Arriving late a wise plan for hummers

Posted: Friday, April 17, 2009

Mother Nature must be thinking about spring. The gulls are once again flying regularly between the rock, across Mendenhall Lake from the visitor center, and the wetlands. Ducks are exploring the Valley for ice-free ponds, but finding very few as of the end of the first week in April.

Although the snow is still deep in many places, the calendar on the wall says it's time to think about hummingbirds, which generally begin to arrive some time between the end of March and mid-April. A local birder, Gus van Vliet, has kept records for over 15 years of the first rufous hummingbird arrivals at his feeder. The records clearly show that first arrivals tend to be earlier in years with warmer conditions prevailing - as indicated by an index of cool La Niña versus warm El Niño conditions. As of April 15, Gus's feeder has not had any hummingbirds, so this looks to be a late-arrival year.

A late arrival is probably a good thing. There aren't even any blueberry flowers yet to offer their negligible drops of nectar. Hummingbirds also eat a lot of insects and spiders, but I don't see many of those yet, either. Smart hummers would stay south for a little while yet.

Rufous hummingbirds come to us in spring from Mexico, where we can hope it rains less than here. According to one expert, they make the longest migration relative to body size of any bird. He calculated that they could fly over 600 miles without refueling. They use the energy in stored fat, but feed along the long migration route is necessary. Rufous hummers come north along the coast, in general matching the flower-blooming season, but eating insects if they get ahead of the flowers. Males arrive before females and stake out their territories.

Rufous hummers are promiscuous; each male courts and mates with several females but there is no bond between them after mating. Males perform their dramatic, swooping, buzzing displays near a female, and also execute a rapid back-and-forth figure-eight right in front of her. After mating, he seeks another female. Females alone build the nest, incubate the eggs, and rear the chicks.

Hummingbird nests are a little bigger than a golf ball, usually placed less than 15 feet up in sheltering vegetation. The exterior is covered with bits of lichen and bark, attached with spider webs, and the inside is lined with downy vegetative material. The two eggs are incubated for about 17-18 days, and chicks stay in the nest about three weeks. Insects and spiders provide the necessary protein for rapid growth.

Hummingbirds are famous for their hovering ability, their tiny wings going to and fro faster than our eyes can register. Hovering is metabolically very expensive: it takes about 11 milligrams of nectar to fuel one minute of hovering; this is several times the volume of their crop, which needs frequent refilling. Hummingbirds have a very high metabolism, large hearts for their body size, lots of capillaries delivering blood and oxygen to muscle cells and muscle cells that are packed with huge numbers of mitochondria - the tiny organelles that turn food into usable energy.

One way hummers can save energy is by reducing energy expenditure as the ambient temperature drops. A rufous hummer's normal body temperature of 102 F can drop as low as 54 F when the ambient temperature goes down to 44 F. Torpor thus can reduce the metabolic rate to as little as 7 percent of normal. Oxygen consumption decreases at lower metabolic rates and the breathing rate also drops.

Hummingbirds feed on nectar of many kinds of flowers, not only the red columbine and pink fireweed and salmonberry, but also flowers of any other color from creamy currents and yellow paintbrush to purple larkspurs. Their most important criterion for flower choice is the level of nectar reward. They also feed on sap and insects at the wells in tree bark carved by sapsuckers - our most common woodpecker. Hummingbirds are important pollinators of several local native flowers, but they also visit flowers that they do not pollinate, such as blueberries.

When mating is finished in late June or so, the male rufous hummingbirds begin to leave. Females, followed a bit later by juveniles, usually leave in late July and early August. They move south in the mountains, feeding on alpine and subalpine flowers.

The rufous hummingbird is our only breeding hummer. But in late summer and fall, we occasionally see a wandering Anna's hummingbird, which nest farther south but sometimes stray northward after the breeding season. Two other species are rare vagrants.

It's time to put out your hummingbird feeders in some place that bears cannot reach. Use only ordinary table sugar - sucrose - and never use honey. Keep the feeder clean, don't let mold grow and have fun watching the hummers as they take up nectar with their fringed tongues and fight each other over visiting rights!

• Mary F. Willson is a retired professor of ecology.

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