We found that egg volume (equivalent to mass) explained less than 3% of the variation in shape, and hence constitutes only very weak evidence for the hypothesis that larger (i.e. heavier) guillemot eggs are more pointed than lighter ones. In other words, since over 97% of the variation in egg shape is unexplained, size clearly has little effect on egg shape. A more plausible explanation for the slight positive relationship between volume and pointedness is that egg shape becomes slightly more pyriform as size increases as a result of some constraint on maximum egg diameter within the oviduct.
More critically, even after controlling for egg size, the eggs of Brünnich’s Guillemot are less pointed than those of the Common Guillemot. Brünnich’s Guillemots breed on much narrower ledges than Common Guillemots, and therefore if egg shape is an adaptation to facilitate rolling in an arc, we expect Brünnich’s Guillemot eggs to be more pointed, not less, than those of Common Guillemots. Our result thus provides no support for the idea that guillemot egg shape is an adaptation to minimise the risk of rolling.
The factors influencing how the avian oviduct determines the shape of eggs are not well known. Egg shape is likely determined by the eggshell membrane before the shell is formed, and that membrane is formed within the isthmus region of the oviduct. It is assumed that one end of this region is more constricted as the membrane is being formed (Bradfield 1951; Smart 1991). In addition, it is known that egg length and breadth (diameter) in particular are consistent within female birds (Romanoff and Romanoff 1949), including guillemots and the Razorbill (Birkhead and Nettleship 1984). Here, we show that egg shape within females, at least for Common Guillemots, is also repeatable, although the reasons for this are unknown.
It is striking that, with very few exceptions, almost all previous researchers have attempted to explain the pyriform shape of guillemot eggs as an adaptation to minimise the risk of rolling (Belopol’skii 1957; Tschanz et al. 1969; Ingold 1980). This narrow focus may be a consequence of the way guillemot colonies were studied and exploited during most of the twentieth century. In the past, those collecting eggs or studying guillemots typically climbed onto breeding ledges causing the incubating birds to depart in panic, and many of their eggs rolled off the ledge (Belopol’skii 1957; Uspenski 1956; Tuck 1961). Some studies even looked at the effect of gunshots, which caused an immediate mass departure of incubating birds in panic and a concomitant loss of eggs (Belopol’skii 1957). All this suggested that egg loss through rolling must be a major mortality factor for breeding guillemots. More recent observational studies of undisturbed guillemots showed that it is relatively rare for an egg to roll off a ledge (Birkhead 1977; Harris and Wanless 1988). Unless disturbed by large terrestrial predators such as man, guillemots of both species rarely leave their egg unattended: one partner incubates continuously. Escaping from predators that threaten their own life and abandoning their egg is exactly what we might expect from a long-lived species like guillemots.
Ingold’s (1980, 2016) conclusion that both shape and mass affect an egg’s rolling trajectory is based on a rather small sample size (n = 9 Common Guillemot and n = 9 Razorbill eggs). Also, as we show in this present study, eggs of the same volume or mass can vary considerably in shape (Fig. 1), but Ingold provides no information on egg shape, nor does he tell us whether he even matched eggs of similar mass in his rolling experiments. Third, as had been shown previously and confirmed by his own studies, egg mass declines during the course of incubation, yet he does not state that the eggs used in these experiments were at the same stage of incubation. With at least three different factors affecting an egg’s rolling trajectory, Ingold’s (1980) sample size of 9 is almost certainly too low to draw any firm conclusions.
Ingold (1980) acknowledged that other selection pressures, such as ‘weather conditions, predators and conspecifics’, might explain the pyriform shape of the Common Guillemot’s egg, but he did not elaborate nor test any other hypotheses. He also showed that parental behavior, including keeping the egg between their legs with the blunt end directed away from the bird, was important in keeping the egg on the ledge (Ingold 1980; see also Tschanz 1990; Ingold 2016). Elsewhere, we consider several other hypotheses for the pyriform shape of guillemot eggs (Birkhead et al. 2017).