Seasonal abundance of migrant shorebirds in Baja California Peninsula, Mexico, and California, USA ROBERTO CARMONA1,3, GORGONIO RUIZ-CAMPOS2 & GEORGINA BRABATA1 •Universidad Autdnoma de Baja CaliforniaSur, Departamentode BiologfaMarina, Apdo. Postal 19-B, La Paz, Baja CaliforniaSur, C.P. 23080, Mexico. [email protected] 2Facultadde Ciencias;3Facultad de CienciasMarinas, Universidad Autdnoma de Baja California,Apdo, Postal 1653, Ensenada, Baja California, C.P. 22800, Mexico. U.S. mailingaddress: PMB 064, PO Box 189003-064, Coronado, California,92178, USA. Carmona,R., Ruiz-Campos,G. & Brabata,G. 2004. Seasonalabundance of migrant shorebirdsin Baja CaliforniaPeninsula, Mexico, and California,USA. WaderStudy Group Bull. 105: 65-70. Nearctic shorebirdsuse three main flyways (Pacific, Centraland Atlantic) and the largestnumbers are found in the Pacific Flyway, which includesthe Baja California Peninsula.There, at variousdifferent sites,the followingpatterns of abundancehave been observed: very low numbersin summer;large numbers in autumn; low and stablenumbers in winter; and moderatenumbers in spring.In contrast,further north in California, numbersduring spring migration are ashigh as or evenhigher than those in autumn.We contrastthe differential utilizationby shorebirdsof Baja Californiaduring spring and autumn. In autumn,southbound migrants arrive at sitesin southernCalifornia and the upperGulf of California,and then either continue along the mainland coastof Mexico or alongthe Baja CaliforniaPeninsula. Some continue migrating further south while others stayfor the winter.During the returnmigration in spring,modest numbers occur throughout the peninsula, with rathergreater numbers in the north-west,probably associated with the northwardmigration through the extensive wetlands of Sonora and Sinaloa on the mainland coast. INTRODUCTION STUDY AREA AND METHODS Of the 49 shorebirdspecies that breed in the Nearctic, 40 We have analysedshorebird abundance data for five sites migrateto temperateand tropicalareas of Mexico, Central alongthe south-westcoast of the United Statesand in north- and South America for the northern winter (Myers et al. westMexico thathave been previously identified as support- 1987). ing importantpopulations. These are HumboldtBay (north At typicalshorebird stopover and wintering sites through- California),the CentralValley of Californiaand three sites outthe PacificFlyway, there are distinct patterns of seasonal in the Baja California peninsula:Punta Banda, Guerrero abundance.In summer,numbers are very low becausemost Negro and Ensenadade La Paz (Fig. 1). are breedingin the north. In autumn,most sitesrecord the Although there are many siteswith extensiveintertidal largest numbersas first adults and later juveniles migrate mudflatsalong the PacificFlyway in California,we selected south.Lower numbersare present in winter,but theyremain just two (Humboldt Bay and Central Valley) to represent relatively stable becauselittle movement takes place. In thosethat are typical - in termsof bothhabitat and shorebird spring,northward migration leads to increasednumbers as abundance- of the onesused by migrantshorebirds in that birdsreturn to their breedingareas (Page et al. 1979). area (Colwell 1994, Shuford et al. 1998). Of the three major shorebirdflyways in North America All five sitesare influencedby a fairly stable,tropical, (Pacific, Atlantic and Central), the mostimportant in terms high-pressurebelt that bringsdry winds (Coda 1997). Cali- of numbersis the Pacific Flyway (Myers et al. 1987) and fornia andnorth-west Baja Californiahave a Mediterranean- midway along the Pacific Flyway lies the 1,100-kin Baja type climate,while the southernpart of the peninsula(around California peninsula(Fig. 1). There, duringthe pastdecade, 30øN) has an arid climate, except for its southerntip (the systematicdata on shorebirdabundance have beencollected Cape region) which is subtropical(Garcia 1981). for several different coastallocalities (e.g. Palacioset al. The largest areasof intertidal mudflats in California are 1991, Gonzfilez 1996, Carmona& Danemann, 1998). These found in Humboldt, San Francisco and San Diego bays studieshave shownthat the peninsulais particularlyimpor- (Peinadoet al. 1995), while on the west coastof Baja Cali- tant duringautumn migration and of lessimportance during fornia there are mudflats in the bays of Todos Santos,San spring migration (Carmona 1997, Fernfindezet al. 1998, Quint/n, Ojo de Liebre-GuerreroNegro, San Ignacio and Carmona& Danemann1998). In this paper, we presenta Magdalena. On the east coastthe largest mudflats are at generalhypothesis to explainthe patternsof seasonalabun- Ensenada de La Paz in Bahia de La Paz (Carmona et al. danceof migrantshorebirds along the Baja Californiapenin- 