DROUGHT AND PREDATION CAUSE AVOCET AND STILT BREEDING FAILURE IN NEVADA JULIE A. R. ALBERICO, Universityof Nevada,Ecology, Evolution and Conservation BiologyProgram, 1000 Valley Rd., Reno, NV 89512 The LahontanValley wetlands of Nevadaare criticalbreeding, wintering: and migratorystopover sites for shorebirdsand waterfowland have been classifiedas a HemisphericReserve within the WesternHemisphere Shore- bird ReserveNetwork (Myerset al. 1987, Harringtonet al. 1989). From 1905 through 1987, wetlands in the Lahontan Valley declined from 34,800 to 6150 ha (Hoffman et al. 1990:5). In 1991, thesewetlands were reducedfurther as Nevadaexperienced its fifth consecutiveyear of drought (Schaefer1991). During 1991, I monitoredbreeding by AmericanAvocets (R'ecurvirostra americana) and Black-necked Stilts (Himantopus mexicanus) in the extremelylimited suitable habitat available to them in the LahontanValley. Here I report numbersof breedingrecurvirostrids and discusspossible reasons for their successor failurein 1991. Avocetsand stiltsfeed in shallowwetlands and nest together in loose colonies(Hamilton 1975). Neighboringpairs work together in voicing alarm, mobbingpotential predators, and performingdistraction displays (Sordahl 1986, 1990). Like the well-studiedPied Avocet (/•. avosetta) in Europe and Black Stilt (H. novaezealandiae)in New Zealand,they prob- ably first breed at an age of 2 or 3 years and are relativelylong-lived (Cadburyand Olney 1978, ChristineReed pers. comm.). Sordahl(1984) marked individual American Avocets and Black-necked Stilts and observed 20-30% return in the followingbreeding season; 60-90% of thesebirds had previouslybred successfully. METHODS I compiledcount data for AmericanAvocets and Black-neckedStilts from a variety of sources:Alcorn (1988), nestingseason reports in Audubon Field Notes andAmerican Birds, andunpublished data provided by Larry Neel of the Nevada Department of Wildlife and Bill Henry of Stillwater NationalWildlife Refuge. I usedonly count data that distinguishedbreeding birdsfrom nonbreedersor migrantsbecause there is extremevariability in migrationdates for thesebirds (Bill Henry unpubl.data; datesin American Birds regional reports), and I observed large numbers of resident nonbreedersduring the 1991 breedingseason. However, breedingbirds can be undercountedif adultsuse alternate foraging sites (Y&sou and Girard 1988), and incubatingbirds can be difficultto see at some sites(Gibson 1971, his Figure3). They can alsobe overcountedbecause birds sounding an alarm attract others(pers. obs.). In 1991, I monitoredavocets and stiltsat all LahontanValley wetlands with conditionspotentially appropriate for breeding(Figure 1). I categorized birdsas prenesting (defending feeding territories and copulating but without a nest), nesting (with nests and defending feeding territories), or nonbreeding(showing no defensiveor copulatorybehavior) (Gibson 1971, Western Birds 24:43-51, 1993 43 AVOCET AND STILT BREEDING FAILURE Hamilton 1975). I foundmost nestsduring the layingperiod. For nests foundafter the clutchwas complete,I estimatedinitiation dates by floating the eggs(Westerkov 1950). Whenpossible, I identified nest predators from evidenceleft in the nest, tracks, and direct observation.Nests at Mahala Sloughwere checkedevery 2 daysas part of behavioralmonitoring, and nestsat S-Line seepwere monitoredevery 5 days.For nestsat Mahala Slough,I took data on severalvariables I expectedto influencethe likeli- hoodof nestpredation: distance to land,number of avocetsin the colony, numberof stiltsin the colony,and distanceto nearestneighbor. RESULTS AND DISCUSSION Prior BreedingNumbers and ReproductiveSuccess During 18 of the years from 1949 to 1975, numbersof breeding recurvirostridsat StillwaterWildlife Management Area (SWMA) were esti- mated and reportedin SWMA Annual or QuarterlyNarrative Reports (Figure2). Kingery (1974) reportedcounts at SWMA of 2400, 2100, I •5miles 0 ,•km Hi I GunClub Mahala ß Reservoir• Slouo_h 0S-Line -,,•,•1•,seed Fal•Ion • Rice Carson Lake SprigPond U• Figure 1. Map of LahontanValley wetlands near Fallon, Nevada,adapted from Hoffman et al. (1990) and U.S. GeologicalSurvey maps. Labeledweftands held water duringall or part of the 1991 breedingseason; underlined labels indicate weftandswith known recurvirostridbreeding attempts in 1991. 