Occurrence of Supernormal Clutches in the Laridae

Wilson Bull., 96(2), 1984, pp. 249-267 OCCURRENCE OF SUPERNORMAL THE LARIDAE CLUTCHES IN MICHAEL R. CONOVER In recent years, female-female pairings have been discovered in Western Gulls (Larus occidentalis)(Hunt and Hunt 1977), Ring-billed Gulls (L. delawurensis)(Conover et al. 1979, Ryder and Somppi 1979), California Gulls (L. culijbrnicus) (Conover et al. 1979), and Herring Gulls (L. argentatus) (Fitch 1979). These associations occur when two females pair together rather than with male mates and both lay eggs in a mutual nest. It has yet to be determined whether female pairs are restricted to a small number of gull species or are widespread among the normally monogamous Laridae. Supporting the latter possibility is the recent discovery of female pairs among breeding Caspian Terns (Sterna cuspia) (Conover 1983). Such female pairs are often identifiable because their nests contain supernormal clutches (4-6 eggs), double the normal clutch-size. Not all supernormal clutches (SNCs), however, result from female pairings. In a study of Ring-billed Gulls, 17% of the 5-6 egg clutches resulted from polygynous associations in which two or more females lay eggs in the same nest (Conover, in press). Furthermore, a polygynous group was found attending a SNC in the Brown Skua (Catharucta skua) (Bonner 1964). Hence, the presence of SNCs in a particular species may indicate that either female pairings or polygynous associations occur in that species. This is not necessarily the case, however, for a few SNCs may also result from adults rolling a foreign egg into their nest or from nest parasitism although normally neither event should cause a doubling of clutch-size, except for one-egg clutches. In this study, I used clutch-size data to estimate the frequency of SNCs in different gull and tern species. Such information is useful in identifying those species in which female pairings or polygyny may occur. These data were also used to test the hypothesis that DDT contamination produces female pairings by feminizing male embryos (Fry and Toone 198 1). If this is so, the frequency of female pairs and consequently SNCs should have increased since the late 1940s when DDT’s widespread usage began. METHODS Dejinition of supernormalclutch. -Supernormal clutch,an impreciseterm, has historically signifiedan unusuallylargeclutch. This term has been applied mostly to birds which usually lay three-eggclutches;for these birds, most authors agree that clutchescontaining five or more eggsare supernormal but disagree on the inclusion of four-egg clutches. Hunt and 249 250 THE WILSON BULLETIN l Vol. 96, No. 2, June 1984 Hunt ( 1977) and Conover et al. (1979) have included them while Ryder and Somppi ( 1979) and Shugart (1980) have not. Therefore, a more precise definition is necessary,especially for studiesof other specieswhich do not lay a three-eggclutch. Bonner (1964) considered a three-eggclutch to be supernormalin the Brown Skua, a specieswhich lays two eggs. I usethe following definition: a supernormalclutch exceedsthe modal clutch-sizeby more than 50% with the exception that a two-egg clutch shall only be consideredsupernormalif 90% of the individual females lay a single egg. Hence, if the modal clutch-size contains 2, 3, or 4 eggs,then SNCs must contain at least 4, 5, or 7 eggs,respectively. Data collection procedures. -Clutch-size data used in this study came from two sources. First, data were gatheredfrom most of the major museum eggcollectionsin North America. For each egg set examined, I recorded the clutch-size, species,year, and collection site. While either I or my techniciansexamined most eggsets,someadditional data were furnished by the staff of museums which I was unable to visit. Data from egg collections represent pre-DDT conditions becausefew egg setshave been collected since 1940. The remainingdata on clutch-sizefrequenciesfor gullsand ternswere obtainedby literature search. Excluded from this analysis were some reports in which: (1) fewer than 50 nests were sampled in those specieswhere sample sizes were already so large that such studies added little; (2) the number of nests of each clutch-size was not given or was estimated rather than counted