1994). sula. 65 Buffetin 105 December 2004 66 Wader Study Group Bulletin I 110 ø N 1 35 ø C•© UnitedStates PB --25 ø Mexico Pacific Ocean Fig, 1. Study area and shorebirdcensus sites. (HB = HumboldtBay (north California);CV = Central Valley of California;PB = Punta Banda(north-west Baja California);GN = GuerreroNegro (mid-west Baja California); and LP = Ensenada de La Paz (south Baja California.)) Our analysisrelates to datafrom six studiesduring 1991- becauseno data are availablefor Central Valley and Hum- 1998. Two were carried out at Estero Punta Banda (north- boldtBay), and(2) seasonalabundance of the speciesamong westBaja California)by Palacioset al. (1991) andGonzfilez the threesites having data for all four seasons.In bothcases, (1996) and coveredsampling periods of sevenmonths and whenthe independencehypothesis was rejected in the first one year, respectively. test, we determined which site contributed the most to this The study sites(Fig. 1) were: (1) Humboldt Bay, north rejection,and then removed it from the analysis.This proc- California (38øN, Colwell 1994); (2) CentralValley of Cali- ess was repeateduntil the hypothesiswas accepted(and fornia (35øN, Shufordet al. 1998); (3) Punta Banda, north- thereforea homogeneousgroup was formed). In addition,the west Baja California (31øN, Palacioset al. 1991, Gonzfilez excluded sites from the initial analysis were compared 1996); (4) GuerreroNegro (28øN, Carmona & Danemann mutually,restarting the procedure(ct = 0.05 in all the cases; 1998); and (5) Ensenadade La Paz (24øN, Carmona 1997). Zar 1999). The frequencyof samplingat eachsite was rather variable anddepended on the particularobjectives of eachstudy (see RESULTS Appendix).Therefore, to facilitatecomparisons on bothspa- tial andtemporal scales (i.e. sitesand seasons)we calculated In our monthlyanalysis, Humboldt Bay, California,had the the monthly average abundanceof each speciesand then greatestabundance of shorebirdsin autumn(November) and convertedto percentageson the basisthat the aggregateof spring(April), with lowernumbers in winter(February) (Fig. the monthlyaverages for eachsite was 100%. 2a). In the CentralValley of California, abundanceincreased The datafor HumboldtBay includea numberof separate graduallyfrom autumn(November) to winter (January)to countsfor siteswithin the bay (Colwell 1994), so for each spring (April) (Fig. 2b). At Punta Banda, Baja California, monththese were aggregated.For two sites,Humboldt Bay therewas greater evenness in seasonalabundances, with only and the CentralValley of California, therewere no datafor smallincreases in autumn(October & November)and spring severalmonths. Therefore, for these,the datawere analysed (March) and a notabledecrease in summer(May & June;Fig. on a seasonalrather than monthlybasis as follows: winter 2c). At Guerrero Negro, highest abundanceoccurred in (December-February), spring (March-April), summer autumn(August & September)with much lower but more (May-June) and autumn (July-November). Even on this stablenumbers in winter (December-February)and a small basis,there were no datafor HumboldtBay or CentralVal- increasein spring(April; Fig. 2d). At Ensenadade La Paz, ley for summer(see Appendix). the highestabundance occurred in autumn(September & An independenceZ2 test (ct = 0.05;Zar, 1999)was used October) and winter (January),with no increasein spring to comparethe abundancesof shorebirdsat PuntaBanda as (March & April; Fig. 2e). reportedby Palacioset al. (1991) andGonz•lez (1996). Like- In our analysisby season(Fig. 3), Humboldt Bay, Cali- wise, two types of comparisonsof shorebird abundance fornia, had the largestnumbers during springand autumn amongsites were made using Z 2 tests:(1) seasonalabun- migration,with only a smalldecrease in winter (Fig. 3a). In danceof the speciesamong the five sites(except for summer contrast,numbers in the CentralValley of California were Buffetin 105 December 2004 Carmonaet al.: Seasonalabundance of migrantshorebirds in Baja CaliforniaPeninsula, Mexico, and California,USA 67 Fig.2. Meanmonthly relative abundance (%) forthe five study sites. Fig.3. Meanseasonal relative abundance (%) for the five study sites. a. Humboldt Bay a. Humboldt Bay 30 4 15 10 0 Aug. Sept, Oct, Nov. Dec, Jan. Feb, Mar. Apr. May June July SPRING SUMMER AUTUMN WINTER 40 b, CentralValley b. Central Valley 35 3O • 25 lO 5 ND ND ND ND o Aug. SepL Oct. Nov. Dec. Jan. Feb. Mar, Apr. May June July SPRING SUMMER AUTUMN WINTER 14 1 40 c. Punta Bande Punta Banda 35 30 • 25 !5 10 5 0 Aug. Sept. Oct, Nov. Dec. Jan. Feb. Mar. Apr. May June July SPRING SUMMER AUTUMN W INTER 60 • d. Guerrero Negro d. Guen•emNegro 2oI to o Aug. Sept. Oct. Nov. Dec. Jan, Feb. Mar. Apr. May June July SPRING SUMMER AUTUMN WINTER 25 e. La Par e. La Paz 20 5 0 Aug. Sept.