J. Field Ornithol., 64(3):341-345 ERYTHRISTIC EGGS IN THE COMMON TERN HELEN HAYS American Museum of Natural History Central Park West at 79th St. New York, New York 1002d USA KENNETH C. PARKES CarnegieMuseum of Natural History ddO0 Forbes Ave. Pittsburgh,Pennsylvania 15213 USA Abstract.--Erythrism, an abnormallyreddish color, is known in the eggsof a few Laridae and Stercorariidae.It apparentlyresults from the absenceof one of the normal pigments and possiblyan overdoseof another.Erythrism in eggsof the CommonTern (Sternahirundo) is exceedinglyrare. Data from the long-termstudy of the large tern colonyon Great Gull Island,New York, provideaccurate information about relative frequency of the phenomenon and demonstrateits continuityin two femaleswith changingmates. ERITRISMO EN HUEVOS DE STERNA HIRUNDO Sinopsis.--Seha informadoeritrismo (coloraci6nrojiza anormal) en los huevosde algunas especiesde Laridae y Stercorariidae.Esta condici6nparece ser el resultadode la ausencia de uno de los pigmentosnormales y la sobreproducci6nde otro. E1 eritrismoes sumamente raro en huevosde Sternahirundo. Datos tomadosde un estudioa largo alcanceen una colonia de estasgaviotas en Great Gull Island, New York, ha provistode informaci0nprecisa sobre la frecuenciade estefen0meno y ha demonstradola continuidaddel mismoen doshembras que cambiaron de pareja. Erythristic eggsare known in one speciesof jaeger (Stercorariuspar- asiticus),several species of gulls (Larus), and two speciesof terns (Sterna) (Bates et al. 1976, Jourdain and Borrer 1914, Sutherland1980). Bates et al. explain the phenomenonas follows: "No more than two kinds of pigment are present in the eggshellsof most birds ..., and often one alone or practically none. One of thesepigments is blue or greenishand when presentoccurs throughout the shell structure.The other is brownish, ranging from red to black, and when presentis usuallynear the surface. A small amountmakes a white shellyellowish or pale brown and a blue shell greenor olive. The brownishpigment also produces any markings that are presentwhich may occurat variousdepths as the shellis formed, varyingin shadeaccording to their depthand the shellcolour... erythrism may be defined as having the reddish colour exaggeratedor abnormally replacing the bluish pigment." At least one early writer erroneously attributedthe reddishcolor of sucheggs to their being "stained... with blood-pigment" (Borrer 1913). Erythristic larid eggsare rare in museumcollections. Given the assid- uousnessof egg collectorsin the past, the recordsthat exist, as reviewed by Jourdain and Borrer (1914) and Bateset al. (1976), probablygive at least an approximationof the frequencyof occurrenceof this color ab- normality in this family. Publishedrecords of erythristiceggs of larids 341 342] H. Haysand K. C. Parkes J.Field Ornithol. Summer 1993 are basedon isolatedsets collected at widely scatteredlocalities as well as, in a few instances,collections made over a period of years in areasat which red eggswere found on a regular basis.For example,in the last centuryand the early part of this century,erythristic eggs of five larids werecollected at or near two localitiesalong the northerncoast of Norway (Bateset al. 1976, Jourdain and Borrer 1914). Theseincluded a number of setsof erythristicgull eggs,most of which were thoseof Herring Gull (Larusargentatus), but alsosome of Great Black-backedGull (L. marinus). At one localityin England, red eggsof CommonTern (Sternahirundo) were found everyseason for five or six years.Borrer (1913) believedthat "a strain of birds had establisheditself in this colonywhose tendency it was to lay" red eggs,as he did not believethat the samepair of birds could have survivedand returnedfor five or six yearsto the samespot, but he was obviouslyunaware of the great longevityof CommonTerns. Bateset al. (1976), noting severalinstances in which setsof red eggsof a particular specieswere found in successiveyears at the same site, suggestedthat the samefemales might be responsiblefor thesesuccessive clutches. There are fewer recordsof erythristic eggs for terns than for gulls. Jourdainand Borrer (1914) and Bateset al. (1976) reporteda few records of erythristicsets of eggsfor Commonand Arctic (S. paradisaea)terns in Britain and Europe.A mail surveyof the institutionslisted by Bankset al. (1973) as havingthe largestegg collections in North America yielded only two erythristicsets of CommonTern eggsfrom Great Britain and one from North America. The latter, now in the San BernardinoCounty Museum, California, was collectedon 13 Jun. 1919 at North Nauset Beach,Chatham, Massachusetts.As abnormallypigmented eggs would undoubtedlyhave been selectively taken by collectors,as suggestedby the series of gull eggs from the two Norwegian localities,the