Where Have All The Songbirds Gone? (A Brief Introduction to Monsoon Molt Migration)

Though the thermometer outside hits triple digits with alarming frequency in July and August here in California's Great Central Valley, according to the birds, fall migration is already underway!

The "fall" migration of shorebirds is well-known for its early start in July, as individuals begin returning from Arctic breeding grounds while summer is still at its peak.  Failed nesters may arrive the earliest, but even those that raise young successfully in the short northern summers begin returning south when the calendar indicates that summer is far from over.  

But what about songbirds?  How early do they pack up and head south?  

Visit your favorite birding patch in August, even during the cool hours just after dawn, and you will most likely be met with... silence.  Why is it so quiet out there in late summer?  The birds seem to have just... vanished.  But are they really gone?  If so, where have they gone?  Have they migrated south already?

Like migration itself, the answers to those questions are complex!

Western Kingbird: Abundant in the Valley from April through June, these flycatchers of arid grasslands and shrublands become scarce by August and September.

The simple answer, for many species of resident (non-migratory) breeding birds, is they haven't gone anywhere at all.  In fact, in late summer, after scads of young birds have fledged, there are actually more birds out there than earlier in the season!  But after the frenetic pace of establishing and defending breeding territories, attracting mates, and raising a brood or two of young, all of which are quite noisy endeavors, songbirds  seriously quiet down during the long, hot days of late summer.  They simply no longer need to make much noise.  Some jays are still squawking, of course, and you might hear a few young birds trying out their first quiet songs, but the avian soundscape is still much, much quieter in August than it was a couple of months ago.  

Okay, so one answer is that the birds are just a little trickier to find, as they quietly and discreetly go about their late summer activities.  

But the more time I spent observing the birds in my favorite local patches, the more convinced I became that, come August, many of the familiar migratory species really are just... gone.

And so, I started doing some research.

The rollicking warbled song of the Bullock's Oriole is a familiar sound in riparian woodlands in the spring and early summer.  But come August, these same trees are eerily quiet.

The lives of birds are intricately linked to the seasons in ways that we, as humans, can't understand.  In addition to the influences of the seasonal cycle, the lives of birds are also governed by three basic annual life events: breeding, which includes establishing and defending territory, attracting and bonding with a mate, nest building, egg laying, incubation and chick-rearing; molt, a period during which a bird replaces old, worn-out feathers by growing new ones; and migration.  

As you may imagine, all three of these events are extremely taxing on individual birds.  To meet the demands of any one of these three events, birds require access to a steady supply of nutritious food, in ample quantities.  (Contrary to popular vernacular, to "eat like a bird" is actually to eat quite a large amount of food!  Daily requirements are highly variable amongst species, but generally, small birds need to consume something like a quarter to half of their bodyweight - or more - in food every day!)  

To ensure that songbirds are able to get energy they need to fuel these annual events, birds are designed to tackle them at different times of the year, rather than trying to, for example, molt while raising young.  Just imagine the stress!!  

For many years, it was understood that migratory songbirds that undergo a post-breeding, prebasic molt in the fall (that is, a molt into basic (nonbreeding) plumage, involving the replacement of most body and flight feathers), do so either on their breeding grounds, before heading south, or on their wintering grounds, after having completed their southern migration.  And in many species, this is the case.

But as with most things, it's not quite so simple.  (Molt in birds is never simple!)

Relatively recently, ornithologists began discovering that songbird molt might be much more complex than they realized.  Research shows that passerines across North America may in fact move around quite a bit after the breeding season is over, in a sort of post breeding dispersal or "molt migration."  In some cases, birds molt neither on their breeding grounds nor on their wintering grounds, but somewhere else entirely.  

In waterfowl, it has long been understood that male ducks depart from their breeding grounds shortly after breeding to migrate to specific locations to molt.  While molting, ducks are briefly rendered flightless as they wait for new flight feathers to grow in, so their molting sites must be rich in food, as well as provide cover and protection from predators.  

