Habitat Relationships of Winter Wrens in Northern California

HABITAT RELATIONSHIPS OF WINTER WRENS IN NORTHERN CALIFORNIA

CAMERON W. BARROWS, P.O. Box 478, LaQuinta, California 92253

The Winter Wren (Troglodytestroglodytes) has the most widespread distributionof any wren and is the only member of the Troglodytidaefound in Asia, Europe and northern Africa, as well as North America. In Europe, where it is known simplyas the Wren, this bird has been well studied (Arm- strong1955). In North America, however,there hasbeen littleinvestigation of Winter Wren ecology. Despitetheir cosmopolitan distribution, Winter Wrens in Californiaappear restricted in their habitat selection. Dawson (1925, Vol. 1:680-681) des- cribedthe Winter Wren as an" . . . inhabitantof rugged streambeds . . . throughoutthe somberdepths of the fir and Redwoodforests... pine coun- try beingaltogether too dry." Grinnell(1915: 160) referredto thiswren as "...a common residentof the humid coastbelt... sparinglyin the northern highSierra... occursmore widelyin mid-winter."Beyond such general descriptions,information on the habitatof Winter Wrens in Californiacomes from broad spectrum inventories. Breeding bird censusespublished in American Birdsand elsewhereare currentlythe only sourceof Winter Wren habitatdata for this region. In thispaper I presentdata on the habitatrelationships of Winter Wrensin northern California. I document the wrens' seasonal habitat selection in the contextof forestage and temperature-moisturegradients.

STUDY AREA AND METHODS I gathered most of the data presentedhere at the Nature Conservancy's Northern California Coast Range Preserve in Mendocino County. Interior siteswhere additional, opportunisticobservations were made include the King RangeConservation Area and ButteCreek Old GrowthForest Reserve in Humboldt County, The Nature Conservancy'sMcCIoud River Preservein ShastaCounty and YosemiteNational Park in MariposaCounty. Mostof the data from coastalareas were collectedat the Jughandle State Reserve in Mendocino County. Additional, opportunisticobservations were made of coastalWinter Wrens in the city of Arcata and at the Nature Conservancy's Lanphere-ChristiansenDunes Preservein Humboldt County. I made censusesof Winter Wrens from 1982 to 1984 at the Coast Range Preserveand JughandleCreek sites.At thesesites wrens were counted along predeterminedtransects which traversedrepresentative habitats occurring in the region. The transectat the Coast Range Preservewas 4.8 km long; at Jughandle Creek it was 2.8 km long. Censuseswere stratifiedbetween the fall-winter(late Septemberto early April) and spring-summer(late April to early September)seasons. The CoastRange Preservetransect was censused 24 timesduring the fall-winterperiod, and 14 timesduring spring-summer. The JughandleCreek transectwas censused14 timesduring the fall-winter season,and 5 times during the spring-summer. Western Birds 17: 17-20. 1986 17 WINTER WREN HABITAT

Habitat types found at the coastalcensus site are characterizedby the following descriptions: DisturbedPine Forest:A heavilydisturbed habitat dominated by native Bishop Pine {Pinus muricata) and introducedMonterey Pine {P. radiata). Overall canopy closure of these coniferswas lessthan 30%. Gaps in the canopy were filled with blackberry{Rubus sp.) and various introduced grasses. Sitka Spruce Forest: A dense multi-layeredconifer forest consistingof Sitka Spruce (Picea $itchen$i$),Bishop Pine, Grand Fir (Abie$ grand/s), WesternHemlock (Tsugaheterophy!!a) and Douglas-fir(Pseudotsuga men- •ie$ii). Canopy closurefor these speciesapproached 100%. An extensive understorywas comprisedof Cascara(Rharnnu$ purshiana), Labrador Tea (Ledurn glandulosum), rhododendron (Azalea macrophy!!um),Oregon Grape (Mahonia netrosa), and Red and Blue huckleberry(Vaccinium par- vilroliumand V. ovaturn). Riparian:This habitatwas restrictedto a thin stripalong JughandleCreek and wasdominated by Red Alder (Alnusrubra) and willows(Salix spp.). The understoryhere consistedof SwordFern (Polystichummunitum) and Yellow Skunk Cabbage(Lysichitum americanurn). The inland censussite at the Coast Range Preservewas within the mixed evergreenforest zone (Sawyer et al, 1977). The subdivisionsof this forest where the census was conducted are characterizedby the following descriptions: Old Growth Forest:Generally north-facing slopes dominated by Douglas- fir and occasionallysome Redwood (Sequoiasemperoirens) trees aged 200 years and older. A well developed middle canopy existed, consistingof Tanoak (Lithocarpusdensifiora), Canyon Live Oak (Quercuschrysolepis) and Madtone (Arbutus menziesii). The forest floor was littered with large downedlogs; Blue Huckleberry,Oregon Grape and Sword Fern often oc- curred as a ground cover. Young GrowthForest: A singlelayered canopy of hardwoodsand conifers usuallyaveraging much lessthan 200 yearsin age. Few if any largedowned logs. Meadow: Grassland areas with scattered shrubs, often with human dwellingspresent. RiparianForest: A thin stripof forestalong the South Fork of the Eel River dominatedby Red Alder and Big Leaf Maple (Acer macrophyllum).Sedges (Carex spp.) occurredalong the watersedge. Frequencyof habitattypes along the transectswas determinedby pacing the distancethe transectpassed through each habitattype.