44 AVOCET AND STILT BREEDING FAILURE õ • o o BunoA 45 AVOCET AND STILT BREEDING FAILURE 1800, and 675 avocetyoung and 1300, 1000, 720, and 225 stilt young from 1971 to 1974, respectively.Though actual counts are unavailablefor these years, numbersof recurvirostridsin the Lahontan Valley declined duringthe 1976-1977 drought(Rogers 1977, Kingery1977) and during the current drought (Bill Henry unpubl. data). In the 1976 and 1977 breeding seasons,recurvirostrids increased at Great Basin wetlands in Oregonand Utah, suggestingthey had movedfrom drought-strickenareas suchas the LahontanValley (Kingery1977, Rogers1977) Limiteddata on breedingsuccess in the LahontanValley suggest that few young shorebirdshave been producedsince the currentdrought began. Stanton(1988) observed"moderate" stilt hatching success at SWMA but no younglater in the season.Stilts did not fledgeat SWMA in 1989 (Kingery 1989), and "fared poorly" there in 1990 (Kingery 1990). Avocetsestab- lishedonly four nestsat SWMA in 1989, all of whichfailed (Kingery 1989). Thus it is surprisingthat Kingery (1990) reported "excellentnesting suc- cess"for 2700 avocetsin the LahontanValley, though he gaveno countsof young. Thesedata showthat in manyyears thousands of avocetand stiltyoung are producedin the LahontanValley. Duringthe droughtyears of 1988 to 1990, numbersof breedingrecurvirostrids and their successwere reduced, exceptin 1990, when avocetsapparently had appreciablesuccess in spite of the drought. RecurvirostridBreeding Numbers, 1991 Several wetlandswhere recurvirostridsnested in previousyears (Bill Henry and Larry Neel unpubl.data) were dry in 1991, includingSheckler Reservoir,large portionsof SWMA (includingStillwater National Wildlife Refuge),and much of CarsonLake (seeFigure 1). Only about 52 avocets and 16 stilts nested. At the CanvasbackGun Clubon 27 May, I saw22 prenestingavocets, an islandwith many scrapes,and two avocetnests on hummocksof cattail (Typhalatifolia) detritus. However, by 14 June,nests had beendepredated and avocetswere no longerexhibiting breeding behavior. Behavior of a stilt pair suggestedthat they had a neston 14 June,but the pair disappeared8 dayslater. At CarsonLake on 27 April, I counted319 prenestingavocets on Sprig Pond, 179 on the eastside of RiceUnit, and 30 alongcanal banks. Because no water was deliveredto CarsonLake wetlandsuntil mid-July,Rice Unit dried up by 29 May, preventingbreeding there. Although I regularly searchedfor nestsat SprigPond, I neverfound any. By 10 Juneall breeding behavior had ceased, and from 1240 to 3000 avocets remained as nonbreedersthrough June. Breeding behavior by avocetsalong canals ceasedat approximatelythe sametime. I counted50 stiltsat CarsonLake on 10 June but saw no evidenceof reproduction. The only other LahontanValley siteswhere recurvirostridsattempted breedingwere Mahala Slough(between Fallon and Hazen)and a seeparea behindS-Line Reservoir(Figure 1). Nest initiationchronology at thesesites for nestswith knownclutch initiation dates is shownin Figure3a. At Mahala 46 AVOCET AND STILT BREEDING FAILURE Sloughthere were 59 totalavocet nests (including renestings) and 10 stilt nests.Mahala Slough began to dryup from 50 ha on31 May;by 30 June 2O Black-necked Stilt American Avocet 10 2O 10 10 15 20 25 I 5 10 15 20 May June Date Figure3. a, Clutchinitiation dates; and b, depredationdates for AmericanAvocets andBlack-necked Stilts at MahalaSlough and S-Line Reservoir seep, 1991. Clutches includedare those for which initiation or depredationdates could be approximated. 47 AVOCET AND STILT BREEDING FAILURE allbut several roadside borrow pits (< 100 m2 in totalarea) were completely dry. At S-LineReservoir seep, where water was stable at 4 ha, therewere one avocet and six stilt nests. It is difficultto assesshow the droughtwill affect futurerecurvirostrid numbersin Nevada. Site fidelity of avocetsand stilts (Sordahl 1984) suggestsprevious experience affects breeding site choices.If birdsbreed elsewhereduring drought conditions and choosenot to return,it may take manywet years before breeding populations are comparableto thoseof the 1960s and 1970s. RecurvirostridBreeding Success, 1991 All 59 avocetnests at MahalaSlough were unproductive. Of these,13 weredepredated by coyotes(Canis latrans), 42 were knownor suspected to have been depredatedby birds,three were abandoned,and in one the eggsfailed to hatch.Of 10 stiltnests at MahalaSlough, two weredepre- datedby birds,four were abandoned,and in four the eggshatched. At S- Line seepthe singleavocet nest was depredated by a bird;of sixstilt nests there,four were depredated by an unknownpredator (probably a bird),and in two the eggs hatched. The Common Raven (Corvus corax) was the primary avian species that preyed on nests. California Gulls (Larus californicus)were probablyoccasional nest predators (e.g., Hill 1988), but I hadno evidenceconfirming predation by gulls.Nest depredation chronol- ogyat MahalaSlough and S-Lineseep combined is shownin Figure3b. Nest persistencetime was not significantlycorrelated with distanceto land, numberof avocetsin the colony,number of stiltsin the colony,or distanceto nearestneighbor (Spearman rank correlations,P > 0.3, n -- 50 for all). Stilt nestingsuccess