Valley Habitats: Number Six

WILDLIFE RESOURCES OF THE CENTRAL VALLEY, CALIFORNIA BIRDS - PART II: Winter Residents and Transients

INTRODUCTION

"Wildlife Resources of the Central Valley, California: Birds - Part 1"discussed the hydrology and habitat history of the Central Valley, thus settingthe framework to discuss birdlife. The Central Valley supports nearly 250species of birds. This vast resource can be characterized into four groupsbased on annual occurrences: permanent residents, summer residents, wintervisitors, summer visitors, and transients.

Part I focused on permanent residents and summer residents in the CentralValley. These species form the breeding component of the Valley's avifauna andhelp to define the California ornithological region. The California realm isalso characterized by a high diversity and density of winter residents andtransient species. Part II discusses winter residents and transient birds thatcontribute to this high diversity.

CENTRAL VALLEY BIRDLIFE

Winter residents and transients exploit the rich, seasonal resources providedby the Valley's various natural habitats and agricultural lands. In fall andearly winter, migratory waterfowl feed on waste grains of harvested corn,wheat, rice, and other crops. Raptors hunt for prey on harvested beet fields,irrigated pasture, and other fallow croplands. The winter rains bring morebirds moving south from northern breeding grounds. The seasonal wetlands,flooded fields, and managed wetlands provide these birds with feeding andresting habitat. In spring, migration again follows the accelerated vegetativegrowth and invertebrate blooms seen in natural wetlands, fields, and croplands.

For planned restoration and wildlife-friendly farm management activities, it isimportant to understand the seasonal patterns of bird movements in the CentralValley. Today, protection and restoration of the remaining resources are theprimary goals of some private and public wildlife conservation organizations.However, it is the private landowner that can and continues to make thedifference in providing seasonal habitat for wildlife. This is especially truefor the winter residents and transient species. Only through private land andpublic agency cooperation can we maintain the diversity of wildlife in theValley.

WINTER RESIDENTS

Winter residents breed in latitudes north of the Central Valley or in montanehabitats. They migrate to the Central Valley during fall to spend the wintermonths (Fig. 1). They depart in spring for their breeding grounds. Examplesof winter residents include: sandhill crane, American wigeon, rough-leggedhawk, long-billed dowitcher, ring-billed gull, ruby-crowned kinglet,yellow-rumped warbler, and white-crowned sparrow. The Central Valley supports75 winter resident species (Appendix). Therefore, during winter, the Valleyrealizes its highest diversity of birdlife with nearly 170 species (combinedpermanent residents and winter residents).

Click here for Figure 1.

The arrival of wintering species is perhaps the most noticeable and importantavian phenomenon in the Central Valley. The movement of cranes, ducks,shorebirds, wintering sparrows, and raptors bring overall diversity and speciesnumbers in the Valley to its highest, particularly during the migrationalperiods (September - November and March - May). Their movements are triggeredby several factors, including genetic programming, day length, temperature, andincreased frequency of storm events.

Altitudinal migrations from the Sierra Nevada to the Valley floor are anotherimportant bird movement. Species such as yellow-rumped warbler, ruby-crownedkinglet, hermit thrush, and dark-eyed junco nest in the Sierra Nevada andwinter throughout the Valley. Some resident species become more abundant inwinter than in summer because of the infusion of northern breeders. Examplesinclude: red-tailed hawk, American kestrel, northern harrier, red-wingedblackbird, mallard, and song sparrow.

Sandhill Cranes: A Winter Resident

Sandhill cranes that winter in the Central Valley represent two distinctsubpopulations that are characterized by body size and where they breed (Fig.1).

The greater sandhill crane (Grus canadensis tabida), a state threatenedspecies, is rare and declining in the Pacific Flyway. The Central Valleywinters 3,500 "greaters" or 98% of the known Pacific Flyway winteringpopulation. These birds originate from northern California and eastern Oregon(Fig. 1). Greater sandhill cranes are, on average, 10% larger than lessersandhill cranes. Their bills are noticeably longer. Otherwise, bothpopulations are nearly identical in coloration and behavior.

The lesser sandhill crane (Grus canadensis canadensis) nests in the high Arcticand northern latitudes of Canada (Fig. 1) and outnumber the "greaters" in theValley by 5 to 1. Mid-winter surveys conducted by the U.S. Fish and WildlifeService indicate a population of nearly 25,000 "lessers," of which 85% winterin the Delta.

