It depends a lot on the weather. They are looking for potential nest sites now and as soon as the weather permits they will begin nest building. If bad weather rolls in they will suspend building until it rolls out again.
Birds that migrate further south for the winter, (wrens, hummingbirds, swallows, orioles, buntings, grosbeaks, and warblers) usually become situated in their summer territories and look for nesting material in May.
Some birds have only one family. Others can have two or three clutches and will look for more nesting material throughout the summer. And the American Goldfinches don’t even begin to nest until late summer. So it is good to have nesting material out from March until October to attract a wide variety of birds to nest in your area.
At Wild Birds Unlimited East Lansing, MI store we have cotton nesting balls, bison down bags, alpaca wool, and Birdie bells full of grasses, cotton, animal hair and feathers. Or you can collect: twigs, cotton or wool yarn cut less than 3 inches, human hair or animal fur, feathers or dried decorative grasses. Thread, plastic material and lint are the 3 big no, nos for nesting material.
5 Tips to Attract Birds to Nest in your Bird Houses http://bit.ly/xETceZ
Common Bird House Problems http://bit.ly/wrWzyN
Which Way Do You Face a Birdhouse? http://bit.ly/AD43TW
Don’t use treated lumber to build a birdhouse http://bit.ly/x2pIG0
When do birds begin nesting? http://bit.ly/wbJ3kE
DO NOT Collect Dryer Lint for the birds to use as nesting material! http://bit.ly/wC5HcO