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1 Codor, 8: The Cooper Orithological Society 1978 SNOW CONDITIONS AND THE ONSET OF BREEDING IN THE MOUNTAIN WHITE-CROWNED SPARROW MARTIN L. MORTON Sice the summer of 1968, my studets ad I have bee studyig the breedig biology of the Moutai White-crowed Sparrow (Zootrichia leucophrys oriatha) i the Moo Basi of the Sierra Nevada of Califoria, especially at Tioga Pass. This bird summers i high moutai meadows of wester North America ad witers i the southwester Uited States ad orther Mexico. Earlier we reported o reproductive schedule ad success of oriatha durig the cosecutive seasos of 1968 through 197 (Morto et al. 1972). Of all evirometal factors affectig the reproductive efforts of this bird, sow coditios seem to be most importat. I may years sowpack seems to determie arrival ad occupacy schedules at breedig grouds. Duratio of sowpack ad rate of disappearace affect the availability ad suitability of habitat throughout much of the breedig seaso, The 1969 seaso of our study followed a witer with the heaviest sowpack ever recorded i the Moo Basi ad the 1976 seaso followed a witer with oe of the lightest sowpacks recorded. Herei I compare data from these seasos, ad others, i order to provide isight as to how migratory birds are adapted for breedig uder the rage of coditios foud i high moutais. Most of our study was coducted at Tioga Pass Meadow, a.5 X 1. km subalpie meadow that exteds orthward from Tioga Pass ( elev. 9,5-1,3 ft ). RESULTS SNOW CONDITIONS Begiig i 1927, ad for all but three years sice, sow depth has bee measured, usually several times a year, at Tioga Pass Meadow by State ad private agecies. Sowpack is usually deepest about 1 April but measuremets o that date have varied greatly durig the past 5 years ( Fig. 1). I terms of sow depths durig this study, 1968 ad 1976 (especially 1976) were dry years ad 1969 was extremely wet. 197 was itermediate (compare Table 1 with Fig. 1) ad closest to the log-term average sow depth of cm o 1 April. The rate at which sowpack disappears each sprig depeds o may factors icludig pack depth, D851 water cotet, ad the weather. Durig our study, sow disappeared earliest i the year of least sowpack (1976) ad latest i the year of greatest sowpack ( 1969 ). Sowcover, the percet of groud covered by sow, expresses this variatio i sow disappearace (Fig. 2). The percet of sowcover idicates the quality ad availability of habitat for estig. The date whe egg layig bega each year i our study populatio varied accordig to sowpack, but ot to the extet oe might have predicted (Table 1 ad Fig. 2). I fact, egg layig bega o the same date, 4 Jue, i 1968 ad i 197. It bega five days later i 1969 ad seve days earlier i 1976, thus it 351 :: 3 :: -: _ E _ o 25- l P AZ- CI fl Q) 2- is s 15- c/) IOO- _ l l 1931, 1976 FIGURE 1. Frequecy of sow depths measured o or about 1 April at Tioga Pass, Moo Co., Califoria, Data take from State of Califoria, Bull. 12. : :

