I would be surprised to see a large number of birds arriving early
due to El Nino. Spring migration and other breeding behaviors seem to be
controled by day length, not temperature, particularly in passerine birds.
Research by avian physiologists suggests increasing day length
causes the release of hormones in the brain of the bird. These hormones
travel to the pituatary and cause the release of gonadotropin horomones
which go to the gonads and tell them to start growing and produce
testosterone and estrogen. (In birds, the gonads regress during the
non-breeding season and grow before the breeding season.) Most of theses
studies were done on sparrows (particularly White-crowned Sparrows) but
similar results have been found for a wide range of birds that have been
looked at.
Birds use day-length as a signal because it is a good predictor of
spring, better than temperature. As we all know, the temperature fluctuates
greatly, day to day and year to year. The birds need a reliable predictor
of upcoming warm weather and increasing day length does that. El Nino and
other large-scale weather patterns alter temperature but have no effect on
day length.
Obviously, this works best for birds that breed and winter in the
north temperate zones and restrict breeding to the spring and summer months.
Birds that winter along the equator may have an additional factor, probably
a circannual rythmn, that makes them more sensitive to sunlight a certain
time. Birds that winter south of the equator may be "turned on" by shorter
day lengths. Birds that nest in unpredictable environments or depend on
unpredictable food source (such as crossbills) may use a variation of this
pattern or may use a totally different system we haven't yet found.
Variation in arrival dates arise due to conditions along the
migration route. The day length starts the process. Then the bird starts
migrating. If food supplies along the route are below average, the bird
might have to spend more time at stopovers. If food is abundant, the trip
may go faster. However, I doubt the process sped up by several weeks.
An alternate explanation for apparent "early migrants" are winter
stragglers that never left and were never discovered. It is possible that
every winter a few birds just don't leave; winter weather usually takes care
of them. However, during mild winters, particularly El Nino winters, they
can survive and are "found" on those early dates.

I predict that most birds will be right on time.

Deb Beutler
Department of Zoology
P.O. Box 644236
Washington State Univerisity
Pullman, Whitman Co., WA
dbeutler at wsunix.wsu.edu