Black-crowned Night-Herons, Snowy Egrets, Great Egrets, and to a lesser extent, Glossy Ibis, breed among the islands as well. These wading birds nest in the trees along the coastline. Sarah Island has the largest wading bird colony in the harbor. Recently, American Oystercatchers were recorded on Slate, Lovells, and Snake Islands.

As suspected, smaller islands have less diverse populations of breeding landbirds than larger islands. It is believed that Peddocks Island has as many as 34 breeding species. Landbirds such as Song Sparrows, Red-winged Blackbirds, and Yellow Warblers are common to the area. The more widespread Savannah Sparrow can be found nesting on the restored grasslands of Spectacle Island.

Migrating shorebirds are most commonly found on Snake, Rainsford, and Great Brewster Islands from July through August. In the past, sightings have included Black-bellied Plovers, Purple Sandpipers, and Ruddy Turnstones.

Park managers use the size of the breeding bird populations and the number of nests, eggs, and chicks to gauge the environmental health of the park. Breeding season occurs from May through the end of July. The public is asked to respect this delicate stage in the life cycle and not to disturb the colonies while they are at work.

A female Eider (top) and a Snowy Egret are two of the more than 100 kinds of birds found on the Boston Harbor Islands.

Bird lovers should check out eBird, a database for birdwatchers. Contributing your observations will help scientists understand species distribution and movement patterns in Massachusetts and across the continent. www.ebird.org.

Photos: Christopher Klein