effects of temperature on ice conditions
For both areas considered, it was clear that late winter and spring temperatures had a strong effect on ice conditions in late June. Although, with the exception of spring temperatures at Coral Harbour, weather station temperatures showed no significant warming trend during 1970–2003, remote sensing observations suggest that there has been significant warming in these areas over the period considered (Jones et al. 1999; Comiso 2003; Jones & Moberg 2003). Similarly, although our measures of ice cover showed a significant reduction only in Hudson Bay, remote sensing observations suggest a lengthening of the open water period in both areas since 1980 (Parkinson et al. 1999; Parkinson & Cavalieri 2002).
The trends in seasonal sea ice cover in the eastern Canadian Arctic are part of a general decrease in Arctic sea ice since the 1970s (Johannessen, Shalina & Miles 1999; Cavalieri, Parkinson & Vinnikov 2003) and are supported by large-scale thermodynamic models (Hilmer & Lemke 2000). The connection between temperature and Arctic ice cover is predictable, given that ice thickness is determined primarily by summer melting (Laxon, Peacock & Smith 2003). Model predictions suggest that these trends are likely to intensify over the next 50 years (Gregory et al. 2002; Comiso 2003; Laxon et al. 2003).
effect of ice conditions on breeding
Ice conditions affected timing of breeding in both areas, with ice cover and distance to ice edge being positively correlated with median dates of laying. This result accords with observations for black-legged kittiwakes (Rissa tridactyla) at a colony in the Chukchi Sea, where variation in spring temperature, probably caused by variation in the timing of ice break-up, caused variation in the timing of laying of up to 23 days over a 15-year period (Murphy, Springer & Roseneau 1991). At Prince Leopold Island, the timing of kittiwake breeding is strongly correlated with that of Brunnich's guillemots (Gaston et al. 2005). The polynomial fit with ice cover for laying date at Coats Island (Fig. 7) hints that laying at that colony also may be delayed in years with very early ice clearance. However, that result is highly dependent on a single year (1999) and hence requires confirmation.
The effect of ice cover on timing of laying was modified at both sites by temperature, with increased temperature advancing date of laying. At Prince Leopold Island, the accelerating slope of the relationship between timing of laying and ice edge position (Fig. 5) suggested that conditions in extreme years were close to those in which laying, within the relatively short open-water season, would have been impossible. This late breeding was associated with lower adult mass during incubation and brooding and lower chick growth rates. The combined effect of these observations is to suggest that, at Prince Leopold Island, late ice years are associated with poor reproduction.
In 1978, the year of most extreme ice conditions, reproductive success was lower than in the preceding three years; some nestlings starved on the colony and post-fledging survival is believed to have been very low (Nettleship, Birkhead & Gaston 1984). In 2002, 63% of chicks failed to reach 14 days old, most because they starved to death. The mean mass of chicks at 14 days was the lowest among 34 colony-years reported from Canadian colonies (Gaston & Nettleship 1981; Gaston, Chapdelaine & Noble 1983; A.J.G. unpublished data). Even among chicks that left the colony, probably few survived, given their very low body mass and poor feather development (some grew no feathers at all). Ice records suggest that conditions similar to those seen in 1978 and 2002 occurred in two other years during 1970–2003. Hence, Brunnich's guillemots at Prince Leopold Island have probably suffered near or complete reproduction failure related to heavy ice conditions in 12% of years since 1970.
In contrast with the situation at Prince Leopold Island, years of heavy ice in Hudson Bay had little effect on adult condition or chick growth at Coats Island. Instead, the years of lowest adult mass and nestling 14 days’ mass were those when summer ice extent in Hudson Bay was smallest. The observed trend suggests that increased temperatures, leading to reduced ice cover, will have a negative effect on nestling growth at Coats Island. Slower-growing chicks at Coats Island are less likely to be resighted at the colony as adults (U. Steiner unpublished data), suggesting that slow growth is associated with reduced survival. In fact, a generally increasing trend in the population at Coats Island from 1985 to 1997 was succeeded by a phase in which the population remained roughly stable from 1998 to 2004 (Gaston 2002 and unpublished data). As Brunnich's guillemots mainly begin to breed at 4–6 years (Gaston et al. 1994), the change in population trajectory between 1997 and 1998 corresponds well with the date at which post-1992 cohorts would have begun to recruit to the colony and suggests lower recruitment since 1997.