2 286 MARTIN L. MORTON TABLE 1. Measuremets of sow ad habitat coditios, estig pheology, est placemet, ad umber of breedig pairs of Zootrichia leucophrys oriatha. Data take from Tioga Pass ad adjacet areas of Lee Viig Creek watershed i Moo Basi, Moo Co., Califoria SNOW CONDITIONS Sowpack, 1 April (cm) Date of 25% sowcover NESTING PHENOLOGY Number of clutch starts Date of first clutch start Date of last clutch start Mea date of clutch starts Spa of clutch starts (days) HABITAT CONDITIONS Date willows were leafed out Date oios bloomed Date of peak mosquito abudace NEST PLACEMENT Numbers of ests measured Percet o groud Percet above groud Mea est height (cm) PAIRS ON TIOGA PASS MEADOW Jue 4 July 14 Jue 17 May Jue 9 Jue 4 Jue 28 May 9 July 23 July 24 July 15 July 16 Jue 28 Jue 24 Jue 11 Jue Jue 12 July 25 Jue 2 Jue 7 July 3 July 17 July 7 July 25 Jue 1 July 24 Jue 25 Jue had a rage of 12 days. The ed of egg layig, first egg of the last clutch, icludig reests ad secod clutches, raged over 15 days (9 July i July i 197, see Table 1). I 1976, egg layig cotiued util 15 July but clutches begu i July were located i oly a few hillside areas kept gree by groud water. Sowcover varied greatly from year to year at the time whe egg layig bega: ear 1% i 1969 but less tha 1% i Meltig was prologed i 1969, as expected from pack depth, ad also i 197. A week-log storm i early Jue 197 brought more sow, ad relatively cool temperatures teded to preserve the pack ad prolog the melt period. The timig of vegetatioal developmet differed the most betwee 1968 ad The approximate dates of two cospicuous vegetatioal chages-whe willows (S&x spp.) fiished leafig out ad whe swamp oio ( Allium validur) bega flowerig-are give i Table 1. Growth of willows ad other vegetatio seemed to be retarded by the extremely dry coditios of This did ot ihibit estig, however, because i the absece of sowcover, may 2.1. ogutha ested o the groud i patches of old grass, usually at the base of small, scraggly willows. Mosquitos became most abudat about seve to te days after they bega emergig i all years. This date was about 25 Jue every year except 1969, whe it was 15 days later (Table 1). Sow coditios seemed to affect the sparrows estig i at least three ways: 1. Timig. Assumig that the temporal progress of the breedig cycle was best idicated by mea date of all clutch starts, the estig schedule varied oly 17 days amog years, whereas sow coditios, as calculated from date of 25% sowcover, varied by 48 days (Table 1). These rages were calculated by comparig 1969 ad 1976 seasos. 2. Nest site. If thick, dry grass was available, most females built their ests o the groud. If the grass was wet or covered by sow, the birds ofte ested above groud i pies or willows, eve uleafed willows. This adaptability is show most clearly by comparig data o locatio ad average height of ests i 1969 ad 1976 ( Table 1). 3. Number of breedig pairs. If the birds were preset durig the mai period of thaw, patches of emergig vegetatio, particularly willows, formed foci for establishmet of territories. The first clutches of the seaso were started i these first patches of ope vegetatio If sowcover persisted through the last half of Jue, as i 1969 ad 197, some meadow areas that provided territories i other years were ot used. This had the effect of reducig total umbers of breedig pairs because there was o apparet compesatory decrease i territory size. For example, i 1968 ad 1976, years whe sow melted early, Tioga Pass Meadow supported 28 ad 27 pairs of White-crowed Sparrows, respectively (Table 1). I 1969 ad 197, years whe sow melted late, 23 ad 25 pairs were preset.

