Nest Boxes & Cavity Nesting Birds

Birdhouses come in all shapes, sizes, and styles.  They can be eye-catching pieces of art that delight humans.  Or they can be very plain boxes that focus solely on what a bird might want, with little concern for what a human might think.  And they can fall anywhere between those two extremes.  But a very basic question underlies this continuum:  Why might any bird use a box made by a human?

Most birds raise their young in open nests that they construct from scratch each spring.  But some species nest in cavities.  Primary cavity nesters, such as Nuttall’s, Downy, and Acorn woodpeckers, excavate their own cavities every year.  Secondary cavity nesters use naturally occurring cavities or those that have been made, then abandoned, by primary cavity nesters.  Wood Ducks, American Kestrels, Barn Owls, Western Bluebirds, Tree Swallows, House Wrens, House Sparrows, and European Starlings rank among the more common secondary cavity nesters in our area.  (The last two species in that list can create extra challenges for other cavity nesters.  But that’s a topic for another time.) Because naturally occurring cavities often run scarce,many secondary cavity nesters will use a human-made birdhouse, if it’s the right size and in a good habitat for them.  And, take it from me, you can derive some real delight watching a pair of birds settle in and raise a family in a structure you’ve provided.

When I lived in Colorado, I monitored more than 100 nest boxes every week during breeding season.  That’s right—every week.  Kind of addicted, you might say.  But come with me on a nest box check and you might understand why.  Walking to a nest box, I listen for nearby singing or call notes, watch for sudden movement.  I stand to the side, tap lightly on the box, and wait a few seconds.  A soft, descending “phew, phew” to my left alerts me to a Mountain Bluebird in the vicinity; a few seconds later, a female zooms to a tree beyond the nestbox.  “Ah,” I say softly.  “Is this your box?” Opening it, I see thin strips of juniper bark woven into a sculptural swirl,confirming that this is indeed a Mountain Bluebird nest with one lovely, light blue egg.  I quickly close the box, move away, make a note on my clipboard, and head to the next box.

Repeat that experience scores more times in one afternoon and you get a sense of the wonder and delight that can await a birdhouse “landlord.”

Monitoring nest boxes provides many fascinating and joyful experiences.   Babies grow from wrinkled, naked, dinosaur-look-alikes into fully feathered young birds in a mere 2 or 3 weeks.  Hard-working parents bring insects, insects, and more insects to their hungry nestlings, all while vigilantly protecting them from predators and other harms.  Every now and then, you’re in the right place at the right time to see nestlings actually fledge, launching, tumbling, or lurching from the safety of the box into the vast world beyond.  And I will never forget my glimpses of the diminutive Northern Saw-whet Owl with her 3 adorable nestlings—extraordinary interactions few people ever experience.

Don’t get me wrong, though.  Monitoring nest boxes is not all rainbows and unicorns.  One summer’s brutal heat killed a clutch of 10-day-old Western Bluebirds.  Occasionally a small mammal or a tree-climbing snake takes eggs or nestlings.  If one parent falls prey to the local Cooper’s Hawk,who is also trying to feed its growing family, the other parent can’t always care for all of the nestlings alone and some die in the nest.  While some people agonize over any loss, I take a more objective view.  Some birds effectively protect their offspring from all sorts of threats.  Those that don’t?  They’re out of the gene pool, at least for this year.  Estimates suggest that 75% – 90% of all birds that hatch in one year never make it to the next.  Such high mortality rates may not seem right or fair, but they are nature’s way.  And honestly—if every baby songbird that hatched each breeding season lived to breed the next year, we’d be sweeping songbirds out of our way with every step.

Whether you’re simply interested in some lovely or whimsical art or you’d like to help out a bird family looking for a cavity to call home for a few weeks, birdhouses offer something for everyone.  Mark your calendar for the Buena Vista Audubon Society’s Seventh Annual Bird House Auction at the Nature Center on Saturday, March 18, from 5:00 – 8:00 p.m.  Come join us!


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