Observations of nestling diets at Coats Island since 1981 showed that a marked change occurred in the mid-1990s, with Arctic cod (Boreogadus saida) being progressively replaced by capelin (Mallotus villosus) and sandlance (Ammodytes spp.) between 1992 and 1997. A similar decrease in Arctic cod and increase in capelin has been observed at the nearby Digges Island colony (Gaston, Woo & Hipfner 2003). This corresponds with the observed reduction in sea ice extent. Arctic cod is a characteristic species of Arctic waters and the main prey of Brunnich's guillemot at Prince Leopold Island (Gaston & Nettleship 1981), while capelin and sandlance are the predominant schooling fish in waters off Atlantic Canada (Liem & Scott 1966; Hunter et al. 1984; Morin & Dodson 1985; Carscadden, Frank & Leggett 2001). The change in diet indicates a switch from a predominantly high Arctic food web to one more characteristic of low Arctic waters.
Although capelin appears to be a potentially suitable replacement for Arctic cod in the diet of Brunnich's guillemot nestlings, it is notable that the mean mass of Arctic cod observed delivered to nestlings at the Coats Island colony was roughly three times that of capelin delivered (A.J.G. & K. Woo unpublished data). As Brunnich's guillemots normally deliver only one fish at a time (Gaston & Hipfner 2000), the difference in mass suggests that murres may have to commute more frequently between colony and feeding areas when only capelin are available. Moreover, although capelin are the predominant food of common guillemots (Uria aalge), razorbills (Alca torda) and Atlantic puffins (Fratercula arctica) in Atlantic Canada (Bradstreet & Brown 1985; Piatt 1990), they are exploited little there by Brunnich's guillemots (Birkhead & Nettleship 1987). It seems likely that the switch in prey that has accompanied diminishing summer ice cover in Hudson Bay has been responsible, at least in part, for the lower nestling growth rates observed, as suggested previously by Gaston & Hipfner (1998).
predictions for the future
The rise in temperature predicted by climate models and observed for the Canadian Arctic over the past 2–3 decades will lead to a reduction in the extent of sea ice in summer (Gregory et al. 2002; Comiso 2003; Laxon, Peacock & Smith 2003). Substantial changes have already taken place (Parkinson 2000; Falkingham, Chagnon & McCourt 2002). Some model scenarios for Hudson Bay predict the disappearance of sea ice altogether within the next century (Gough & Wolfe 2001). At Coats Island, in the Low Arctic, changes in the physical environment over less than 20 years have already led to a marked change in the local marine fish community (Gaston, Woo & Hipfner 2003; A.J.G. unpublished data). Trends in growth rates of nestling Brunnich's guillemots suggest that the change in fish fauna has had a negative effect on conditions for chick-rearing. As conditions continue to shift towards those characteristic of Atlantic Canada, this trend will probably intensify, while at the same time colonization of the area by species typical of the low Arctic (razorbill, common guillemot) may intensify competition.
Conversely, at Prince Leopold Island, where conditions for reproduction have been constrained by heavy ice conditions in some years since 1970, we may expect conditions for reproduction to improve. As years when the ice edge in late June remains > 200 km east of the colony become fewer, breeding should become more predictable. In addition, breeding birds should be able to maintain their body condition more consistently, perhaps enhancing post-breeding survival.
The net outcome of processes that we have identified will be to reduce recruitment at the low Arctic margin of the species range, while increasing it at the high Arctic periphery. A combination of these two processes should result in a gradual decline of the species in Hudson Bay and an expansion among the high Arctic islands. This thesis is supported by recent changes in the population trajectory at Coats Island from increasing to stable (Gaston 2002; A.J.G. unpublished data).
Our scenario of deteriorating ice conditions in the southern part of the Brunnich's guillemot range and improvement at the northern edge accords with predictions for the Adélie Penguin (Pygoscelis adeliae) by Smith et al. (1999). Like Brunnich's guillemot, the Adelie Penguin is an obligate ice-associated species, but within its range populations do best in situations of intermediate ice-cover. This type of population shift in relation to global warming has been widely predicted (Brown 1991; Boyd & Diamond 1994; Boyd & Madsen 1997), but we believe that this is one of the first cases where at least one of the population mechanisms at work has been identified.