A leafcutter bee visits my garden.

On a warm, sunny day in late spring last year, I discovered I’d unwittingly constructed a condo for one of our precious native pollinators. Some years before, I’d stacked a pile of twigs and small branches against the south side of the house to use as kindling. Well, we hadn’t had a fire in the fireplace for a few years, and in the meantime, dozens of teeny bees had found a home. The shiny, black bees were squeezing themselves into the tiniest cracks and holes in the twigs. After some research, I determined they were small carpenter bees in the genus Ceratina. They were mothers provisioning their nests.

About 30 percent of North America’s 4,000 bee species nest aboveground in twigs, stems, old beetle burrows in wood, or other narrow crevices. These gentle, nonaggressive bees create tiny nursery rooms inside the stems. They fashion partitions from various materials; for example, small carpenter bees use pith and their own saliva, mason bees use mud, and leafcutter bees use pieces of leaves or petals. The mother places a little ball of pollen mixed with nectar in each cell and lays an egg on it. The larvae will develop on the food their mother has left for them, overwinter in pupal form inside the stem, and emerge the following year as adults to start the cycle over again.