Behavior of Late-Nesting Black Skimmers at Salton Sea, California

WESTERN BIRDS

Volume 7, Number 3, 1976

BEHAVIOR OF LATE-NESTING BLACK SKIMMERS AT SALTON SEA, CALIFORNIA

GILBERT S. GRANT and NORMAN HOGG, Department of Biology, University of California, Los Angeles,California 90024

The Black Skimmer (Rynchopsniger) was first found nestingat the south end of Salton Sea, Imperial County, California in 1972 (McCaskie et al. 1974). Five nests were found in 1972, three in 1973 (McCaskie 1973), and three in 1974 (McCaskie1974). Nestinghad been suspected at the north end of the sea,but wasnot confirmeduntil 16 August1975 when the authors,Sandra Grant and JeanneHogg discovered eight nests on a small barnacle-covered island in the northeast corner of the sea. Biweekly visits were made to the island and several aspectsof their breedingbehavior are reported here.

RES ULTS

Nests. When first discovered, the 20 m x 40 m island was about 15-20 cm abovewater level. It is doubtful that nestingcould haveoc- curredthere in June and early July (all previousegg dates of the skimmer at Salton Sea fall within this time period) as the islandwas probably submergedthen. Evaporationexceeds irrigation effluent in late June-July and the sea level falls from then until about December. The nestingsite was about 50 m southeastof the end of Grant Street drain in the WhitewaterCove area of Salton Sea, RiversideCounty. The un- vegetatedisland was made primarily of Acorn Barnacle (Balanusam- phitrite) shellswith a few shellsof a freshwatersnail (Helisoma sp.). Eight nestswith eggswere found on 16 Augustand by 30 Augustfive more nests had been added (Table 1). On these early visits several empty scrapeswere found. The meandepth of the 13 activenests was 64 mm (range40-90 mm) and the mean diameterwas 296 mm (range260-400 mm). No lining, other than the seashellspresent, was added. Nestswere placedalong the highercontours of the islandand proximityto the nearestneighbor Western Birds 7:73-80, 1976 73 BLACK SKIMMERS

Table1. Populationcounts and nesting data of BlackSkimmers at the northend of the Salton Sea in 1975. Scrapes Eggs Total Flying Nest- with Eggsin Out of Eggsand Adults Juv. lings Eggs ScrapesScrapes lff•stlings 16 Aug 37 1 0 8 14 0 14 30 Aug 39 2 0 13 38 4 42 13 Sep 48 1 8 -- 17 101 35 27Sep 46 4 82 - • 198/ 27 1. Includes2 eggspunctured probably by heron or gull 2. Includes I chick (1 day old) dead near nest 3. Includes 3 eggspunctured by skimmers seemedhaphazard; mean distance was 242 cm (range135-457 cm). The distancefrom the neststo the water rangedfrom I m to about 10 m. A behavior in skimmers was observed that seems similar to the "choking behavior"illustrated by Tinbergen(1960:59, 60, 76) for the Herring Gull (Larusargentatus) and the LaughingGull (Lo atricilla). The male and female skimmersstanding near a scrapeoften faced each other, pointed the bill nearly vertically downwardand rhythmically moved the headup and down. From our observationpoint, we wereunable to distinguishany vocalizationsassociated with this behavior. This behavior was observedin severalpairs of skimmers,but choking wasnot observedin more than one pair at a time. Choking preceededseveral episodesof diggingby one pairof skimmersat the siteof an unfinished scrapeon 16 August. Skimmersformed the nest scrapeby lowering the breast,raising the tail, and kicking barnacleshells outward with the feet. After severalkicks the bird movedto another positionwithin the scrapeand kicked outward again. To a casualobserver it mightbe assumedthat a scrape is formed by slowly spinningaround on the breast feathers but this would cause excessive abrasion of the feathers (Tomkins 1933 ). Eggs. The mean clutchsize of the 13 nestswas 2.9 (range1-4 eggs). Eggsfound outside of scrapes(4 on 30 August;Table 1) werenot includ- ed in this calculation. They may have comefrom someof the 13 nests, from an empty scrape,or laid inadvertentlyaway from the bird'sscrape. Preston(1962) reporteda skimmerlaying an eggaboard his boat while it was strandedon a bar near a nestingcolony of skimmersand terns. We measuredand weighed38 eggson 30 August. The meanlength was 45.4 mm (range40.5-50.0 mm), meanwidth was34.1 mm (range31.5- 36.0 mm), and the mean weight was 26.8 g (range 22.0-32.5 g). Egg weightswere not all freshweights as 14 eggswere known to be at least 14 days old. Unfortunately, we had no meansof numberingeggs and nestsnor weighingeggs on our first visit. Eggdimensions agree well with those reported by Bent (1921) 74 BLACK SKIMMERS

