This Guest post was written by my good friend in Corpus Christi, Mr. John Barrow - purple martin landlord expert and all-around good-guy-extraordinaire. Thanks for sharing, John!
Winnowing Definition: (referring to a group of things, people, animals) to reduce something's size by separating the ones that are useful or relevant from the ones that are not.
I have been
contemplating the effects of the post-fledge natural winnowing process on a
colony this season and, I am becoming convinced that a great deal (or majority)
of hatch year (HY) loss occurs in the week or two after fledging occurs.
At
my colony we had moderate drought in 2008, severe drought in 2009, and a banner
year in 2010. In 2008 we had numerous jumpers--at one point we were rehabbing a
dozen nestlings. In 2009, we had severe brood reduction due to weather related
events--nearly all in the nature of fewer eggs produced; many that were not incubated.
I recall we had only one jumper, but had 62 nest starts from 45 pairs. This
year (2010) we had laying and incubation of large clutches of eggs, with only
one jumper that was returned to fledge.
Fledging
each year, occurred in most cases over several days. That scenario, which is
probably typical, results in separation of the family unit as adults take their
new fledges to a presumably safe location and, starting individually, teach
them to find and catch food. Prior to fledging adults have already reduced
feeding of nestlings to encourage fledging, and as the post-fledging training
occurs, those left in the nest lose weight, often to critical levels. This is
what, in most instances absent mites, etc., I believe, causes jumpers. Not an
easy problem to deal with. You can stuff the jumper with food and stick it back
in the compartment you believe it belongs in. But that doesn't increase the feeding
by its parents. These late fledges are really in a battle against time to get
out of the nest, fly capably, and remain with their family group, in order to
survive.
Add
to this other typical behavior like the adults returning to the cavities with newly
fledged young--a form of post fledging memory stamping--and the ensuing
confusion as these newly fledged young are attacked, chased and threatened when
trying to reenter what is often someone else's turf, you almost certainly have
more family unit separation and confusion.
There
has been little study done of the effects of this post fledge winnowing period,
and I suspect study would be difficult and for the most part subjective. But it
is something I have been reflecting on. Did my adult pairs of martins that
fledged 3 young/per pair in 2009, and were probably better equipped to train
that lesser number and keep the family unit intact, have a substantially lesser
effect on the overall population, then those pairs in 2010 that fledged 6 or 7
per nest; but, that had more difficulty in training the individual fledges,
locating and preserving a safe and secure locale for staging that provided
a reliable food source; and finally, in
keeping the entire family unit intact for the requisite training period to allow
the young to become independent? I think there are signals that the overall
population fluctuation of a certain area in a good year, may not be
significantly different than in a marginal year.
What
does it matter? Assuming there are substantial losses during this post fledging
period, what can be done to reduce it? Winnowing in this light is defined as
reducing the number of birds in a species until only the best ones are left. It
is natural selection at its purest.
It
is not maintaining a colony so large that it becomes a magnet for predators,
not only endangering adults during the nesting process, but more so, becoming
an attack zone for recently fledged hatch year birds whose survival is
dependent on their having a safe and secure training area to learn basic
survival skills as part of a family unit. It is not offering housing that
serves to maximize the confusion of family units returning to housing as part
of post fledging memory stamping at the colony, be it housing that is too
closely configured, or stacked row on top of row; it is not building a colony
of the size or in an area beyond the capacity of the immediate food supply.
