As winter relaxed its grip earlier this month, six consecutive days of bright sunshine and blue skies left no doubt that spring was on its way. Red maple buds burst open, and a pair of eastern bluebirds began visiting a nest box on the edge of the yard.

They were just beginning to investigate and evaluate potential nest sites. First, the duller female landed on the top of the box and then hopped down to the entrance hole. She stuck her head in a few times and then disappeared into the box. She did this several times

Then the bluer than blue male inspected the box. It's impossible to know why, but clearly they found the box appealing. Perhaps it was because they successfully raised two broods in it last year.

In any case, they repeated this behavior on each of the recent warm sunny days. They haven't yet carried any nesting material to the box, but that will come, possibly this week.

I look for the season's first bluebird eggs in early April. I remember that on my wife's birthday (April 6) one year, we had a heavy wet snow fall, and one bluebird nest had four eggs. My notes remind me that it was 1988, and the nest was ultimately successful.

After 30 years of building and monitoring nest boxes, bluebirds are one of my favorite birds. Though I never saw one as a child, my mother often reminisced about the bluebirds she saw when she was young in the 1930s. I suspect it was why blue was always her favorite color.

By the time I grew up in the 1950s and 1960s in southeastern Pennsylvania, bluebirds were rare. Pesticides contaminated their invertebrate foods, and nesting cavities were in short supply.

Since the first Earth Day in 1970, however, we have restricted the use of broadly toxic chemicals and discovered the bluebird's willingness to use man-made nest boxes. Today, bluebirds are common in suitable habitat -- open areas with scattered trees. Pastures, hayfields, cemeteries, and golf courses are ideal.

For bluebird box plans, visit www.nabluebirdsociety.org/eastwestbox.htm.

If you're not a do-it-yourselfer, bluebird nest boxes can be purchased at wild bird stores and nature centers.

First Published: March 21, 2010, 8:00 a.m.