directly; (3) nests were not randomly selected or for some were not representativeof that speciesdue to some unusual situation (such as studiesprompted by an unusualevent like birds nesting out of seasonor after a natural disaster);(4) the count was made either before most of the clutcheswere completed or after many of the eggshad hatched; or (5) the reliability of the data was uncertain (i.e., misidentified species,mathematical errors,etc.). Reports, however, that containedsmall inconsistenciesin the data (such astotalsnot addingup exactly) wereincluded if the error had little effecton SNC frequencies. When reports gave sequentialclutch-sizedata from the same colonies, I selecteddata from the day when the highest proportion of the clutches was complete or when the greatest number of nestswas counted, providing that this day did not occur after many of the eggs had hatched. To determine whether SNCs have increasedin frequency since DDT’s widespread use, all clutch-sizedata in the post- 1950 literature and post- 1950 breedingreportsfrom the Royal Ontario Museum were compared to that in the pre- 1940 literature and to the museum data using a Chi-square test corrected for continuity. If this test proved invalid for a particular species,due to a low expected frequency, the Fisher-exact probability test was used. Data collectedduring the 1940swere usually excluded, as DDT wasjust coming into use during this decade; this exclusion had little effect on the results becauseonly a small number of studieswere conductedduring this period. RESULTS SNCs in gulls.-Evidence of supernormal clutches of five or more eggs was found in both museum egg collections and in the literature (Appendix) for seven gull species: Laughing (Larus atricilla), Common Black-headed (L. ridibundus), Mew (L. canus), Ring-billed, California, Herring, and Western gulls. In three additional species, the Glaucous-winged (L. gluucescens), Glaucous (L. hyperboreus),and Great Black-backed (L. marinus) gulls, some SNCs were found in the museum egg collections though not in the literature, owing to a lack of published reports on the clutch-size distribution for the latter two species. Furthermore, SNCs consisting of four eggs were reported in the Black-billed Gull (L. bullen], Silver Gull Conover l SUPERNORMAL CLUTCHES 251 (L. novaehollandiae),and Black-legged Kittiwake (Rissa tridactyla), species which predominantly lay two-egg clutches. Likewise, some two-egg SNCs have been found in the Swallow-tailed Gull (Creagrusfurcata), a species that usually lays only one egg. No SNCs were found in either the literature reports or egg collections for Lesser Black-backed (L. fiscus), Kelp (L. dominicanus), and Sabine’s (Xema sabinz]gulls. In egg collections, SNCs were most common in Ring-billed (4.0% of all clutches), California (2.7%) and Laughing (1.9%) gulls. In the pre- 1940 literature, SNCs were most frequent in Laughing (1 .O%) and Ring-billed (0.5%) gulls, and in the post- 1950 literature were most common in Ringbilled (1.9%), Western (1.8%) and Herring (0.3%) gulls. SNCs in terns.-SNCs occurred in almost all tern species (Appendix). In both the museum egg collections and in the literature, SNCs of five or more eggs were found for the Gull-billed (Sterna nilotica), Common (S. hirundo), Roseate (S. dougalliz],and Least (S. antillarum) terns. Furthermore, 5-6 egg clutches occurred in museum egg sets of the Forster’s Tern (S. forsteri) although no literature reports on clutch-size distribution were found for this bird. In contrast, no SNCs were found in the egg collections of Black Terns (Chlidonias niger) but SNCs were reported in the nesting data collected by the Royal Ontario Museum. In those tern species where two eggs formed the most common clutchsize-the Caspian, Arctic (S. paradisaea), Sandwich (S. sandvicensis), and Least terns- four-egg clutches (which are considered supernormal) were found in all species. Likewise, one-egg clutches predominate in the Royal (S. maxima), Elegant (S. elegans),White-fronted (S. striata), Black-naped (S. sumatrana), Sooty (S. fuscata) terns and the Brown Noddy (Anous stolidus);two-egg supernormal clutches have been reported in all of these species but the White-fronted Tern. Hence, SNCs occurred in all terns