Today, the Black Tern is not so familiar, as its breeding range in California has retracted substantially. In the San Joaquin Valley, with the loss of historical wetlands, breeding is now so infrequent that the species is all but extirpated. Currently Black Terns breed primarily in marshes scattered across the hinterlands of northeastern California and in rice fields of the Sacramento Valley.
|Shallow-water marshes provide Black Terns with traditional nesting habitat in Modoc County. Photo by W. David Shuford / PRBO.|
For breeding, its haunts in the Modoc Plateau (see map, below) are shallow-water marshes with low-growing rushes. But, like many waterbirds that depend on shallow-water habitats, the Black Tern has proven adaptable and now breeds in cultivated rice paddies, which mimic conditions in its native marshes.
The species can be quick to make use of ephemeral habitats. In 1998, after an El Niņo winter, snowmelt exceeded the capacity of Sierran dams and flooded barley stubble fields downstream, in the Tulare Basin of the southern San Joaquin Valley. Such replicas of suitable nesting habitat (once widespread and regular) occur roughly once every ten years, but Black Terns promptly nested there in 1998.
|Black Tern nesting on a mat of floating vegetation. Photo © G. McElroy / VIREO/ PRBO|
Black Terns breed semicolonially, clustering in loose colonies in one or more locations in a marsh. They build their nests over water and typically nestle them within or weave them around growing vegetation. Nests may also be supported from below. In rice, birds may place nests on large dirt clods that protrude above the water's surface after fields are flooded for planting. In the Modoc region, grazing cattle venture further into marshes as the season progresses and water levels drop.Then when snowmelt and spring rains reflood the marsh, presto, dried cow pies float to the surface to serve as nest platforms.
The Black Tern must shift breeding locations with the vagaries of drought and flood. Hence, the number of breeding sites may fluctuate over time. PRBO surveys documented about 60 breeding sites in northeastern California in 1997 but only 40 in this drought-stricken region in 2010.
After breeding, Black Terns move to favorable water bodies in California, two of which are exceptionally important. Tule Lake National Wildlife Refuge, in the Klamath Basin along the California-Oregon border, is a key post-breeding or migratory stopover: thousands of terns congregate there in fall, apparently to feast on large numbers of damselflies. The state's only other major fall stopover site is the Salton Sea, just north of the Mexican border, and the thousands of terns on saline waters there speak of the species' divergent habitat use in the non-breeding season. Breeders from the United States and Canada migrate to winter mainly in marine and marine-coastal areas of Central and northern South America, with the Gulf of Panama of particular importance.
|This enigmatic sprite of a tern swoops swallow-like over marshes to capture aquatic insects from the air or the vegetation's surface. Photo © G.Bartley / VIREO.|
Adaptability has its limits, of course. What are this tern's prospects, given the risks of long-distance migration, increasing human demand for water, and a warming climate? For terns in northeastern California, the likely effects of global warming are unclear, because this area lies between regions to the north, projected to have more precipitation under climate-change scenarios, and those to the south, projected to have less. In the Klamath Basin, over-allocation of water likely will see some relief once a recent settlement agreement is funded. By contrast, restoration of tern breeding habitat in the Central Valley faces strong headwinds: severe limits on the availability of shallow water result from mosquito control programs, high summer evaporation rates, and stiff competition for water for agricultural and urban uses. Even so, the Sacramento Valley may continue to host substantial numbers of breeding terns--unless growers are forced to convert to less-water-intensive crops by global grain prices, tight water supplies, or warming temperatures.
To maintain tern breeding habitat in California will require determination and adaptive response to changing environmental and human influences. The terns are up to the challenge, but are we?