Some songbirds, particularly those that spend winter in Central America, also undertake a similar molt migration.  

In the arid Western United States, this annual molt migration is especially pronounced.  As the heat of summer increases in intensity after the breeding season, the landscape becomes even drier and food sources become increasingly more difficult to find.  And so, birds do what they do best: they fly away in search of greener pastures.  

As it turns out, it wasn't my imagination (or lack of skills as a birder) that favorite neotropical migrants, like Western Kingbirds, Bullock's Orioles and Lazuli Buntings, were largely absent from my lists of bird observations in late July and August.

Some eBird sleuthing revealed that bar charts of species abundance in three of the counties in the northern San Joaquin Valley where I do much of my birding (San Joaquin, Stanislaus and Merced) reflect my own observations perfectly: The numbers of certain neotropical migrants decrease significantly in late summer, after the breeding season.  

In the following composite image I captured from eBird.org, notice how the bar chart for these six species reflects a gradual decrease in abundance as the summer advances into fall.  Ash-throated Flycatcher, Western Kingbird, Bullock's Oriole, Black-headed Grosbeak, Blue Grosbeak and Lazuli Bunting all breed in the riparian forests, shrublands and/or grassland edges of the San Joaquin Valley.  The general trend shows that numbers peak during the May-June breeding season, then gradually taper off through July, August and September.  Some individuals, particularly juveniles, stick around in the Valley through the end of September and even into October, at which time their fall migration instincts kick in properly and the last stragglers finally make their way south for the winter.

(I should note that Black-headed Grosbeaks are actually more common breeders at higher elevations in the Sierra Nevada mountains, and the chart reflects an influx in May as they migrate through the Valley.  Other migratory birds that breed in dry, mid-elevation forests of the Sierra also follow this pattern of molt migration, including Western Tanagers and Warbling Vireos.)      

Bar chart showing the abundance of six species of migratory songbirds that breed in San Joaquin, Stanislaus and Merced counties, then disperse as the arid summer progresses.  Notice declining abundance from mid-July through September, delineated by the red boxes.
Source: ebird.org

So, where do these birds go after their young fledge and once-verdant breeding grounds become too hot and dry to tolerate? 

The answer to the riddle lies in the annual weather patterns of the desert southwest, and the unique phenomenon known as the monsoon season.  

In the arid southwestern United States and northern Mexico, late summer is monsoon season.  During this "second spring," rain falls in abundance, initiating a flush of new vegetative growth at a time when the rest of the arid west is still parched and barren.  

During the Mexican monsoon season, deserts burst into bloom and insects show up in droves.  Lush growth and an influx of insects draw migratory birds, including our Lazuli Buntings and Black-headed Grosbeaks, from across the arid western U.S. to a rich buffet, just when they need it most.  In these food-rich regions, migratory birds fill up on the high-energy food they need to fuel their energy-intensive molt.  

Once molt is complete and the birds' new feathers are in, they continue south on their journeys to wintering grounds in Central America. 

This map illustrates the breeding (green) and wintering (blue) ranges of California's Black-headed Grosbeaks.  After departing their breeding grounds in mid-late summer, Black-headed Grosbeaks undergo a molt migration, spending time in the food-rich regions in between (red) before continuing on to wintering grounds further south.  
Source: https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.8934

And so, while birders in the Central Valley broil through the blazing hot, quiet days of August, we can rest assured that at least some of our favorite birds are already far to the south, enjoying the luxuriant growth and insect feast brought about by much-needed monsoon rains.  

For more on molt migration, check out this article from the ABA.  And to get a better sense of what diverse groups of birds, like hummingbirds, waders and shearwaters, are up to at this time of year, take a look at this short article from Audubon.

And for a refresher on the topic of bird migration in general, I suggest my article Unraveling the Mysteries of Migration.