Boston’s Franklin Park Zoo is home to three Costa’s hummingbirds, but that number may soon grow.
The zoo’s only female Costa’s hummingbird built a nest on June 26, and the staff was “pleasantly surprised’’ to find an egg early Wednesday morning because the birds rarely breed in captivity, said Frederick Beall, general curator for Zoo New England, which runs the zoo.
He said the egg is especially rare because the birds are out of their normal range, which includes the western United States and Mexico. All three of the zoo’s Costa’s hummingbirds were born at zoos on the West Coast.
Beall attributed the nest building and egg laying in captivity to the zoo’s efforts to provide an ideal environment for the birds.
“It’s what we had hoped for, and so it would appear that what we are providing are things that the bird needs in order to successfully breed,’’ he said.
Zoo staff watched as the female bird gathered feathers, plant fibers, animal hairs, and moss to make her nest, which is then strengthened by spider webs, which have the elasticity to allow the structure to expand as her chick grows.
Costa’s hummingbirds typically lay two eggs at a time, each one usually measuring half an inch by a quarter of an inch. An egg takes about two weeks to hatch. Beall said “fingers are crossed’’ that the bird lays another egg within 48 hours of the first. Of two eggs, he said, “We would hope at least one of these would be a female.’’
In the meantime, the female bird will regulate the temperature of her egg, sitting on it to keep it warm, or standing above it, blocking the sun to cool it down.
The male hummingbird does not help care for the eggs because the birds are solitary animals.
“He’s off scot-free,’’ Beall said.
Beall hopes the unique occurrence drives more visitors to the zoo. He said the birds can be seen in the Butterfly Landing exhibit during the summer, but are taken inside once winter hits.
“Our winters are just way too cold for them to stay out,’’ he said.
Although it is getting kind of late in the season, Beall said, it is possible the bird could lay another clutch of eggs this summer.