3 SNOW CONDITIONS AND BREEDING IN SPARROWS m R ; 3 1 zj 5 loo t i v IO 2 May 3 IO Jue FIGURE 2. Sowcover at Tioga Pass ad schedule of clutch starts i Zootrichia leucophrys oriatha. Arrows mark mea date whe clutches were started, icludig reests. AIR TEMPERATURE Air temperature was measured daily by the U.S. Park Service at Tioga Pass with a maximum-miimum thermometer. Daily oscillatios i temperature formed o defiable patter i sprig ad early summer with respect TABLE 2. Mea daily low ad high air temperature ( C) at Tioga Pass, ad May Jue l July l Low SD High SD -1.G to sowpack coditios. Miimum ad maximum temperatures rose from mid-may through Jue (Table 2). Frost-free coditios geerally prevailed from late Jue through August although freezig temperatures might occur at ay time durig the summer. Daily temperature was typically coldest i early morig, the time of egg layig. Durig the layig period miimum temperatures were ofte at or ear freezig. I oe extreme istace, 5-7 Jue 1968, three clutches were started whe daily highs were 2 to 4 C ad lows were -12 to -13 C. DISCUSSION WEATHER AND ONSET OF BREEDING Goadal growth i Zootrichia leucophrys is primarily cotrolled by day legth (see re-

4 288 MARTIN L. MORTON views by Farer ad Follett 1966, Farer ad Lewis 1973) although social iteractios (Lewis ad Orcutt 1971) ad air temperature (Farer ad Mewaldt 1952, 1953, Lewis ad Farer 1973) may additioally affect it. I 2.1. oriatha cosiderable goadal growth occurs after arrival at the breedig grouds (Morto, Horstma ad Osbor 1972, Morto 1976). Evirometal coditios prevailig at that time might be expected to affect growth of the goads. Weather modifies the birds breedig schedule idirectly through effects o vegetatio ad such crucial factors as availability of cover, estig sites, ad food supply. Sowpack seems to be the most importat of all weather-related coditios that affect the aual cycle of plat growth at high altitude (Weaver 1974, Owe 1976). The earlier developmetal phases of plats i subalpie meadows are delayed up to eight days for every 1% icrease i sowpack above the log-term average. I years of greatest sowpack herbaceous plats may grow ad mature as much as six weeks later tha ormal. The growig seaso may be shorteed appreciably at such times because cessatio of plat growth is regular from year to year (Owe 1976). Male White-crowed Sparrows usually appear ad begi establishig territories at Tioga Pass i mid-may (Morto 1976), but i 1976 several sigig males were preset o 2 May (Paul Sherma, pers. comm.). This suggests that some birds may arrive i lowlad valleys at the latitude of the breedig groud i late April or earlier. They may the asced to breedig areas as soo as habitat becomes available, but retreat if storms i the moutais close the areas. Although males ted to arrive before females at Tioga Pass, there are a few females amog the vaguard (Morto 1976). Therefore, i a dry year, estig ca begi as soo as goadal maturatio is complete. These sparrows are omivorous ad feed i various ways. This icreases their ability to lay ad icubate eggs while much sow is still preset, although storms of several days duratio may iterrupt estig (Morto et al. 1972) gamhelii estig i Alaska ca also edure cosiderable bad weather while layig eggs ( Oakeso 1954). Birds arrive at the breedig grouds with substatial fat stores that they are usually able to maitai (Morto et al. 1973, Morto 1976). Thus they have a eergetic reserve for survivig poor weather ad for vitellogeesis. Habitat becomes available uevely because of differeces i sowmelt. South-facig kolls clear first. Sparrows i sow-free areas proceed with estig while those i adjacet sow-covered areas must wait. WEATHER AND PRODUCTIVITY The effects of weather o productivity of a oriatha populatio are difficult to evaluate. Deep sow ca delay the estig seaso ad alter the choice of est sites, possibly resultig i icreased predatio. Storms durig the estig seaso ca cause cosiderable mortality i both eggs ad chicks (Morto et al. 1972). These adverse effects are offset by some ukow margi, however, because the birds routiely reest if eggs or chicks are lost, several times if ecessary. I believe that they are physiologically able to breed, ad do so, as log as coditios i their territories are favorable. Toward the ed of