Between13 and 27 Septemberskimmers evidently punctured three eggswith their knife-likebills (Figure 1). Theseeggs were outsideof scrapeswhen found and it is not known whether nestingor non-nesting birds were responsible. Inside one puncturedegg was a well-formed embryo. Eggswere displacedfrom nestsof late-nestingskimmers in Louisiana (Chamberlain 1959), but it was not indicated whether any eggs had been punctured. Bongiorno(1968) found LaughingGulls punctur- ing eggsof their own specieswhen their nestshad been displacedby a storm. Incubation. Both sexes incubate, but much variation occurred. It was hot (34-38øC) and nest relief occurred every 3-4 minutes when we first discoveredthe colony on 16 August. Our initial disturbancemay have causedthe rapid nest relief. It was at this time that we observed "foot-wetting" (Turner and Gerhart 1971) and other coolingmechan- isms(to be discussedmore fully elsewhere).Three nestswere watched closelyon 30 Augustfrom 1543 to 1820 to determinethe amount of incubation by each sex (Table 2). Tomkins (1933) and Bent (1921) felt that femalesdid all of the incubatingand Pettingill(1937) felt these choreswere sharedequally. Modha and Coe (1969) reported that in the AfricanSkimmer (Rynchopsflavirostr•s), the parentstook turns incubating. Sexescan be determinedin the field bv sizedifferences. The male appearslarger both on the ground and in the air (when both sexesare

Figure 1. Punctured egg of Black Skimmer at Salton Sea, California. Photo by G. S. Grant 75 BLACK SKIMMERS

Table 2. Percentageof incubationperformed by eachsex from 1543 to 1820 on 30 August 1975. Male Female Uncovered Nest No. 6 (4 eggs) 6.5 86.1 7.4 NestNo. 12 (3 eggs) 38.9 53.5 7.6 Nest No. 13 (3 eggs) 19.1 71.3 9.6 presentfor criticalcomparison). The maximuminterval of uncovered eggswas 3 minutes. Duringthe heat of the day a shortinterval of disturbancemay provelethal for the eggs.To minimizedisturbance most of our observations were made from a boat anchored offshore and ex- amination of eggsand young was limited to early morningand late afternoon visits. Incubatingbirds orientedinto the wind, almostwithout exception, but in still air orientation with respectto other skimmerswas random. From 0715 to 1100 on 27 Septemberall skimmerswere facinginto a NW wind (tail to sun) regardlessof whetherincubating, brooding, or in a restingflock. From 1100 to 1 130 no wind occurredand bird posi- tions were random. At 1130 the wind came from the SE and all birds faced into the wind and sun. Our observationposition was unchanged duringthis time interval. Wind flow overthe feathersurface facilitates heat lossby convection.Ruffled feathers and orientation into the wind were important behavioraladjustments for desertnesting Gray Gulls (Lar•s modestus) in Chile (Howell et al. 1974).

Fibre 2. Some heat lossmechanisms of the Black Skimmer. Note panting;elevated crown, nape, and scapularfeathers; shadingof legsand feet; and slightly droopedand outwarddisplaced win•. Pboto by G. S. Grant 76 BLACK SKIMMERS