which were examined, except this last species. In the egg collections, SNCs were most common in the Royal Tern (38.8%), Sandwich Tern (6.7%), Roseate Tern (3.7%), and Brown Noddy (3.4%). In the pre- 1940 literature, SNCs were most common in the Least (7.1 O/o),Roseate (1.9%), Royal (1.4%), Caspian (0.9%), and Sandwich (0.6%) terns. Since 1950, however, the highest frequencies of SNCs have been reported in the Sooty (12.3%), Elegant (6.7%) Caspian (2. lo/o), and Royal (1.7%) terns. Change in SNC frequenciessince 1950. -Based on pre- 1940 and post1950 literature reports, SNC frequencies have decreased significantly since 1950 in the Sandwich (x2 = 5.63, P < 0.05) Roseate (x2 = 9.88, P < 0.01) and Least (P < 0.05, Fisher test) terns. In contrast, SNCs have significantly increased in post- 1950 literature reports in the Caspian Terns nestingintheU.S.(~~ = 5.73, P < 0.05)andinHerringGulls(x2 = 23.71, P < 0.0 1). Surprisingly, all 43 of the reported SNCs in Herring Gulls since 252 THE WILSON BULLETIN l Vol. 96, No. 2, June 1984 1950 have been found in the Great Lakes and Ontario; 0.3% of the over 15,000 nests surveyed in this region since 1950 consisted of SNCs. Prior to 1940, no Herring Gull SNCs were found among the 72 museum egg sets collected from the Great Lakes or the 777 nests from that area reported in the literature. Comparing the post- 1950 literature with the pre- 1940 egg collections revealed a higher SNC frequency in the egg collections for most gull and tern species, a result which was not surprising since many egg collectors did not collect clutches randomly. Nonetheless, in one species, the Western Gull, the SNC frequency was significantly higher in the post-1950 literature than in the egg collections (P < 0.0 1, Fisher test). DISCUSSION Supernormal clutches occurred in most of the gull and tern species examined, albeit in low frequencies. SNCs were found in both literature reports and egg collections for many species, thus providing independent confirmation of their occurrence. These two sources of clutch-size data, however, pose potential problems which necessitate caution in using the results of this study to estimate SNC frequencies for many of these species. For example, in the literature reports, the paucity of clutch-size studies for most species results in a high proportion of data coming from a small area and for only a few breeding seasons. Given possible geographic and yearly variation in intraspecific clutch-size, such data may not be representative of the species as a whole. Moreover, SNC frequencies may change between the early and late part of the incubation period causing variability among studies if the eggs were counted at different times (Ryder and Somppi 1979; Conover, in press). I tried to minimize this latter problem by excluding those studies known to be conducted either early in the incubation period or when many of the eggs had already hatched. While these procedures may minimize the problem, the potential bias remains and may affect interspecific comparisons of clutch-sizes based on literature reports. Problems with clutch-size data from museum egg collections are somewhat different. Here, hundreds of egg collectors have gathered eggs from numerous locations during the last century, minimizing the bias of singlesource data. Egg collection data are probably more representative of each species as a whole, allowing for more accurate interspecific comparisons of clutch-size and SNC frequencies. However, SNCs are probably overrepresented in the egg collection data, for many collectors were not concerned with randomly sampling clutches in a colony and, instead, sought completed and unusual clutches. Consequently, SNC frequencies for any given species cannot be accurately determined from egg collections, and Conover l SUPERNORMAL CLUTCHES 253 comparisons between pre- 1940 egg collections and the post-1950 literature can be used only to document substantial increases but not decreases in SNC frequencies since 1950. Origin of SNCs. -Many authors have speculated on the origin of SNCs. Some have felt these clutches are the work of a single female (Glegg 1925, Marples and Marples 1934, Robinson 1934b). Most authors, however, have suggested that these large clutches