Wednesday, August 5, 2020

Ruby-throated Hummingbirds

"Why is this bird different from all other birds?" Hummingbirds represent an evolutionary expansion into a niche frequented by flower-pollenating moths and butterflies. Downsizing and an elongated beak and tongue combination allows these birds to feed on flower nectar. The adaptation initially took place in tropical climates where the ability to retain body heat – typically a function of larger body size – was not critical.

More than 300 hummingbird species inhabit the Central American tropics. Only 16 frequent the U.S., and of these only one species - the ruby-throated hummingbird - migrates east of the Mississippi River. The rest go west. The risks of a longer eastward migration are offset by having a monopoly on the hummingbird niche once they arrive.

Painting of a male Ruby-throated Hummingbird
Ruby-throated hummingbirds winter in Mexico, then migrate as far north as Canada in the spring. Their journey begins with gorging on insects until body weight is almost doubled, then launching into a night-long flight from the Yucatan peninsula of Mexico to the U.S. Gulf Coast. The fuel for this comes from gorging on small insects and flower nectar. Once over land again, they revert to daylight eating every 10-15 minutes, consuming more than half their body weight over the course of each day.

In eastern Massachusetts these birds are in residence from May through September. The slightly larger, less brightly colored females arrive a week or two after the males. Males do not help with nest construction, egg incubation or feeding. Hummingbirds do not flock. A yard with multiple feeders can be host to several birds at the same time – a “charm” of hummingbirds – but in reality, what is seen is a temporary truce around a shared resource. Each bird, sated, goes back to its own territory.

Females lay two eggs. Nests are the size of a ping-pong ball, eggs the size of a coffee bean. Gestation is two weeks. Chicks are ready to leave the nest three weeks after hatching. Fledgling survival to adulthood is high compared to other species, and lifespan about five years if the fledglings survive the first migration. By comparison, a nesting pair of golden-crowned kinglets (a small, Maine-dwelling bird twice the weight of a hummingbird) will hatch two broods over the course of a summer, each numbering 8-10 fledglings, yet only one or two survive to the following year’s breeding season.

Hummingbirds are omnivores - they get carbohydrate calories from flowers - but all of their protein, fat, vitamins and minerals from eating gnat-sized insects. Sugar-water feeders supplement the energy needs also met by flower nectar. A well-managed feeder should attract visitors from May to September. For a natural alternative, plant monarda, honeysuckle, trumpet vine, and other hummingbird-friendly perennials. Do not use pesticides, as these will kill the small bugs hummingbirds also need to consume. Feeders do not deter hummingbirds from the urge to migrate south in the fall. 

Ruby-throated hummingbirds have some interesting dependencies on other species. Males arrive in the northeast before most of the flowers have opened. Instead of relying on nectar, the birds also feed on tree sap from holes drilled in trees by yellow-bellied sapsuckers. Females construct nests incorporating spiderweb silk.

Part of mural on the building at the corner of  Parker 
& Waltham Streets = male, broad-billed hummingbird.
In addition to small size and hover capacity hummingbirds employ an unusual energy conservation strategy - extreme body temperature modulation. During cool summer nights these birds allow their body temperature to drop from 105 to about 70 degrees Fahrenheit. Resting heart rate drops from 500 to 50 beats per minute (rate while flying can exceed 1,000). Without this trick of lowering energy expenditure via torpor they risk overnight starvation. Muscles are rewarmed in the morning by shivering.

Maynard is blessed with a hummingbird mural on the south side of the building at the corner of Waltham and Parker Streets. The bird in question is a male, broad-billed hummingbird, a species that winters in central Mexico but migrates as far north as southern Arizona during breeding season.   

A version of this column was published in 2012

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