Summer World_ A Season of Bounty - Bernd Heinrich [66]
A male ruby-throat weighs about one-tenth of an ounce, only slightly more than a penny (about 0.08 ounce) and several times lighter than many large moth caterpillars. Its heart and wings beat at twenty-one and sixty times per second, respectively, while it flies north on a journey of about 2,000 miles and then flies the same journey south in the fall. At any given moment it is within hours of starvation, as it needs to consume about twice its body weight of food per day. No other hummingbird attempts what would appear to be a very risky journey, to a destination often devoid of nectar-bearing flowers. When hummingbirds are not traveling, their body temperature often drops until they are torpid, to conserve energy when they can’t forage, as at night. They prepare for their long nocturnal fast by fattening up during the day. During migration, they replenish in the morning, leaving the middle of the day for traveling. Indeed, at Hawk Mountain in Pennsylvania in the fall, most of the migrant hummingbirds come through at midday (Willimont et al. 1988). But apparently they change their strategy on another leg of their migratory route, when they cross the Gulf of Mexico.
The Gulf of Mexico presents 520 miles of open water that must be crossed at one go and without refueling. At their top sustained flight speed of about thirty miles per hour, they face a nonstop flight of about seventeen hours. And here, before tackling that distance, the hummers depart from Alabama in late afternoon for a night flight directly across the gulf (Robinson et al. 1996). In the spring the birds returning to Alabama (at the banding station at Fort Morgan) arrive in the dark of night (Sargent 1999). Apparently some of them also take the longer route along the coast of Texas, where they could migrate by short hops and presumably refuel. Do they decide on one or the other option on the basis of their fat reserves? Do they know what they are up against before they take off over the open water of the gulf?
Although migration is hazardous, it can’t be excessively so for the hummingbirds, since they have one of the lowest reproductive rates of any northern bird migrants. They raise only one clutch of two young per year (perhaps because the female alone does all the work). By contrast, a pair of northern warblers will raise four to five young in a clutch, and a pair of golden-crowned kinglets will raise eight to twelve chicks per brood and nest twice per summer. Since on average these bird populations are stable over time, the number of offspring they raise provides a measure of their mortality rates; the hummingbirds must therefore have a relatively low death rate. We know they come north to build their tiny nest cups of lichens held together by spiderwebs, where the female rears her clutch of two young. But why not do it in the south? Why not stay in their ancestral home along with most others of their group? There are many theories, but no answers. There are, however, some answers regarding how they get by once they reach the north.
The male hummingbirds are the vanguard in the northward migration, as is true of most other bird migrants (Stichter 2004). The standard explanation for this phenomenon is that the males compete to establish territories in order to attract better females. It sounds reasonable. But then in the migration south at the end of the breeding season, the males again precede the females and the young of the current year.
For many years I thought that the hummingbirds I had seen in the northern Maine spruce fir woods in May must have returned much too early by mistake. Later I learned they were right on schedule. Their timing is synchronized with the return of a woodpecker, the yellow-bellied sapsucker, Sphyrapicus varius, which is their main food provider then, and all summer long.
The sapsucker is one of the most visually striking of birds. Its bold black and white markings contrast with a scarlet crown, a scarlet throat in males, and a soft lemon yellow–tinged belly. Sapsuckers differ from other woodpeckers