occur because eggs from different fernares end up in the same nest, basing this hypothesis on both the large clutch-sizes and the unusual variation in egg coloration and marking patterns within some SNCs (Jones 1906, Willett 19 19, Bancroft 1927, Munro 1936, Beretzk 1957, Smith 1975, Conover et al. 1979, Penland 1981). Some authors have hypothesized that these large clutches were created when a foreign egg fell or was rolled into a nest, a phenomenon which has been demonstrated by marking eggs in only the Mew Gull (Trubridge 1980), Caspian Tern (Penland 198 l), and Sooty Tern (Brown 1975). While some SNCs undoubtedly result from the above and similar occurrenceseggs falling into nests, being rolled in by the incubating birds, or from nest parasitism-in most cases these events should only increase the original clutch-size by one egg and hence not create a SNC, except in species that normally lay one-egg clutches. Another explanation for SNCs was proposed by Hunt and Hunt (1977) when they showed that most SNCs in Western Gulls were produced by female-female pairings. Other authors have supported this conclusion, finding female-female pairs in Ring-billed Gulls (Ryder and Somppi 1979, Conover et al. 1979) California Gulls (Conover et al. 1979) Herring Gulls (Fitch 1979) Black-legged Kittiwakes (K. Chardine, pers. comm.), and Caspian Terns (Conover 1983). Some SNCs also result from polygyny in the Ring-billed Gull (Conover et al. 1979; Conover, in press), and in the Sandwich Tern (Smith 1975). Further, Nethersole-Thompson (1946) observed three adults attending a four-egg clutch in Mew Gulls; unfortunately the birds were not sexed, rendering it unclear if this was a polygynous association or a three-female group similar to the one discovered in Ring-billed Gulls by Conover (in press). Hence in most species which have been investigated, SNCs have usually resulted from multi-female associations (either polygyny or female-female pairings). The results of this study raise the possibility that female-female pairings or polygyny may occur in many gull and tern species. Furthermore, SNCs are not unique to the Laridae. Unusually large clutches have also been reported in Procellariiformes (Allen 196 1, Rice and Kenyon 1962, Warham 1962, Tickell and Pinder 1966, Fisher 1968), Pelecaniformes (Snow 1960), and other Charadriiformes (Charateris 1927, Eggeling 1929, Jourdain 1936, 254 THE WILSON BULLETIN l Vol. 96, No. 2, June 1984 Bonner 1964, Drent et al. 1964, Hussell and Woodford 1965, Scott 1974, Sealy 1976, Erwin 1977). Clearly, more research is needed on the origin of SNCs in many species. Changesin SNC frequenciessince 1950. --Fry and Toone (198 1) speculated that DDT may have caused an increase in female-female pairings in gulls by feminizing male embryos to the extent that these individuals do not breed as adults. If correct, the frequency of female-female pairs, and therefore SNCs, should be higher since the 1940s when DDT became widely used. For most gull and tern species, my results do not support this hypothesis. Since 1950, SNC frequencies have increased significantly in only three species: Western Gulls and Herring Gulls nesting in the Great Lakes, and Caspian Terns breeding in the United States. Of course, this recent increase in SNC frequencies in these three species may be due to causes other than DDT or female pairings. Both of these gull populations, however, nest in areas which have had problems with DDT pollution. In the Western Gull, SNCs have been reported primarily in colonies along the southern coast of California, an area which has suffered from high DDT pollution originating from the sewer systems of Los Angeles (Fry and Toone 198 1). Also indicative of the high organochlorine levels in the food chain of this area is the documented problem of eggshell thinning in Brown Pelicans (Pelecanuserythrorhynchos)(Risebrough et al. 197 1, Jehl 1973) and Double-crested Cormorants (Phalacrocorax auritus) (Gress et al. 1973) nesting in the area. In the Herring Gull, SNCs have only been reported in the Great Lakes but not in European colonies or in colonies located along the east coast of North America (Appendix). Herring Gulls nesting in the Great Lakes also have had a lower reproductive