About a month ago I described an active nest box in my grandson’s backyard. A pair of tufted titmice had built a nest and incubated and hatched five eggs. I promised an update on the nest’s fate; all five young titmice fledged successfully. That’s the definition of “nest success.”

Uncertainty haunts the fate of every nest box. Will the eggs hatch? Will the chicks fledge? Will weather wreak havoc on the nest? Will the parents survive the nesting cycle? Or, will a climbing snake or raccoon help itself to the box’s contents?

A nest box landlord’s greatest concern is to do no harm to the nest. Too many disturbances or poorly timed disruptions can be disastrous.

It’s OK to periodically monitor active bird nests but exercise common sense. For example, during the egg-laying period, females visit boxes early in the morning and lay a single egg. So during egg-laying, check nests after mid-morning while females are off foraging to prepare for the bouts of extended incubation to come.

Count, study and photograph the eggs. Take notes. Show the kids. But stay no longer than 60 seconds and emphasize to children the serious nature of this form of citizen science.

Never disturb a nest while a female incubates the eggs. Incubation is a critical time during which females are sensitive to disturbance. If you see a female leave the box, a quick check is OK to confirm final clutch size. If you open a box and find an incubating female warming her eggs, quietly and quickly closed the box and walk away.

After the eggs hatch, which takes about 14 days for most small cavity-nesters, you may again check the box every few days without fear of harming the nest. You will know when the eggs hatch because suddenly both parents bring food to the nest at two- to four-minute intervals. Visit an active nest only after you see an adult leave. Again, keep visits to less than one minute, but feel free to photograph the contents. Record your observations after you’ve left the box.

For about 10 days after hatching, parents keep the nest surprisingly clean by removing the fecal sacs nestlings excrete after swallowing a meal. A fecal sac is simply a neat packet of poop encased in a mucous membrane. This makes waste easy to remove so the nest stays clean.

After the chicks are about two weeks old, they stop forming fecal sacs. Instead they position their back ends toward the rim of the nest and expel a squirt of whitewash onto the inside of the box.

By the time they leave the nest in another week or so, the interior of the box is plastered in whitewash. This provides near conclusive evidence of the nest’s fate. If the inner walls of a cavity are clean, the chicks did not live long enough to apply the whitewash. If the inner walls are covered in whitewash, the chicks likely lived long enough to leave the nest safely.

When the chicks are about 12 days old, the nest again becomes sensitive to disturbance. If bothered, older chicks sometimes leave the nest before they should. Nestlings that fledge prematurely will die. If exposure doesn’t get them, predators will. So never risk premature fledging by disturbing a nest box after the chicks are 12 days old.

Most small cavity-nesters (bluebirds, chickadees, titmice, wrens) fledge at 16 to 21 days. By that time, their feathers are well developed, and they can fly well enough to reach the safety of a tree.

Fledglings do not return to the nest after leaving the box, so it’s OK to remove old nests to encourage more nesting attempts. Though chickadees and titmice raise just one brood per year, bluebirds and wrens raise two or three.

One final word of caution: check nest boxes carefully. Wasps, mice, flying squirrels, and snakes sometimes move in.

Rodents and rat snakes may startle you, but they won’t hurt you. Wasps and copperheads, which have occasionally been reported in nest boxes, are another matter.

Consider a nest box to be an outdoor laboratory to learn more about the private lives of some favorite backyard birds.

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Send questions and comments to Dr. Shalaway at sshalaway@aol.com or 229 Cider Mill Dr., Apt. 102, Hendersonville, NC 28792.