Both cranes have similar life histories. Cranes are highly gregarious inwinter, forming large, foraging flocks that range for miles around theirprinciple roosting sites. However, the most important social element is thepair. Sandhill cranes mate for the life of the partner. They can be seenthroughout the winter reaffirming their pair bonds through elaborate courtshipdances.

Cranes feed on a variety of items. They seek a diet rich in carbohydratesthrough the winter. They frequent plowed and unplowed grain fields (corn andrice) to feed on waste grains. In addition, cranes will focus their foragingefforts on greening winter wheat fields, feeding on young shoots. Natural foodincludes tuberous plants which they dig up with their bills. Cranes havebecome dependent upon agricultural fields (particularly irrigated pastures andfallow fields) for securing winter foods. Cranes seek shallow-flooded marshesand fields to roost. The marshes must have open vistas, providing cranes withgood visual range.

The arrival and departure of cranes in the Central Valley is amazinglypredictable. The first "scout birds" arrive by the second week of September,with the main body pushing in by mid-October. The arrival of all the cranesthat winter in the Valley varies with weather events affecting their stagingareas to the north. Crane migration patterns are similar to geese. They movein long migration leaps to staging areas where they will rest and refuel.Cranes depart the Valley during the first week of March and are usually gone bythe middle of that month.

Waterfowl Patterns

The patterns of waterfowl movements in the Valley are highly variable anddependent upon wetland resources and weather patterns (Table 1). The 250,000acres of marshes, and thousands of acres of flooded and fallow agriculturallands, are now providing resources year-round. Increases in habitat, throughrestoration of wetlands, should increase local carrying capacities and durationof use (e.g., to accommodate more birds for a longer time). For example, alocal increase is already documented at the newly restored Cosumnes RiverPreserve, 20 miles south of Sacramento. With a base wetland habitat of 1,140acres, total bird use days have increased from over 1.4 million to over 1.9million birds in five years (1989 through 1993). Bird use days are an estimateof the total number of birds multiplied by their total number of days visitingthe Preserve.

Click here for Table 1.

Swans

Tundra swans arrive in the Central Valley by early November. For the remainderof November through mid-December swans continue to arrive, peaking between40,000 and 60,000 birds. Of the total number wintering in California, over 90%do so in the Delta. By mid-January, their numbers slowly taper off, with thelast individuals generally departing by mid-March. Tundra swans seem to prefershallow-flooded agricultural fields, where they feed on waste grains (e.g.,corn and rice) and aquatic vegetation. Cholera annually affects swanswintering in the Delta.

Geese

The Central Valley is one of North America's most important wintering regionsfor geese. Mid-winter counts in the 1990's (USFWS) indicate a 10-year averageof 860,000 geese in the Pacific Flyway. The Central Valley winters nearly600,000 geese annually, 70% of the flyway total (USFWS 1991). The Valleywinters nearly 400,000 white geese (snow and Ross'), or 30% of the continent'stotal; 75,000 white-fronted geese, greater than 50% of the continent's total;and 35,000 cackling Canada geese, or 80% of the continent's total.

Geese concentrate their foraging activities on agricultural lands, seekingwaste grain, uplands, and green browse (e.g., winter wheat). They loaf on openwater in Central Valley refuges and flooded agricultural lands. Most northernnesting geese arrive by mid-November and depart by mid-March.

Only the great basin Canada goose breeds in the Central Valley, predominantlyin the Sacramento Valley and adjacent foothills. Many Sierra Nevada reservoirssupport Canada geese during the breeding season.

Ducks

In the fall, northern breeding ducks join locally produced ducks, creating thesingle largest movement of waterfowl in the state. The Central Valley wintersnearly 3 - 5 million ducks, or 60% of the total in the Pacific Flyway.Northern pintail are the most numerous, with a winter population fluctuatingaround 1.6 million birds. The mallard is next in abundance, with a winteringpopulation exceeding 400,000 birds. Other puddle ducks make up the balance.In descending order of abundance, these ducks include: American wigeon,northern shoveler, green-winged teal, gadwall, and cinnamon teal. Diving ducksaccount for nearly 70,000 birds, with ruddy duck and canvasback accounting for75% of total wintering divers.

