150 



E. H. DUNN 



(1961) for eiders; Snow (1963) for shag; Thore- 

 sen (1964) for Cassin's auklet; Vermeer (1963) 

 and James-Veitch and Booth (1974) for glau- 

 cous-winged gull (Larus glaucescens); and 

 Andersson (1973) for jaegers. 



Nest Building 



Although many northern seabirds have es- 

 sentially no nest, they may spend consider- 

 able time working or displaying at the site 

 (Belopol'skii 1961). Black-legged kittiwakes 

 (Rissa tridactyla) have substantial nests, but 

 they are built in a comparatively short time 

 (about a week) soon after the birds arrive 

 (Fig. 6). Shags also have substantial nests, 

 but they are not completed until about 1 or 2 

 weeks before the first egg is laid (Snow 1963). 

 Herring gulls build smaller nests, 5 to 10 days 

 before laying, although in the Far North they 

 and glaucous gulls may not start building the 

 nest until the first egg is laid (Belopol'skii 

 1961). The eider always begins preparing the 

 nest when the first egg is laid (Belopol'skii 

 1961; Schamel 1974), and terns and skuas, 

 which build no nests, choose their sites at that 

 time. Murres, which frequently lay their eggs 

 directly on snow, choose a site somewhat 

 earlier and spend considerable time protecting 

 it (Belopol'skii 1961). Burrows may be dug 

 within a period as short as 3 days for Leach's 

 storm-petrel (Gross 1935) to one as long as 

 several weeks in Cassin's auklets (Manuwal 

 1974a). Overall, the prelaying period is longer 

 for burrow nesters than for those using 

 crevices (Sealy 1973a). 



The amount of time and energy spent by the 

 male and female in nest building differs 

 among species. In Leach's storm-petrel, the 

 male digs the burrow (Gross 1935), whereas in 

 eiders, the nest is built entirely by the female. 

 In most seabird species, the sexes share in 

 nest construction, but roles may still be sepa- 

 rated. For example, in shags the male collects 

 the nest material and the female builds the 

 nest (Snow 1963). 



Egg Laying 



Timing of egg laying is influenced not only 

 by weather (Erskine 1972; Sealy 1975c), but 

 also by numerous biotic factors. Smith (1966) 

 showed that where glaucous gulls, herring 

 gulls, and Thayer's gulls (Larus thayeri) breed 



GLAUCOUS 

 GULL 



TEN-DAY PERIODS 



Fig. 7. Timing of peak sexual activity (a combined 

 measure of egg laying and testes size) in colonies 

 of arctic gulls of different species composition. 

 From Smith (1966). 



in mixed colonies, the peak of sexual activity 

 and egg laying in Thayer's gull is about mid- 

 way between the peaks for the other two 

 species (Fig. 7). In nearby colonies where 

 herring gulls are absent, however, the peak of 

 sexual activity in Thayer's gulls is delayed 

 about a week, and activity continues for a sig- 

 nificantly longer period (Fig. 7). 



Annual variations in food supply also will 

 affect the start of the egg-laying season. Belo- 

 pol'skii (1961) cited an example from the 

 Barents Sea in 1940 when a series of storms 

 made it difficult for certain seabirds to find 

 food. Murres and kittiwakes, which were able 

 to catch fish, started reproductive activities 

 on schedule. Gull breeding was delayed, how- 

 ever, and egg laying began in force only after 

 fishing boats arrived and started discarding 

 offal. Onset of egg laying in great cormorants 

 (Phalacrocorax carbo) is correlated to April air 

 temperatures (Erskine 1972), and this may 

 also be related to variations in spring increase 

 of food availability. In certain birds the breed- 

 ing season has been shown to start particu- 

 larly early when food supplies are unusually 

 abundant (Hogstedt 1974; Kallender 1974), 

 but this has not yet been demonstrated in 

 seabirds. 



Lastly, age and sex of seabirds are known to 

 affect the timing of egg laying (e.g., Coulson 

 and White 1960; Lack 1966); older, more ex- 

 perienced birds tend to lay earlier than do 

 younger ones. In shags, males tend to breed 

 progressively earlier as they increase in age, 

 but females do not (Snow 1963). 