success than gulls nesting elsewhere, a problem which has been attributed to high levels of organochlorines in their tissues and eggs (Keith 1966, Gilbertson 1974, Gilbertson and Hale 1974, Gilbertson and Fox 1977, Gilman et al. 1977). In the Caspian Tern, SNCs have increased in frequency since 1950 in U.S. colonies but not in those located in Canada and Finland (Conover 1983). One might suspect that Caspian Terns breeding in the U.S. had higher DDT concentrations than those breeding in the more pristine parts of Canada or Finland, but there is little information on the subject. Thus, DDT or some other pollutant may have caused an increase in SNCs in Western Gulls, Herring Gulls breeding in the Great Lakes, and possibly in Caspian Terns breeding in the U.S.; for most gulls and terns, this apparently is not the case. SUMMARY This study examined the frequency of supernormal clutches (SNCs) in gulls and terns by checking egg collections and literature reports. Supernormal clutches were defined as clutches that contained at least 50% more eggs than the modal clutch-size, except that a two-egg Conover l SUPERNORMAL CLUTCHES 255 clutch was considered supernormal if 90% of the individual females lay a single egg. While SNCs were found in 15 of 18 examined gull species and 14 of 15 tern species, they constituted less than 1% of the clutches in most of these species. Most studies which have documented the causes of SNCs have shown that SNCs usually resulted from female pairings or polygynous associations. These results suggestthat female pairings or polygyny may be widespread among the Laridae, as well as other normally-monogamous waterbirds. This study also tested the hypothesis that DDT has caused an increase in female pairings. If correct, there should be an increase in SNC frequencies since the late 1940s when DDT became widely used. In the Western Gull, the Great Lakes population ofHerring Gulls, and the U.S. Caspian Tern population, there has been a significant increase in SNC frequencies since 1950. These two gull populations breed in areas which have had pollution problems and where high levels of organochlorines have been found in the eggs and tissues of these birds. For most gull and tern species, however, there has been no significant increase in SNC frequencies since 1950. Hence, this study’s findings do not disprove this hypothesis but indicate that any pollutant-induced increase in female pairings probably is limited to a small number of species. ACKNOWLEDGMENTS I thank the following museums for furnishing data on egg sets in their collections and for allowing me accessto their collections: Academy of Natural Sciences of Philadelphia, American Museum of Natural History, Boston Museum of Science, British Columbia Provincial Museum, California Academy of Science, Cowan Vertebrate Museum, Delaware Museum of Natural History, Denver Museum of Natural History, Field Museum of Natural History, Florida State Museum, Los Angeles County Museum, Museum of Vertebrate Zoology (University of California, Berkeley), Museum of Zoology (University of Michigan), Museum of Zoology (Louisiana State University), National Museum of Natural History, National Museums of Canada, New York State Museum, Peabody Museum, Princeton University, Reading Public Museum, Royal Ontario Museum, Santa Barbara Museum of Natural History, and the Western Foundation of Vertebrate Zoology. D. E. Aylor, D. 0. Conover, and P. A. 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F. 1910. Breeding habits of Common Terns and Black-headedGulls. Br. Birds 4:32. WOLFE,L. R. 1923. The Herring Gulls of Lake Champlain. Auk 40:621-626. WOOD, A. H., JR. 1924. Water birds breeding on Pierce Pond, Maine. Wilson Bull. 36: 132-134. YTREBERG, N.-J. 1956. Contributions to the breeding biology of the Black-headed Gull (L. ridibundusL.) in Norway. Nest, eggs,and incubation. Nytt Mag. Zool. 4:5-106. -. 1960. Some observationson egg-layingin the Black-headedGull (L. ridibundus L.) and the Common Gull (L. canusL.). Nytt Mag. Zool. 9:5-l 5. DEPT. ECOLOGY AND CLIMATOLOGY, CONNECTICUT EXPERIMENT STATION, 123 HUNTINGTON AGRICULTURAL ST., BOX 1106, NEW HAVEN, CONNECTICUT 06504. ACCEPTED 15 JAN. 1984. 262 THE WILSON BULLETIN l Vol. 96, No. 2, June 1984