Puddle ducks exploit shallow marshes and fields, with some species (mallard andwigeon) feeding in uplands. Divers forage in deeper waters, rivers, and ponds.Most roost on open lakes. Canvasback and ring-necked ducks often forage inshallows of Valley wetlands. Mergansers, goldeneyes, and wood ducks exploitriverine habitat, as well as ponds and lakes.

Day-Hunting Raptors

The Central Valley is renowned for supporting dense concentrations ofwintering, day-hunting raptors (hawks, eagles, and falcons). They exploit thevast, open grasslands of the Sierra Nevada foothills, agricultural lands in theValley, and woodlands. Table 2 highlights the raptor species, seasonaloccurrence, and preferred habitat in the Central Valley. Recent observationson restored seasonal wetlands and irrigated pasture on the Cosumnes RiverPreserve confirm active foraging by Swainson's, red-tailed, and red-shoulderedhawks; northern harrier; white-tailed kite; prairie and peregrine falcons;American kestrel; and merlin. More studies are needed to document the seasonaluse of wetlands by raptors.

The red-tailed hawk is the most numerous raptor in the Valley. However, turkeyvulture, American kestrel, northern harrier, and white-tailed kite also arewidespread. The red-shouldered hawk is widespread, but is restricted towoodlands. Winter brings an influx of northern breeders, such as: theferruginous hawk, merlin, peregrine falcon, and bald eagle. The latter twospecies frequent large wetlands to forage on waterfowl.

Click here for Table 2.

TRANSIENTS

Transients are bird species that migrate through the Central Valley enroute tonorthern breeding grounds or southern wintering grounds. These speciestypically nest in more northern habitats and winter in the tropics. They arepresent in the Valley for a short period of time in spring (March - May) andfall (late August - October). The patterns of movements are different for eachspecies group. Shorebirds begin their fall movements in early August (seebelow). Warblers and other insectivorous birds move through inmid-September.

Spring migration in the Central Valley is a protracted event. Swallows, notrobins, herald the beginning of spring migration. Earliest arrivals are cliffswallows, arriving in mid-February, with other swallows soon to follow. Bymid-March, flycatchers and warblers begin to arrive, but it is in mid-Aprilthat the diversity of migrants peaks. Nine species of warblers, seven speciesof flycatchers, three vireos, one tanager, and five hummingbirds, are movingthrough. Some species migrate through the Valley only during one season.

For example, the rufous hummingbird migrates to take advantage of floralblooms. Its northward migration (spring) is through the Central Valley andfoothills, exploiting floral displays for nectar and insects. Their fallmigration takes them predominately through the High Sierra in July and August,thus exploiting a July - August bloom (Fig. 2).

Click here for Figure 2.

Shorebird Patterns

Shorebirds exhibit an interesting migrational pattern, particularly those thatpass through the Central Valley. Numerous studies throughout North Americahave eluded to patterns of migrational replacement (movements by distinctpopulations of birds), not only between species but also between age classes.To illustrate this, we can examine a typical migration in the Central Valley.Migration begins in late August when adult shorebirds (breeding in the Arctic)return to their migrational staging areas. The adults move through the CentralValley, peaking in mid-September. The majority depart after only a few daysand rapidly make their way southward. A second migrational wave (replacement)begins in mid-September. This wave contains the juveniles reared during thesummer breeding season. Their numbers peak by early October. This typicalpattern varies by species.

The Central Valley supports a moderate fall migration as most species migratein numbers along the coast and through Great Basin marshes (Table 3). The dryconditions in fall provide limited shorebird habitat in the Valley. CentralValley wildlife areas and flooded farmlands support the majority of migratingshorebirds in fall. In contrast, the Central Valley is an importantmigrational corridor for shorebirds in spring (Table 3). If the Valleyexperiences a normal rainfall the previous winter, numerous flooded fields,bottoms, and vernal pools provide widespread habitat, rich in invertebrates,for shorebirds. Arrival and departure dates are different and some species aremore numerous in fall than in spring and vice versa. Some interesting examplesare described below:

Click here for Table 3.

Dowitchers:

Long-billed and short-billed dowitchers are closely related andmigrate at different times throughout the Central Valley. Since they aredifficult to identify in the field, knowing when to expect each species helps."Short-bills" are early migrants through the Valley and are uncommon."Short-bills" do not winter in the Valley. Young "short-bills" generally movethrough when the first wave of "long-bills" arrive in the Central Valley inmid-September. During the fall migration, long-billed dowitcher populationspeak in mid-September and again in mid-October (young birds) and many remain tooverwinter. In portions of the Valley, long-billed dowitcher populations peakduring their spring migration in March, with the main body of birds departingfor arctic breeding grounds by the first of April.