Panting,elevated crown, nape, and scapular feathers, slightly drooped and outward displacedwings, and shadingof feet and legs were fre- quently used heat loss mechanisms(Figure 2) of nestingskimmers in this stressfulenvironment. Shadetemperatures above 40øC and ground temperaturesabove 50 øC are often encounteredhere. Nestlings. Intensesolar radiation dictatesegg coverageduring the egg-layingstage and asynchronoushatching would be expected. Evenif the eggswere only shadedduring the egg-layingstage, some develop- ment within the eggwould probablyoccur because of the high ambient temperaturesencountered here. One pair brooded three chicksand there was at least 24 hours differencein the ageof eachone. Tomkins (1933) also reported incubationbeginning with the first egglaid and chickshatching a day or so apart. Measurementsand weightswere ob- tained on all chickscaptured. The upper and lower mandiblesare about the samelength at hatchingand in chicksweighing up to 300 g only 1 mm difference was detected. In the largestchick (340.5 g), the lower mandible extended 3 mm beyond the upper. This individual was near fledgingage. Unevenbill lengthswould handicap the feedingability of chicks,especially if the food item fell to the groundduring the exchange from the parent'sbill to the chick, as alsosuggested by Bent (1921) and Modha and Coe (1969). A rapid growth of the lower mandibleoccurs at fledging or shortly afterward as the bill lengths of flying juveniles that we saw appearedsimilar to those of the adults. We have no data on the extent of parentalfeeding during this intermediatestage. Feed- ing of the young was not observedduring our visits,but discardedSail- fin Mollies (Peocilialatit•inna) were found in some of the scrapes.Skim- mers feed primarily at dusk and at night and presumablythe young are not fed during the day at Salton Sea. Four chicks were banded on 13 September and four more on 27 Septemberwith the hope of learningsomething about their wintering area. On 27 Septemberwe recapturedtwo of the previouslybanded chicks,but the other two could not be found. One chickand two eggs were lost to avian predatorsand 15 other eggsand/or young were not accountedfor between30 Augustand 27 September(Table 1). A chick, about one day old, that wasfound deadon 27 September(specimen to UCLA) had a gapinghole at the baseof its neck which was probably inflicted by a heron or gull. Skimmerswere observed feeding in the WhitewaterRiver delta region of Salton Sea as late as 13 October. Behavior'toward nest markers. Sixteennumbered, wooden tongue depressors(152 mmx 19 mm x 2 ram), with red paint appliedto the top 20 ram, were placed about 15 cm from the nest scrapeson 30 August. Almost immediatelysome birds began actively removing these markers,and by 27 Septemberonly one marker of the original 16 re- mained upright. Sevenothers were scatteredin the vicinity of nests 77 BLACK SKIMMERS and one of these had been displacedover 5 m. The remainingeight were no longer present on the island. To investigatecolor responses we placed five variouslycolored tongue depressorsfrom 15 cm to 1 m from a nestscrape (one eggand two chicksin scrape)on 13 September. Within one hour the pair (male in all the caseswhere sex wasdeter- mined) had knocked down or flown off with all markers in the follow- ing orderand approximatedistance placed from the nest:black (30 cm), yellow (30 cm), green(15 cm), blue (1 m), red (65 cm). The vertical componentof the marker was not the solestimulus as they repeatedly attempted to lift markerslying flat on the groundnear the nest. We attemptedto duplicatethis experimenton 27 Septemberbut the birds did not respondto markersplaced near.the nests. Adult attentions were directed toward the more mobile chicks rather than to the nest sites on this date. With our limited data it would be premature to speculateon the significanceof this behavior.

DISCUSSION

Skimmers characteristicallynest later than most terns and gulls on the Atlantic seaboard,with most egg dates coming from mid-May to late July (Bent 1921). The African Skimmer (Britton and Brown 1974) breeds mainly in April-May at Lake Rudolf, Kenya, but elsewherein East Africa they breed on exposedsandbanks of riverswhere egg-laying beginsin June but may continue into Septemberduring the dry season (Attwell 1959). Falling water and exposureof sandbankshas been sug- gestedas the nestingstimulus for the Black Skimmer along the Amazon River (Preston 1962). Late-nestingskimmers were studied in Louisiana where the majority of eggsdid not hatch until 18 August (Chamberlain 1959). Chamberlainsuggested the late nestingwas probably due to a secondnesting effort. At the south end of Salton Sea, Black Skimmers successfullyraised one young by mid-August 1975 (McCaskie 1975). Any nestsplaced in the drift or high water line are subjectedto flooding by rising water levels until late June-July or by wind-drivenwater at any time during the summer. Skimmersnesting at the north end were probably unsuccessfulin an earlier nestingattempt in 1975 along the shoreline of the sea. Nesting successof this speciesmight be enhancedby constructing small islandsthat remain above June and July water levelsat both ends of the sea. Islandskept relatively clear of vegetationand at a suitable •istancefrom landto preventpredator and human access should prove acceptableto skimmersseeking nest sitesin June. Nest sitesavailable early in the seasonwould allow nestingrelatively free of gull predation. Gull numbersincreased sharply after the first week in July 1975 at the north end of the sea.

