BREEDING WATERBIRDS OF LA PAZ BAY, BAJA CALIFORNIA SUR, MEXICO ROBERTOCARMONA, JUAN GUZM,•N, SAUDIEL RAMIREZ, and GUILLERMO FERN,•NDEZ,Universidad Aut6noma de Baja California Sur, Apartado Postal 19-B, La Paz, Baja CaliforniaSur 23000, M•xico La Paz Bay, BajaCalifornia Sur, M•xico, is locatedin the transitionzone betweentwo mayorbiogeographic areas (based on fish and invertebrate distribution),the Californiaregion and the Cortezprovince (Briggs 1974 in Anderson1983). Theseareas are characterizedby patternsof climaticand latitudinalzonation that influenceseabirds' distribution and breedingranges (Anderson1983), sodata on speciesnesting in and nearthis transition zone may help clarifythe factorsgoverning these distributions. Within the bay there are nine islandsand extensivemangroves, constituting the main sites usedby breedingwaterbirds (Figure 1). This part of the BajaCalifornia peninsula is characterizedby a semidesert climate with a mean annual temperatureof 23 ø C (Garcia and Mosifio 1969) and an averageannual precipitation of <200 mm (Jim•nez 1989). Vegetationon the islandsis sparse,typified by chollas(Opuntia spp.), sour pitahaya (blachaerocereusgummossus), old-man cactus(Lophocereus schottii), saltbush(Atriplex spp.), box-thorn(Lycium spp.), Adam's tree (Fouquieriactiguetti), and card6n(Pachycereus pringlei) (Roberts1989). Estuaries,surrounded by mangroveforests, cover about 25% of the coastlineof La Paz Bay, mainly at its southernend (Llinaset al. 1989). Mangrovesoccur also on EspirituSanto Island.There are three speciesof mangrovesin thisarea: the Red (Rizophoramangle), the Black(Avicennia germinans),and, in smallnumbers, the White (Lagunculariaracemosa). The statusand conservationof breedingseabirds in the Gulfof California have been reviewedby Everett and Anderson(1991) and Velarde and Anderson(1993). The waterbirdsof La Paz Bay havebeen studied by many researchers(Mendoza 1983, Llinas 1986, Galindo 1987, Palacios 1988, Jim•nez 1989, Llinaset al. 1989, Llinasand Galindo 1990, Carmona and Z•rate 1992, Carmona 1993, Fernandez1993), but much of their infor- mation is unpublished.Here we compile and add to these records.This report is based on 14 years (1979-1993) of field work by the Avian Laboratoryof the UniversidadAut6noma de Baja CaliforniaSur, La Paz. We censusedbirds along the coastsof the baymonthly from 1980 to 1985. The lastthree years our censuseswere performedtwice every month during the nestingseason (February-July) and extendedto the west coast of EspirituSanto Island and the wholeperiphew of the smallislands E1 Gallo, La Gallina, La Lobera, La Ballena, La Gaviota, El Merito and San Rafaelito (Figure1), where mostwaterbirds nest. SPECIES ACCOUNTS BlackStorm-Petrel (Oceanoclroma rnelania). This specieshas been nesting on La Lobera since at least 1988. There are over 150 nests,but the number is hard to Western Birds 25:151-157, 1994 151 WATERBIRDS OF LA PAZ BAY estimate accuratelybecause this speciesnests in crevicesand under boulders,on cliffs,and in otherdifficult sites. The nestingseason goes from Februaryto June. Least Storm-Petrel(O. microsoma).This speciestoo has nestedon La Lobera sinceat least1988 (>200 nests).Like the Black,the LeastStorm-Petrel lays one egg aroundmid March, and the chicksleave the nestsin the beginningof June. The breedinghabitat of both speciesof storm-petrelis the same. Brown Pelican(Pelecanus occidentalis). We have nestingrecords since 1979. Although the colony has changedlocation, the Brown Pelican has nested fairly regularlyon La Ballena:1979, 800 nests;1980, 600 nests;1981, 70 nests;1982, 300 nests;1983, 250 nests;1984, no nesting;1985, 750 nests;1986, 300 nests (Jim•nez1989); 1987, no nesting;1988, 170 nests;1989, 800 nests;1990, 1200 nests; 1991, 1000 nests; 1992, no nesting.Occasionally, smaller colonies (5-20 nests)have nestedon La Gailinaand El Gallo. Eggsare laid in Januaryand hatch in February. Fledglingsleave the nests in May. Human disturbancehas resultedin panickingchicks killing themselves by impalingthemselves on chollaspines or falling from cliffs. Blue-lootedBooby ($ula nebouxii).On La Lobera,on high cliffsprotected from the sun,one pair nestedin March 1988, layingthree eggsthat neverhatched. On the sameisland in May 1990, two pairseach laid three eggsand fledgedtwo chicks. Great BlueHeron (Ardeaherodias). This speciesregularly breeds throughout La Paz Bay. Until 1988 it nestedon La Gaviota(8-10 nests)in bushesand choilasno taller than 2 m. Since 1989 most of the nests(10-15) have been found at El Conchalitoin the canopyof Red Mangroves(Table 1). Until 1992 two pairsnested on a metallicstructure on San Rafaelito.Some isolatedpairs nestedon La Gallina andEl Galloin March1988, anda colonyof 10 pairsnested on SanGabriel in March 1988. The nestswere