Peeps:

The small sandpipers in the genus Calidris are called peeps (in Europethey are called stints). In the Central Valley, there are three common speciesof peeps: least sandpiper, western sandpiper, and dunlin. Their fallmigration is one of replacement and provides an interesting pattern of seasonaldistribution. Both the dunlin and least sandpiper overwinter in large numberswithin the Valley. The western sandpiper is, for the most part, a transientthrough the Valley, but some do overwinter. Each of the three species havedifferent bill and leg lengths that allow them to forage in different waterdepths. Dunlins are the largest and often forage in water up to their chest (3- 4 inches), probing with their long, decurved bills. Western sandpipersforage in shallow water (less than 3 inches) at the shoreline, while leastsandpipers prefer the upper wet mud, picking at the surface. If dunlins areabsent, westerns will move into their niche to forage. Least sandpipers arethe earliest fall arrivals, while dunlin arrive last (Table 3). Westerns aremuch more numerous in the Valley during spring migration than in fall.

Curlews:

Two species of curlews visit the Central Valley: long-billed curlewand whimbrel. Both frequent irrigated pastures and shallow wetlands.Long-billed curlews arrive in early fall and their numbers peak duringmid-winter. In many respects, they have a migrational pattern similar tocranes and ducks. Curlews travel in large flocks and feed on invertebratesthey find while probing the moist ground, irrigated fields, and pastures. Thesmaller whimbrel has a very compressed migration through the Valley, movingthrough in large flocks in April. They also frequent irrigated fields andpastures.

Yellowlegs:

Two species of yellowlegs visit the Central Valley: greateryellowlegs and lesser yellowlegs. Greater yellowlegs are perhaps the mostubiquitous shorebird occurring in any type of wetland or irrigated field. Theyarrive in healthy numbers, peaking in October, with many remaining tooverwinter. Lesser yellowlegs arrive in early fall, peak in October, butwinter further south (although a few might overwinter). They feed along theedges of marshes with low growing vegetation.

The maintenance of bird diversity in the Central Valley will require theprotection of remaining natural communities. Agricultural lands play acritical role in this effort. As primary habitats and buffers to wildlifeareas, agricultural lands provide resources for numerous winter resident andtransient bird species. If explored, opportunities for habitat enhancement onagricultural lands can add to the natural community protection beingaccomplished by public agencies and wildlife conservation organizations onrefuges. Private landowners can and often develop habitat resources on theirlands. Many find that the natural communities can be managed to complementtheir farming operations. The habitat mosaic created by private landownerswill be essential in the recovery and perpetuation of California's bird life, aunique resource on the North American continent.

Appendix

Winter Residents (75 species)