RESULTS AND DISCUSSION To quantifythe Winter Wrens' habitatrelationships, I initiateda seriesof censusesin bothcoastal and inlandportions of the northernCalifornia Coast Range. A summaryof Winter Wren censusesin the coastalhabitat matrix is shownin Table 1. No significantshifts in the wrens'habitat preferences from the fall-winterto spring-summerseasons were noted. The spring-summer distributionof WinterWrens was not significantlydifferent from random.The 18 WINTER WREN HABITAT

fall-winterdistribution was not random:there appeared to be a slightlysignifi- canttendency for WinterWrens to occurin the deciduousriparian forest and avoid the denseSitka Spruceforest. Overall, Winter Wrens in coastal habitats showed broad habitat preferences.These birdswere observedregularly in young managedRed- wood forests,urban settingsand thicketswithin the salt spray zone along coastal bluffs. A summaryof the censusesconducted at the inland site are sho•vnin Table 2. The distributionof WinterWrens during the fall-wintermonths appeared to be random. In the spring-summerseason Winter Wren habitat selectionshifted significantly to an almost exclusiveoccurrence in the old growthforest type.

Table 1. Resultsof Winter Wren censusesconducted at JughandleCreek State Reserve. Habitat descriptionsare in the text. PERCENT WINTER WREN OCCURRENCE PERCENT HABITAT HABITAT TYPE AVAILABILITY FALL-WINTER SPRING-SUMMER

Disturbed Pine Forest 53 51 43 Sitka Spruce Forest 33 24 36 Riparian Forest 14 25 21

Chi-squarevalue 8.8 1.5

Significancelevel p

Total number of observations 99 14

Table 2. Results of Winter Wren censusesconducted at the Northern California Coast Range Preserve. Habitat descriptionsare in the text. PERCENT WINTER WREN OCCURRENCE PERCENT HABITAT HABITAT TYPE AVAILABILITY FALL-WINTER SPRING-SUMMER

Old Growth Forest 55 60 92 Young Growth Forest 32 25 08 Meadow 10 13 0 Riparian Forest 3 2 0

Chi-squarevalue 4.1 15.6

Significancelevel p :0.25 1:)<0.005

Total number of observations 124 40

19 WINTER WREN HABITAT

Corroboratingdata can be gatheredfrom two sources.An examinationof breedingbird censusesappearing in AmericanBirds over the past 15 years revealed 17 censuses conducted in habitats similar to those covered in this study. Elevenof these 17 censuseswere done in loggedor young-growth mixed evergreenforests and none of thesereported Winter Wrens. The re- mainingsix censuseswere conductedin old growth or mature Douglas-fir dominatedforests; all of these reported breedingWinter Wrens. Also, a 3-year investigationof old growth wildlifehabitat relationships in northern California determined a positive correlationbetween Winter Wren abun- dance and forestage (Martin Raphaelpers. comm.). Winter Wrensappear to selectthe old growth conifer forestsof California'sinterior Coast Range duringthe spring-summermonths. Followingthe wrensalong their dailywanderings in summer,I foundthem more commonly away from creeks during early morning hours or on unseasonablycool days. Duringmid-day warm temperaturesthe wrenswere almostalways within the confinesof creekbeds with water. Thesecreek bed habitatswere more than 5-10 ø C coolerthan surroundingnon-old growth areas. Sixty-eightpercent of my spring-summerobservations of Winter Wrens were along creeksor creekbeds; just 30% of my fall-winterobserva- tions were in these wetter areas. The Winter Wrens' northern distribution in North America (Robbinset al. 1966) adds further credence to the wrens' selection of cooler-wetter climes. Selectionfor cool and moist habitatsis just one of severalhypotheses which could explain the Winter Wrens' distributionin California.Selection for habitatswith higher food resources,less interspecific competition, or a higher densityof nest sitesare other possibleexplanations for the Winter Wrens' limiteddistribution. None of the above hypotheseswere tested.

ACKNOWLEDGMENTS I would like to thank Leonard Brennan, Michael McCrary, Katherine Barrowsand an anonymousreviewer for helpful commentson this paper.

LITERATURE CITED

Armstrong, E.A. 1955. The Wren. Collins, London. Dawson, W.L. 1925. The birds of California. South Moulton Co., San Diego, Los Angeles, San Francisco. Grinnell, J. 1915. A distributionallist of the birds of California. Pac. Coast Avifauna No. 11. Robbins,C.S., B. Bruun & H.S. Zirnrn. 1966. Birds of North America. Golden Press, New York. Sawyer, •1.O., D.A. Thornburg& •I.R. Griffin. 1977. Mixed evergreenforest. Pp. 359-415 in M. Barbour & •1. Major, eds. Terrestrialvegetation of California. Wiley & Sons, New York. Accepted 27 June 1986