the breedig seaso, favorable coditios are highly localized, seemig to deped o the presece of sumps ad sprigs that maitai areas with growig grass. The birds est ad forage i such places as log as they exist. Meltig sow from a heavy pack may keep the meadow wet ad gree util late July, as happeed i 1969 ad 197. Some double-broodig occurred i those years but rarely otherwise. If sow o the groud reduces the amout of habitat available while territories are beig established, fewer pairs will be able to breed. Hece, it seems that, o the average, productivity of the populatio should decrease after witers of heavy sow. SUMMARY Moutai White-crowed Sparrows (Zootrichia leucophrys oriatha) migrate to ad breed i motae areas that sustai large variatios i sowfall. Hece, they ecouter cosiderable aual variatio i evirometal coditios upo arrival at their breedig grouds. At Tioga Pass, breedig is delayed i years of deep sowpack. However, idividual pairs will occupy patches of habitat as soo as they are sow-free. Furthermore, uder such coditios, may females alter a apparet preferece for estig i dese grass o the groud, istead, they est i small pie trees, willows, ad elsewhere. Oce the birds are ready to breed ad habitat is available, estig proceeds relatively uaffected by temperature extremes or storms. I years of heavy sow, some habitat is uavailable util late i the summer. This reduces the umber of breedig pairs i a give area ad may decrease breedig success or productivity for the populatio. The birds re-

5 SNOW CONDITIONS AND BREEDING IN SPARROWS 289 est readily but usually cease breedig after a brood is fledged. Suitable breedig habitat is patchily distributed at the ed of the seaso, wherever surface water cotiues to sustai wet-gree coditios. I such areas efforts at reestig are prologed ad double-broodig may occur. Owig to their schedule of arrival, modes of foragig, maagemet of eergy reserves, flexibility i est placemet, reestig capabilities, ad ability to withstad brief spells of bad weather, these sparrows are able to reproduce successfully i subalpie areas despite large aual variatios i evirometal coditios. ACKNOWLEDGMENTS I thak the followig Occidetal College studets for their help i this study, particularly i searchig for ew ests: Carol Beuchat, Debbie Dema, Sharo Shiba, Ke Sokolski, Koichi Uyemura, ad Tom Wood. LITERATURE CITED FARNER, D. S., AND B. K. FOLLETT Light ad other evirometal factors affectig avia reproductio. J. Aim. Sci. Suppl. 25: FAHNER, D. S., AND R. A. LEWIS Field ad experimetal studies of the aual cycles of White-crowed Sparrows. J. Reprod. Fertil. Suppl. 19 : FARNER, 1). S., AND L. R. MEWALDT The relative roles of photoperiod ad temperature i goadal recrudescece i male Zootrichia Zeucophrys gambelii. Aat. Rec. 113:612. FARNER, D. S., AND L. R. MEWALDT The relative roles of diural periods of activity ad diural photoperiods i goadal activatio i male Zootrichia Zeucophrys gambelii ( Nuttall). Experietia 9: LEWIS. R. A.. AND D. S. FARNER Temueraturc modulatio of photoperiodically iduced veral pheomea i White-crowed Sparrows (Zootrichia leucophrys). Codor 75: LEWIS, R. A., AND F. S. ORCUTT, JR Social behavior ad avia sexual cycles. Scietia 65: MORTON, M. L Adaptive strategies of Zootriclzia breedig at high latitude or high altitude. Proc. 16th It. Orithol. Cogr. (1974) : MORTON, M. L., J. HORSTMANN, AND J. OSBORN Reproductive cycle ad estig success of the Moutai White-crowed Sparrow (ZootTichia Zeucophrys oriatha) i the cetral Sierra Nevada. Codor 74: MORTON, M. L., J. HOHSTMANN, AND C. CAREY Body weights ad lipids of summerig Moutai White-crowed Sparrows i Califoria. Auk 9: OAKESON, B. B The Gambel s Sparrow at Moutai Village. Alaska. Auk 71: OWEN, H. E Pheological developmet of herbaceous plats i relatio to sowmelt date, p I H. W. Steihoo ad J. D. Ives teds.], Ecological impacts of sowpack augmetatio i the Sa Jua Moutais, Colorado. Sa Jua Ecology Project, Fial Report. Colorado State Uiv. Publ., Fort Collis. STATE OF CALIFORNIA. Water coditios i Califoria. Dept. Water Resources, Bull. 12. WEAVER, T Ecological effects of weather modificatio: Effect of late sowmelt o Festuca idahoesis, Elmer meadow. Am. Midl. Nat. 92: Dept. of Biology, Occidetal College, Los Ageles, Califoria 941. Accepted for publicatio 1 July 1977.