paucity of erythristicCommon Tern setsin collectionsmay be taken as an indication of their rarity. Since1966, the tern colonyon Great Gull Island, Long Island Sound, New York (41ø12'N, 72ø07'W), has been intensivelystudied (Cooper et al. 1970, DiCostanzo 1980). During the breedingseason, a team of field assistantsmakes a daily searchof the 6.9 ha island, marking new nests and individual eggs,and banding chickson the day of hatching. A few well concealednests may be missedon thesedaily checks,but the number of these,we feel,is not significant.The numberof nests(including renests) markedper year has risen from about3000 in 1969-1982 to about6000 in 1984-1988 and 7000-9000 in 1989-1991. The thoroughnessof this surveypermits accurateestimates of the frequencyof occurrenceof any variants,such as unusual egg pigmentation. The eggsof the CommonTern are highly variablein color.The ground colorranges from off-whitethrough various shades of buff, gray, and tan to a rich olive-brown,blotched with black,gray and brown. Albino eggs are known (example,Carnegie Museum of Natural History set E-6423, two normal and one albino egg from Starve Island, Lake Erie, Ontario, Vol.64, No. 3 ErythristicEggs in theCommon Tern [343 26 Jun. 1916). Unmarked pale blue eggsare of regular occurrenceon Great Gull Island, usually one or two per year. As in the caseof the albino egg mentionedabove, the pale blue eggsare usually part of an otherwise normal clutch. One female, however, is known to have laid nothing but clutchesof blue eggsfor four or five successiveyears even though shechanged mates. This suggeststhat the female determinesthe colorand pattern of the eggs.Bates et al. (1976) notedthat femalestend to be consistentin the color and pattern of their eggsthroughout their lives, and suggesteda genetic basis for abnormal color in eggs. Our observationsof successiveclutches of blue eggslaid by one female are consistentwith this hypothesis. On 4 Jul. 1975, Parkes found a Common Tern nest on Great Gull Island containingone erythristicegg. The next day the nest containeda secondegg, also erythristic.The eggswere laid in a scrapein weathered concrete,with no vegetation or other lining. The parents, which were never individually identified, eventuallydeserted this nest;the eggswere collectedon 20 Jul. and taken to the American Museum of Natural History, where they were preparedby Henry Pelzl and cataloguedas set 17917. Theseeggs are somewhatreminiscent of falconeggs in appearance. The backgroundis betweencolors 5 (Flesh Color) and 6 (SalmonColor) of Smithe (1975), but paler, and the spotsmatch color 132A (Brick Red) of Smithe(1981). There are no blackishor olivaceousspots, nor is there any olivaceoustone at all in the backgroundcolor. The first egg measured41.14 x 28.42 mm, the second40.09 x 28.04 mm. These measurementsare within thoseof the 82 eggsin the United StatesNational Museum measuredby Bent (1921): length35.5-45, width 27.5-32.5. No tern eggsof this particular color abnormalitywere found subse- quently on Great Gull Island until the summerof 1987, when two such setswere found. The two eggsof the first set (site A) were laid on 17 and 19 Jun., and thoseof the secondset (site B) on 6 and 7 Jul. Although there were slight color differencesamong the eggs,they all conformedto the descriptionabove. We trapped the adults at both sites and subsequentlytrapped birds associatedwith red eggs at these sites each year, with one exception, through 1991. Both of the parentstrapped in 1987 at siteA had hatched in 1984. Subsequentlythe female of this pair, sexedon the basisof bill length comparisonwith its mate (Coulter 1986), paired in 1988 with a bird trapped as an adult on Great Gull Island in 1983. In 1989 she paired with a bird hatched in 1984, remaining with this mate in 1990 and 1991. In 1989 and 1990 this pair had red eggs,but in 1991 they were trapped on a late nest, and the egg in the nest was brown, not red. The 1987 and 1988 mates of the site A female nested with other females subsequently.The eggsin their nestswere normal in appearance. The parents trapped in 1987 at site B were a two-year-old (hatched 1985) and a bird trapped as an adult in 1985. These birds remained togetherand were trapped on red eggseach year through 1990. In 1991 344] H. Haysand K. C.Parkes J.Field Ornithol. Summer 1993 we trapped the 1985-hatchedbird of the pair with a new mate, one that was hatchedin 1984, on two red eggs.Bill measurementsof the pair at site B in 1987 differed by only one millimeter, so we did not attempt to sex the membersof this pair. In 1990 and 1991 we trapped three adults that had hatched from erythristiceggs. Two had hatchedat site A, one in 1988 and one in 1989. On the basisof bill measurementthey both appearedto be females.The third hatched