Cliff Swallows: Aerial Acrobats & Adept Architects

Spring is in full swing here in California's Great Central Valley, and for a naturalist, that means blue skies and warm weather, blooming flowers and budding trees, butterflies and baby animals, and, of course, birds!  Spring brings a whole new assemblage of bird species to the Valley: colorful, subtropical birds that spend winters in Central and South America.  A few of these birds include Western Tanagers and Black-headed Grosbeaks, both of which nest and spend their summers in the conifer forests of the Sierra Nevada; riparian species such as Bullock's and Hooded Orioles; brilliant blue Lazuli Buntings and a host of warbler species.
 
Another group of birds that have returned are among my favorite birds to watch: the swallows.  Some species, such as the Tree Swallow and Violet-green Swallow remain in the Central Valley through the winter.  (Other populations of these swallows are migratory.  But as long as a food supply - namely insects - is present, sticking around through the winter gives these cavity nesters a chance to snag a nest site early, before they're all taken!)  But Barn Swallows, Northern Rough-winged Swallows, Bank Swallows and Cliff Swallows spend winters in warmer environments where the insects supply is assured.   
 
An adult Cliff Swallow peers out from a nest
 
Every spring, owing to the age old mystery of migratory instinct, Cliff Swallows (Petrochelidon pyrrhonota) return to their temperate breeding grounds.  (Famously, they are said to return to Mission San Juan Capistrano every year on March 19th.)  The first individuals arrive in the Great Central Valley in February and March, and by April they are busily constructing mud nests underneath building eaves and bridges all across the Central Valley!  A sure sign of spring indeed!
 
It's a happy day when I spot my "FOS" Cliff Swallow, since that means spring is imminent!  (If you're curious, "FOS" is birding code for "First of Season," the first bird of that species you've seen that season.) 
 
Cliff Swallow nests under construction beneath a bridge over a pond
 
And of course, I intended to write this sometime near the end of February when I saw this season's first Cliff Swallow... but since that didn't happen, I'm doing it now!  But this is an appropriate time as well, since right now there is a flurry of winged activity around any Cliff Swallow nesting site!
 
Construction is a busy time for the swallows!
 
Cliff Swallows are found in open habitats, from farmland and grassland to rivers and lakes in arid landscapes.  They build their nests in the shelter of an overhang on a vertical surface: cliffs and river banks are favored in wild habitats, but with increasing frequency they nest alongside humans.  Often undisturbed spots beneath building eaves and bridges provide ideal nesting habitat.  Nests are built in tightly-packed colonies of hundreds, even thousands of individual birds (the largest colonies appear to be in Nebraska, with thousands of nests!) 
 
Gourd-shaped nests tucked safely beneath sheltering eaves
 
Cliff Swallows require a nearby source of freshwater, as they construct their nests entirely of mud collected in their mouths.  Though closer is better (hence the appeal of bridges for nesting site) these intrepid little architects will fly up to a few miles to gather mud in their bills from lakesides, stream banks or puddles.  One nest requires over 1,000 mouthfuls of mud to build!
 
Cliff Swallows flock to puddles, lakesides and stream banks to collect mouthfuls of mud for nest construction
 
Each nest measures about 8 inches by 6 inches and is roughly gourd-shaped, with a small 2-inch opening at the top or side.  Males and females work together to build their nest, often "renovating" an existing nest from the previous year.  The inside of each nest is lined very sparsely with grass and feathers.
 
 
Water is also a necessary component of suitable habitat because it attracts insects, the primary food source of all swallows.  It is a treat to find yourself in the midst of a foraging flock of swallows, the small birds wheeling and turning and diving all around you in their acrobatic hunt for winged insect prey.  They feed almost entirely on the wing, skimming low over bodies of water or flying high above the ground to nab insects; generally only when bad weather prevents insects from flying will they feed on the ground.
 
Young Cliff Swallows in their cozy nest

Cliff Swallow populations seem to be stable over much of their range; it is promising to see this species adapting well to its human-altered environment!  Introduced from Europe and highly invasive, House Sparrows probably pose the biggest concern to Cliff Swallows, as these feisty little birds spread rapidly and appropriate swallow nests for their own use.


As you're out and about this spring and notice little birds fluttering about near bridges and eaves, take a few minutes to admire their handiwork and watch the aerial acrobatics of these enthralling creatures!

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