Winter in the Sacramento Valley, for PRBO biologists concerned with the health of songbird populations, is a time for examining legs--for combing thickets and groves, some leafy, others jumbled, trying to see birds' legs. Our job is to follow individual Song Sparrows, Hermit Thrushes, Bewick's Wrens--nine focal species in all, to gain a picture of native bird communities in woodland habitats.
|Song Sparrow. Conserving biodiversity involves knowing the status of various subspecies, some local, some migratory. © J.&A./VIREO|
We aim to see birds that we've previously captured, specifically the bands that we placed on their legs. Four colored bands in a unique sequence identify a bird the same way your Social Security number identifies you. By sighting these individual birds, we know they are still alive--the entire point of the survey.
This is year three of a project designed to see how well wintering songbirds survive in both remnant and restored riparian forests on the Sacramento River. From November through February, we collect data at four study plots along some 100 miles of the Sacramento River between the towns of Red Bluff and Colusa. These sites are also part of PRBO's long-term monitoring of native songbirds in the Great Central Valley.
Today I'm searching for banded birds on a plot near Butte City, on land belonging to the Sacramento River National Wildlife Refuge. Thick, lush habitat here brings to mind the "jungle" that once bordered this river.
When PRBO began this winter study, in November 2004, we had two distinct subspecies of Song Sparrows using this study plot. (The species has nearly 20 genetically distinct varieties!) We called them 'Little Red' and 'Big Brown'--and the local subspecies 'The One That Breeds Here.' Little Red and Big Brown are both migratory and breed well to the north. The local breeder is non-migratory, with individuals residing in a particular patch of riparian habitat all year.
Funny thing about the local subspecies, though: in 12 years (1993-2005) of monitoring these plots during the breeding season, we've discovered that The One That Breeds Here actually doesn't, at least not any more. All we record at this site in any season are the wintering migrants, Little Red and Big Brown. The historical distribution of The One That Breeds Here is unclear, but Song Sparrows in the Sacramento Valley now are found primarily in two areas, separated by about 100 river miles. They are exceptionally rare in the area between. Eleven seasons of nest searching on this particular plot turned up exactly zero nests.
But then this past winter, in mid-November 2006, I netted an interesting Song Sparrow. It was bigger and browner than Little Red, and smaller and redder than Big Brown. In fact, I'll lay a wager that this one represented our first capture of The One That Breeds Here. Following this first encounter in more than 13 years here, we caught four more Song Sparrows in our mist nets, and three were definitely the 'local' subspecies.
This indication that critical songbird habitat is recovering along the Sacramento River, while very exciting, also points up a challenge for us. After many years of data collection, we stopped summer work here in 2003, and returned to year-round work here last November after missing two breeding seasons. It's quite possible that breeding Song Sparrows colonized parts of our study area in that time and we missed it.
In June of 2006, while picking blackberries, I located a Song Sparrow nest at a nearby plot that had never had them before but is perfect for a colonizer. It has a big block of habitat with blackberry, cattails, and marshy water. We don't have the funding in place to continue work on these plots in coming years (but we're aggressively seeking some), so next year I plan to pick blackberries in lots of places and figure out exactly what's going on.
What better motivation could there be, and what better evidence of the importance of this research, than having a nearly extirpated bird quietly returning to its historical range?