78 BLACK SKIMMERS

It is interestingto speculateon this suddeninflux of skimmersinto California. One wonders about habitat destruction in the Gulf of Cali- fornia,persecution by man,and change in foodavailability. Two breeding coloniesin Sonoraand severalcolonies in Sinaloa(Friedmann et al. 1950) are the only recordednesting locations for northwesternMexico. About three-fourthsof the annualprecipitation in Sonoraand northern Sinaloafalls betweenJune and September,mainly in July and August (Anon. 1965). The first summer record of the Black Skimmer for Californiaoccurred on 3 July 1968 (McCaskieand Suffel 1971). Per- hapsunusually heavy rains destroyed skimmer nests in northwestern Mexico in 1968 and were responsiblefor the 1968 skimmerinflux into Salton Sea. Rainfall for Guaymas,Sonora in July 1968 exceededthe mean July precipitationby a factor of 2.5 (Anon. 1965, 1968). How- ever,June 1968 had only a trace and we do not know how muchrain occurredbetween I and 3 July 1968.

ACKNOWLEDGMENTS

The authors thank Sandra Grant for her astute observations first calling attention to the possibility of nestingand for typing several drafts of this manuscript,Jeanne Hogg for aid in the field, and Kimball Garrett and Thomas R. Howell for comments on an early draft of this paper.

LITERATURE CITED

Anonymous. 1965. World weather records 1951-1960, vol. 1. North America. U.S. Dept. Commerce,Gov. PrintingOffice, Washington,D.C. Anonymous. 1968. Monthly climatic data for the world. Vol. 21, No. 7. U.S. Dept. Commerce, Gov. Printing Office, Washington,D.C. Attwell, R. I. G. 1959. The African Skimmer Rbyncbops(sic):•avirostris: popu- lation countsand breedingin the Nsefu Game Reserve. Ostrich 30:69-72. Bent, A. C. 1921. Life histories of North American gulls and terns. U.S. Natl. Mus. Bull. 113. Bongiorno,S. F. 1968. Eggpuncturing behavior in LaughingGulls. Auk 85:697- 699. Britton, P. L. and L. H. Brown. 1974. The status and breeding behaviour of East African Lari. Ostrich 45:63-82. Chamberlain,J. L. 1959. A nestingcolony of Forster'sTerns and Black Skim- mers in southwestern Louisiana. Wilson Bull. 71:283-284. Friedmann, H., L. Griscom and R. T. Moore. 1950. Distributional check-list of the birds of Mexico, part 1. Pac.Coast Avif. No. 29. Howell, T. R., B. Araya and W. R. Millie. 1974. Breedingbiology of the Gray Gull, Larus modestus. Univ. Cal;•f.Publ. Zool. 104:1-57. McCaskie, G. 1973. The nesting season. Southern Pacific Coast region. Am. Birds 27:917-920. McCaskie, G. 1974. Ibid. 28:948-951. 79 BLACK SKIMMERS

McCaskie, G. 1975. Ibid. 29:1029-1036. McCaskie, G., S. Liston and W. A. Rapley. 1974. First nesting of Black Skim- mers in California. Condor 76:337-338. McCaskie, G. and S. Suffel. 1971. Black Skimmers at the Salton Sea, California. Calif. Birds 2:69-71. Modha, M. L. and M. J. Coe. 1969. Notes on the breedingof the African Skim- mer Ryncbopsflavirostris on Central Island, Lake Rudolf. Ibis 111: 593-598. Pettingill, O. S. Jr. 1937. Behavior of Black Skimmers at Cardwell Island, Vir- ginia. Auk 54:237-244. Preston,F. W. 1962. A nestingof Amazonianterns and skimmers. WilsonBull. 74:286-287. Tinbergen, N. 1960. The Herring Gull's world. Harper & Row, New York. Tomkins, I. R. 1933. Waysof the Black Skimmer. WilsonBull. 45:147-151. Turner, D. A. and J. Gerhart. 1971. 'Foot-wetting' by incubating African Skim- mersRyncbops flavirostris. Ibis 113:244.