[obol] Bird migration timing and global climate change

[Wayne Weber]

Joel,

Thank you for your very eloquent and thoughtful discussion of global warming
and how it relates to bird migration. Most of what you say is familiar to me
(as an amateur climatologist), but it may be new to many readers of OBOL.

You make some very good points, including the fact that "first arrival"
dates
give a very poor indication of when the main migration of any species
occurs, and that daily counts (either of birds observed or birds caught at a
banding station) are necessary to detect any significant changes in timing
of migration.

The main effects of global (or regional) warming trends on birds are most
likely to be northward shifts in breeding and/or wintering distribution--
and also similar shifts in altitudinal distribution in mountainous areas. We
have already seen such distributional shifts in a number of bird species.

The timing of migration in birds is mainly determined (so physiological
studies have told us) by photoperiod, which affects production of hormones
by the body, which in turn affects migratory behavior. Because changes in
photoperiod (i.e., relative length of day and night) during the year are not
affected by changes in climate, we should expect little if any change in the
timing of spring or fall migration because of global warming.


Wayne C. Weber
Delta, BC
contopus at telus.net