Common Loon - Gavia immer

Horned Grebe - Podiceps auritus

Western Grebe - Aechmorphorus occidentalis

American White Pelican - Pelecanus erythrorhynchos

Tundra Swan - Cygnus columbianus

Trumpeter Swan - Cygnus buccinator

Greater White-fronted Goose - Anser albifrons

Snow Goose - Chen caerulescens

Ross' Goose - Chen rossii

Green-winged teal - Anas crecca

Eurasian Wigeon - Anas penelope

Canvasback - Aythya valisineria

Ring-necked Duck - Aythya collaris

Greater Scaup - Aythya marila

Lesser Scaup - Aythya affinis

Common Goldeneye - Bucephala clangula

Barrow's Goldeneye - Bucephala islandica

Bufflehead - Bucephala albeola

Hooded Merganser - Lophodytes cucullatus

Osprey - Pandion haliaetus

Bald Eagle - Haliaeetus leucocephalus

Sharp-shinned Hawk - Accipiter striatus

Ferruginous Hawk - Buteo regalis

Rough-legged Hawk - Buteo lagopus

Merlin - Falco columbarius

Peregrine Falcon - Falco peregrinus

Sandhill Crane - Grus canadensis

Black-bellied Plover - Pluvialis squatarola

Mountain Plover - Charadrius montanus

Greater Yellowlegs - Tringa melanoleuca

Lesser Yellowlegs - Tringa flavipes

Long-billed Curlew - Numenius americanus

Western Sandpiper - Calidris mauri

Least Sandpiper - Calidris minutilla

Dunlin - Calidris alpina

Long-billed Dowitcher - Limnodromus scolopaceus

Common Snipe - Gallinago gallinago

Bonaparte's Gull - Larus philadelphia

Mew Gull - Larus canus

Ring-billed Gull - Larus delawarensis

California Gull - Larus californicus

Herring Gull - Larus argentatus

Glaucous-winged Gull - Larus glaucescens

Band-tailed Pigeon - Columba fasciata

Northern Pygmy-Owl - Glaucidium gnoma

Long-eared Owl - Asio otus

Lewis' Woodpecker - Melanerpes lewis

Red-breasted Sapsucker - Sphyrapicus ruber

Hairy Woodpecker - Picoides villosus

Say's Phoebe - Sayornis saya

Red-breasted Nuthatch - Sitta canadensis

Brown Creeper - Certhia americana

Winter Wren - Trogolodytes trogolodytes

American Dipper - Cinclus mexicanus

Golden-crowned Kinglet - Regulus satrapa

Ruby-crowned Kinglet - Regulus calendula

Mountain Bluebird - Sialia currucoides

Hermit Thrush - Catharus guttalus

Varied Thursh - Ixoreus naevius

Sage Thrasher - Oreoscoptes montanus

American Pipit - Anthus spinoletta

Cedar Waxwing - Bombycilla cedrorum

Yellow-rumped Warbler - Dendroica coronata

Brewer's Sparrow - Spizella breweri

Vesper Sparrow - Pooecetes gramineus

Savannah Sparrow - Passerculus sandwichensis

Fox Sparrow - Passerella iliaca

Lincoln's Sparrow - Melospiza lincolnii

White-throated Sparrow - Zonothrichia albicollis

Golden-crowned Sparrow - Zonothrichia atricapilla

White-crowned Sparrow - Zonothrichia leucophrys

Dark-eyed Junco - Junco hyemalis

Purple Finch - Carpodacus purpureus

Pine Siskin - Carduelis pinus

Evening Grosbeak - Coccothraustes vespertinus

Transient Species (32 species)

Blue-winged Teal - Anas discors

Pacific Golden-Plover - Pluvialis fulva

Semipalmated Plover - Charadrius semipalmatus

Solitary Sandpiper - Tringa solitaria

Willet - Catoptrophorus semipalmatus

Whimbrel - Numenius phaeopus

Marbled Godwit - Limosa fedoa

Baird's Sandpiper - Calidris bairdii

Pectoral Sandpiper - Calidris melanotos

Stilt Sandpiper - Micropalama himantopus

Short-billed Dowitcher - Limnodromus griseus

Wilson's Phalarope - Phalaropus tricolor

Red-necked Phalarope - Phalaropus lobatus

Vaux's Swift - Chaetura vauxi

Calliope Hummingbird - Stellula calliope

Rufous Hummingbird - Selasphorus rufus

Allen's Hummingbird - Selasphorus sasin

Olive-sided Flycatcher - Contopus borealis

Hammond's Flycatcher - Empidonax hammondii

Dusky Flycatcher - Empidonax oberholseri

Gray Flycatcher - Empidonax wrightii

Swainson's Thrush - Catharus ustulatus

Solitary Vireo - Vireo solitarius

Warbling Vireo - Vireo gilvus

Nashville Warbler - Vermivora ruficapilla

Black-throated Gray Warbler - Dendroica nigrescens

Townsend's Warbler - Dendroica townsendi

Hermit Warbler - Dendroica occidentalis

MacGillivray's Warbler - Oporornis tolmiei

Wilson's Warbler - Wilsonia pusillia

Western Tanager - Piranga ludoviciana

Chipping Sparrow - Spizella passerina

Established Introduced Species (5 species)

Ring-necked Pheasant - Phasianus colchicus

Turkey - Meleagris gallopavo

Rock Dove - Columba livia

Common Starling - Sturnus vulgaris

House Sparrow - Passer domesticus

Extirpated Species (5)

Wood Stork - Mycteria americana

Fulvous Whisling-Duck - Dendrocygna bicolor

California Condor - Gymnogyps californianus

Black Rail - Laterallus jamaicensis

Bell's Vireo - Vireo bellii