In the Fall 1996 issue of Birdwatch Canada, I
told readers about our study of the distribution and abundance of migrant and resident
landbirds during winter in Cuba. We are also interested in finding out some of the things
that most influence wintering populations of migratory birds. Some of these species are
declining, but where does the crunch occur? Migratory birds have complex annual cycles and
populations could face problems on the breeding grounds, the wintering grounds, during
migration, or, quite likely, all three. During the winters of 1995-96 and 1996-97, we
conducted a study of factors that might affect migrants wintering on Cayo Coco, a cay off
Cuba's north coast.
We studied 12 sites, four in each of three forest types: coastal scrub, coppice
woodland, and well-conserved secondary forest. Coastal scrub is low, dense, dry forest
near the coast. Coppice woodland is secondary forest that has been cut over repeatedly to
supply wood for charcoal production. Most of its trees have multiple trunks that emanate
from a central root mass. Well- conserved secondary forest has been relatively undisturbed
in the past 40 years and resembles primary forest; it is also the wettest of the three
forest types.
All sites were visited twice each winter, once in November-December and again in
January- February. During each visit, we captured and banded birds following a
standardized and repeatable protocol. We chose six species for in-depth study: four
migrants (Black-throated Blue Warbler, Black-and-white Warbler, American Redstart, and
Ovenbird) and two residents (Red-legged Thrush and Cuban Vireo). These species received
colour-bands as well as standard number bands, allowing us to individually identify them
in the field.
Using a combination of observation and mist netting to resight and recapture banded
birds allows us to estimate the amount of population turnover. How many of the birds that
were present early in the winter stayed or survived until later in the winter? How many
new individuals arrived? The theory is that the quality of the forest site and its ability
to support the birds through the winter season will be reflected in bird turnover rates. A
high turnover suggests that forest quality is low; birds simply do not use it for long. If
species exhibit differing rates of turnover depending on habitat, it suggests that
migrants may indeed face population pressures on the wintering grounds and that certain
habitats are more worthy of conservation than others. To gauge forest quality at each site
we measured food abundance (insects and fruit), quantitatively described the vegetation
structure, and tracked changes in temperature and humidity.
As it turned out, the two winters were very different from each other, but provided us
with some unexpected opportunities. Winter 1995-96 was a typical dry season with gradual
drying from November to February. On average, the coastal scrub dried out more than
coppice woodland or well-conserved forest. As we had expected, with the desiccation came a
reduction in insects and higher bird turnover rates. Well-conserved secondary forests
dried out the least, retained their insects, had lower turnover, and even attracted new
birds.
In 1996-97, we looked forward to another nice normal season to confirm our findings,
but it was 
not to be. On
18 October 1996, Hurricane Lili slammed into Cayo Coco with 145 km/h winds and torrential
rain. Trees in our sample sites were literally stripped bare of their leaves by the high
winds. As the forest vegetation struggled to recover from its battering, the rain
continued, making for a very wet "dry season" with lots of insects (including
mosquitoes!). In response, the numbers of birds at each site fluctuated wildly,
particularly in the coastal scrub and coppice woodland. A larger than normal number of
inter-site recaptures suggested that birds, particularly the fruit-eating Stripe-headed
Tanager, were flitting from one end of the cay to the other in search of food.
We are still analyzing the data and trying to make sense out of what happened.
Hurricane Lili seems at once a blessing and a curse. On the one hand, our neat little
study was turned upside down. On the other, we were given the opportunity to study first
hand the effects of the most important natural phenomenon shaping Caribbean plant and
animal communities. We'll be sure to keep you informed of our findings. — George E.
Wallace