locatedwithin the canopyof the BlackMangrove trees. In June1993 a colonyof 20 pairsnested on El Merito,on the topsof bushes.During 1992, althoughthe speciesnested on El Conchalito,San Rafaelito, La Gaviota,and San Gabriel,all attemptsfailed. It seemsthat thisardeid has a widespatial nesting niche. Up to 10 speciesof heronsnest in mangrovesat El Conchalito(Table 1). We have recordedonly the GreatBlue Heron nesting elsewhere around La PazBay, however. At El Conchalito,the colonies are arranged largely in monospecificpatches. The only exceptionswere the two nestsof the GreatEgret, which were within the colonyof Cattle Egrets. VirginiaRail (Railuslimi½ola). We recordedone nestat El Conchalitoin May 1988, in a Red Mangrovetree, very closeto the tide level. In 1989 and 1993 courtshipbehavior was observedin the same area, but no nestswere found. AmericanOystercatcher (Haematopus palliatus). Palacios(1988) foundone nest on LasAfeguas. We foundsingle nests with eggson La Gaviotain April 1989 and March 1990, and on San Rafaelitoin 1989 and 1990. Wiison'sPlover (Charardius wilsonia). Palacios (1988) recordedtwo nestson Las Afeguas,four nestsat Chametla,and three nestsat Fidepazduring 1987. We counted25 chicksat Chametlain 1992 and eightchicks at El Conchalitoin 1993. Heermann's Gull (Larus heermanni). We located two nestswith two and three eggson La Bailenain April 1990. Both nestsfailed. Yellow-lootedGull (L. livens).Since at least1983 a colonyhas been settled on La Gaviota;it had 17 nestsin 1989, 15 nestsin 1990, 16 nestsin 1991 and 14 nests in 1993. There are four more coloniesin La Paz Bay, whichhad the following 152 WATERBIRDS OF LA PAZ BAY ll4 e 110ø37I ø I1•I 24 ø 30' LA PAZ BAY ME 24 ø 15' ,: "! . ZA : ß ,,- .-;.-. '. - .- Ensenodo . LoPoz . "*CM - FP . .. ß . Figure1. La PazBay, showing islands and other sites used by nesting waterbirds. Islands:IES, Espiritu Santo; GO, ElGallo; GA, La Gallina;LO, La Lobera;BA, La Ballena;GV, La Gaviota; ME, E1 Merito; RA, San Rafaelito; AF, Las Afeguas (within Ensenadade la Paz).Coastal areas: CO, El Conchalito; FP, Fidepaz; CM, Chametla; ZA, Zacatecas(within Ensenada de La Paz);SG, SanGabriel (southwest coast of EspirituSanto Island). 153 WATERBIRDS OF LA PAZ BAY TaBle 1 NestingArdeids at El ConchalitoEstuary in La Paz Bay, Baja California, 1986-1991 No. Nesting Nestingseason Nesting Species nests regularity (hatch-fledge) habitat• Black-crownedNight-Heron (Nycticoraxnycticorax) _ 10 annual April-June RM (C) Yellow-crownedNight-Heron (Nyctanassaviolacea) __30 annual March-May BM (H) Green Heron (Butoridesstriatus) 1-2 1988, 1989 May-June BM (L) Tricolored Heron (Egrettatricolor) 1 1988 May-June BM (VH) Little Blue Heron (Egrettacaerulea) 1+ 1988, 1993 May-June BM (VH) ReddishEgret (Egrettarufescens) 2 1988 May-June BM (VH) SnowyEgret (Egrettathula) 10-15 1988, 1989 April-June BM (VH) CattleEgret b (Bubulcusibis) _ 60 annual April-June BM (H) GreatEgret (Casrnerodiusalbus) 2 1989 April-June WM (VH) Great Blue Heron c (Ardeaherodias) __15 annual January-June RM (VH) aRM, Red Mangrove;BM, BlackMangrove; WM, White Mangrove;L, low; C, center;H, high; VII, veryhigh. bln1993, thisspecies moved to a location500 m northeast. CThisspecies breeds regularly throughout La Paz Bay (seetext). We estimatethe entire nesting populationin the bay at about50 pairs. numbersin 1989: La Lobera, 6 nests;La Gallina, 25 nests;El Gallo, 20 nests;La Ballena,25 nests.We alsoobserved 20 nestson El Meritoin 1993. The eggsare laid in March, hatch about a month later, and chicksare able to fly after eight weeks (Carmona and Zfirate 1992, Carmona 1993). The nestswithin the coloniesof La Paz Bay are scattered(Carmona and Zfirate 1992), as has been observedin the Yellow-lootedGull coloniesof the centralGulf of California(Hand 1980, Hand et al. 1981). It seemsthat thisscattered pattern is necessitatedby thermalregulation (Hand 1980, Hand et al. 1981, Carmona 1993). Velardeand Anderson(1993) reported1200 nestsonly for EspirituSanto Island alone,but thiswas a typographicerror; in fact, the numbershould be 120 nests(D. Andersonpers. comm.). LeastTern ($terna antillarurn). From 1985 to 1987 an importantcolony (>100 pairs)settled on Las Afeguas(Palacios 1988). Other smallcolonies (one to ten nests) have been found within the Ensendade La Paz (Palacios1988, Mendoza 1994). From 1989 to 1992 pairstried to neston LasAfeguas, but reproductionfailed in all thoseyears: 1989, 80-90 nests;1990, 15-20 nests;1991, 40-50 nests;1992, less than 10 nests(Mendoza 1994). Accordingto Mendoza(1994) the causesof failure 154 WATERBIRDS OF LA PAZ BAY were humandisturbance, depredation, and tide inundations.This speciesnests from May to July. Osprey(Pandion haliaetus). Ospreys have nested on EspirituSanto Island at least since 1984, with 3 nestsin