Early spring in the Sierra Nevada foothills

Spring has arrived full force in the western US, and with it the bird breeding season.  Resident birds and newly arrived migrants are singing up a storm as they set up territories and attract mates, adding an incredible sonorous background to our daily lives.  In early April, I headed over the Sierras to see and hear what was happening in the western foothills.  The purpose of my trip was twofold: first, to get recordings in a new habitat, and second, to meet fellow biologist Dr. Bob Meese and talk about sound recording.  Bob is retired faculty from UC Davis, where he has conducted research on tricolored blackbirds for many years.

We met up at a campground near Georgetown, California, in a heavily managed area of pine, doug-fir, madrone, and oak, with an understory of dogwood, fernbush and a bunch of other plants I can’t name.  A few wildflowers were out, and the grass was starting to grow, letting us know that spring was just arriving at 3000 feet in elevation.  Only a few birds were singing, actually, just a few individuals of quite a few species, so it appeared that the early migrants had just arrived.

This was just what I was hoping for, for a couple of reasons: first, it identifies the earliest migrants and resident breeders, and second, it simplifies the dawn chorus.  At peak breeding season, the dawn chorus can get quite chaotic, and while dynamic, can be overwhelming.  I’ve found that recording the dawn chorus either before or after the peak produces more pleasing recordings.  Recording later in the morning may also produce nicer recordings.

When we arrived, we were greeted with some casual singing by robins, orange-crowned and yellow warblers, and distant drumming by woodpeckers.  Only one or two individuals of each were singing, a pretty good sign that they were the first to arrive.  Dawn choruses were mellow and sweet.  The first was in a grove of oak and pine, and featured a solo by a Cassin’s vireo with a backdrop of wind in the trees and other forest birds:

The second was more open habitat and near a drumming tree or two used by multiple species of woodpeckers and featured an insistent American robin:

We usually think of migrants as wintering in the south (southern US, Mexico, or further south), and migrating north during the spring, as most of the studies of bird migration have been east of the Rockies (for an interesting read, see Kenn Kaufman’s A Season on the Wind).  In the western half of the US, topographies and habitats are much more complex, and many birds migrate altitudinally, and some migrate latitudinally and altitudinally.

Altitudinal migration in birds is much less well known than is latitudinal migration.  A summary of studies altitudinal migration in birds was not published until 2017 (see below).  Latitudinal migrants seem pre-programmed to arrive at certain locations at a certain time each year (often a certain day, but subject to delays due to extreme weather).  Whereas altitudinal migrants move with the weather, that is, as the plants respond to warming temperatures, the birds follow.  Many birds feed their chicks insects, and it’s likely the insects responses to the new foliage that trigger the birds movements.   In some cases, birds may be fleeing scarce resources in their wintering areas, or seeking areas with fewer predators.  Some birds will breed at a particular elevation, then move up or down post-breeding, before returning to their wintering grounds.  Some birds, such as Mountain Quail and Sage Grouse, hike their way up and down mountains.   Others obviously fly, but for many species, there are some individuals that migrate and some that don’t. 

The Sierra Nevada (and most mountain ranges) provide challenges and opportunities for wildlife.  The tremendous changes in altitude impact the weather, which in turn impacts the vegetation communities and their wildlife inhabitants.  Elevations in the Central Valley may be just a few feet above sea level, while the top of the Sierras is more than 14,000 ft.  The Sierras also create a distinct rain shadow on the east side, which is why habitats near my home at the edge between the Sierras and Great Basin are quite different than the same elevation on the west slope.

And as Robert Whittaker described in 1960s, increases in elevation parallel increases in latitude, so that there are similar forms of vegetation going from the tropics to the arctic as there are going from valley bottoms to high elevations mountains.  So, for example, some populations of American pipit summer in the arctic tundra, while others breed in the alpine tundra of the Rockies, Sierras, and Cascades.

So as we listened to an orange-crowned warbler doing a pretty good imitation of a dark-eyed junco, I couldn’t help ponder how it got there.  Some orange-crowned warblers winter on the California coast (where some are permanent residents), and others winter in southern Arizona, Mexico and in the Gulf Coast states.  They breed in the mountains of the west, all the way to Alaska and across Canada.  Some are known to breed at mid-elevation, then move to higher elevations before finally returning to their wintering areas.  They migrate early, so it is probable that the individuals we were listening to had just arrived.  In a few weeks, following the fledging of their young, they would probably be gone, avoiding the hottest parts of the summer by moving to high elevation.

Did this place sound different a decade ago, before so much of the underbrush had been thinned and the trees were a little smaller?  What about 100 years ago, when it might have been recovering from the massive wildfires of 1910?  Few, if any, of the trees in the area look more than 100 years old.  What about 1,000 years ago, when the area might have been actively managed by Native Americans for food and fuel, probably resulting in more oaks and fewer pines?  And what about 10,000 years ago, when the area was in a major transition following the retreat of the glaciers after the last ice age?  And what will it sound like in the future?

References

Boyle, W.A. 2017. Altitudinal bird migration in North America. Auk 134: 443-465.

R.H. Whittaker.  1975. Communities and Ecosystems. MacMillan Publishing Co., Inc. New York.

Orange-crowned warbler.  All about birds: https://www.allaboutbirds.org/guide/Orange-Crowned_Warbler/overview.