NASHUA — A pair of peregrine falcons spotted around downtown Nashua may be looking for a nesting site.

The birds appear to have arrived in the city last year.

“They were in residence through the summer and through the winter as well,” said Christian Martin, senior biologist with the conservation department of the New Hampshire Audubon Society.

Peregrine falcons were removed from the federal endangered species list in 1999. New Hampshire changed their status from endangered to threatened in 2008. Only 20 pairs of Peregrine falcons are known to nest in the state, Martin said.

“We were looking at this pair as a pair prospecting in Nashua and going through courtship,” Martin said.

The pair has been sighted from the Merrimack River west to the Main Street area. The center of activity is north of the BAE Systems facility. The steeple of St. Mary and Archangel Michael Coptic Orthodox Church on Chandler Street seems to be a favorite spot, but they could build a nest within a mile of that territory, Martin said.

“Anywhere in that area is plausible,” Martin said.

Through their markings Audubon volunteers were able to determine that the birds are young. The female had a brown rather than gray back, making her about 1 year old. The male is believed to be about 3 years old. No evidence has been found to determine that the pair built a nest or laid eggs, Martin said. Peregrine falcons typically start breeding around 1 year of age and pairs mate for life.

To nest the couple will need a flat surface on a tall building, Martin said. Peregrine falcons are territorial and won't nest in tight clusters. It would be rare to find nesting pairs within five miles of each other, he said.

“These are long-lived birds and will nest at the same site year after year if it's suitable,” Martin said.

One falcon has nested in Manchester for 12 years and has had two partners during that time, Martin said. On average a pair raises two to four young per year. The new generation stays in the local area for a month or two before migrating away, Martin said.

“They almost certainly won't come back to nest where they were raised,” Martin said.

The Audubon Society trains volunteers across the state to identify peregrine falcons and their nests, and to interpret their behaviors through distant observation. Watching peregrine falcons can be fascinating, Martin said. The birds are tremendously skilled flyers that capture their prey in midair.

Authorities have reason to believe that peregrine falcon populations are growing throughout New England, he said.

New Hampshire Fish and Game and the Audubon Society partner with community members across the state to protect the population. In one instance the agencies worked with local climbers to voluntarily close climbing routes where Peregrine falcons were nesting. When disturbed the birds would leave their nests for periods of time, allowing the eggs to cool and possible preventing them from hatching. “Working with those climbing groups was a major action,” Martin said.

In a city environment such as Nashua, the organization will work with private building owners and property managers to protect a nesting area that may be discovered on the property.