- Important Findings from the Cooper Island colony

A trend to earlier egg-laying as the short arctic summer becomes longer

In northern Alaska Black Guillemots breed in ground-level cavities whose entrances are blocked with snow until early summer temperatures are warm enough to melt the winter accumulation. Because female guillemots do begin to form eggs until they have access to a nesting cavity, the timing of egg laying is sensitive to changes in snow disappearance in the spring. Since 1975 we have monitored breeding chronology at the Cooper Island colony by determining the date of the first egg in the colony and the median date of clutch initiation (the date on which half of the active nests in the colony have eggs). Both are good indicators of breeding chronology, with the latter having the benefit of representing a population response rather than that of a single pair.

Timing of egg-laying has advanced (occurred earlier in the year) over the period of the study at the rate of approximately 3 days per decade. The nature of the advancement shows the importance of conducting long term studies when examining the response of biota to climate change. Egg laying does not always occur earlier than the preceding year and in some periods no advancement is seen.

Timing of egg laying in a given year is well predicted by the timing of snowmelt in Barrow. Researchers at NOAA's Climate Monitoring & Diagnostics Laboratory in Barrow have been monitoring the disappearance of snow at their station just outside Barrow and have found a rate of advancement of snowmelt that is similar to that found in guillemot egg laying over the same period.

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After rapid growth in the 1970s and 1980s colony size declines as warming accelerates in the western Arctic

Black Guillemots colonies are frequently limited by the availability of suitable nest sites as evidenced by rapid occupation of cavities created by researchers and the presence of substantial nonbreeding populations at many colonies. For a decade and a half the Cooper Island colony appeared to be nest site limited. The rapid growth of the population as nest sites were created indicated that there was a large nonbreeding population in the region. After rapid colony growth stopped the number of nonbreeding birds at the colony was between 200 and 300 birds. Competition for vacancies at nest sites was intense.

Surprisingly colony size began to decline in the 1990s. This was first noted in a change in the size of the nonbreeding population but by the mid 1990s the breeding population had declined by almost 100 pairs to 115 pairs. This dramatic numeric change was accompanied by major changes in demography and behavior. Prior to the period of decline, any vacancies in the breeding population from overwinter mortality were quickly filled when birds returned to the island in June. In the mid-1990s, however, some nest-site owners widowed over the winter would be unable to attract a mate during the entire breeding season. As a result of low competition in some instances a single male would pair with two females at two adjacent nest sites. The lack of a nonbreeding population also greatly decreased the time that birds spent in nest site defense.

In the early part of our study the breeding population size was determined by the number of nest sites but now is apparently limited by prey abundance or availability. Beginning in 1990 there has been increased warming in the western Arctic with higher air temperatures and decreases in pack ice extent (Maslanik et al. 1996) (Maslanik et al. 1999). These changes in the primary foraging habitat of Black Guillemots may be the reason for the population decline at the Cooper Island colony.

Although decreased in the extent of ice continue, the 2002 field season saw the largest increase in population in recent years with 150 pairs present in the colony. Surprisingly, this increase may be due to the decreased extent of ice near the colony in 2002. For the first time in the history of the study open water was found directly next to the island when the birds arrived in June. The close proximity of ice free waters may have allowed first time breeders to recruit more readily than in past years.

Maslanik, J. A., M. C. Serreze, and T. Agnew. 1999. On the record reduction in 1998 western Arctic sea-ice cover. Journal of Geophysical Research 26:1905-1908.
Maslanik, J. A., M. C. Serreze, and R. G. Barry. 1996. Recent decreases in Arctic summer ice cover and linkages to atmospheric circulation anomalies. Geophysical Research Letters 23:1677-1680.

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