Migration phenology in 2007

One of the main objectives of the Projet colibris to determine the migration phenology of the Ruby-throated hummingbird (Archilochus colubris).  By plotting the numbers of hummingbirds captured on each day (weather permitting) from May 6 to August 25, 2007, we can get a good idea of the migration phenology for the Estrie region, for birds of both sexes.  It is already known that males of this species migrate earlier than females in spring, and the same is true in the fall.  Figure 1 shows that, even at the northern edge of their range, the spring migration phenology of hummingbirds is comparable to the findings of other studies.  In fact, we can see from the figure that the peak in male captures is about 7 days ahead of that for females.  This confirms that, even though some females are captured early, most of them arrive after the males.  However, the results are less clear for the fall migration.  On the same figure, we see that female captures remain steady throughout July while the number of males captured shows a marked decline over the same period.  We also see that the number of hummingbirds of both sexes captured increases starting in late July, suggesting that the fall migration is well under way.  Another observation: the number of hummingbirds captured in the spring is higher than in the fall.  There are at least three possible explanations for this finding:

hummingbirds’ fall migration routes may be different from the spring routes;
hummingbirds may be making more visits to wildflowers, which are present in greater numbers in the fall, resulting in reduced feeder use and, consequently, fewer captures;
the lower capture rate may be due to natural mortality of hummingbirds after the mating period.  Indeed, it is known that males have a lower survival rate than females, which could reduce the number of adult hummingbirds in our population, affecting the possibility of capture.

Of course, we can get more detailed knowledge about this by compiling migration tracking results for a number of years, and maybe, one day, by increasing the number of hummingbird banding stations in Québec.

What about the migration of young hummingbirds?

In 2007, we captured our first young hummingbird, a female, on July 30.  The total number of young females captured was considerably lower than the number of young males (15 vs. 38).  From Figure 2, we can see that young females appear to migrate earlier than young males.  This result may be due to a potential sex ratio bias towards more young males in the population.  Indeed, more young males than young females may be born in our hummingbird populations.  This would compensate for the higher mortality rate of adult males, bringing the sex ratio for the population into balance.  Also, an effort to keep capturing birds right up to the end of the migration (around September 20) may be needed in order to establish the phenology of the end of the fall migration for young hummingbirds with more certainty.  One thing is sure: young hummingbirds are the last to visit our feeders.  As data accumulates on various demographic parameters such as sex ratio and recruitment, we will gain a better understanding of differences that may exist in the migration sequence of these wonderful, fascinating tiny beings.