance hole (by racoons probably), and by the sudden disappearance
of nest contents
I concluded that a lot of predation occurs (clutches that disappear) and that
baffles could only help. I am switching entitrely to a system recommended by
the VA Bluebird Society whereby the box is mounted on a galvanized metal pole
with a sheet metal baffle underneath. This is simpler to build than some other
similar systems:
http://www.virginiabluebirds.org/pdf/guards_2.pdf
I also highly recommend the inexpensive cedar nest boxes sold by Wal-Mart (and
built in Ohio!) that have a front-opening door that greatly facilitates
checking the nest contents. I have discarded all my old booxes that do not
provide easy access for examining the nest. Without this technique I would
have remained unaware of the predation that was occurring.
I doubt that we have an unusually large number of predators here (foxes,
possums, racoons, rat snakes, coyotes, etc.) and I do exercise a higher degree
of predator control than some might in more civilized surroundings. Thus I am
quite surprised that birds nesting in boxes have such a tough time in avoiding
predation. Those birds that nest in our yard under more natural circumstances
(carolina wrens, brown thrashers, mockingbirds, cardinals, song sparrows, etc.)
seem to have been successful in bringing off fledgings. So perhaps the
unatural conditions of nest boxes ,especially when arranged along fence
hedgerows, focus the attention of predators on nesting birds (snack paks for
snakes?). The other thing I have learned from this experience is that
bluebirds and tree swallows are incredibly persistent in re-nesting after
repeated losses of their eggs or babies.
Bill Dunson
Galax, VA
