Nesting Blues: A Challenging Year for Nesting Seabirds

June 30, 2016 Posted by: Tania Lewis, Wildlife Biologist
2016 does not appear to be a successful year for breeding gulls and terns in Glacier Bay. In early June researchers conducted bird nest surveys at six major seabird breeding colonies and found only a fraction of the usual numbers of nests containing eggs. Many broken egg bits were found scattered throughout the colonies indicating high levels of egg predation; likely from crows, eagles, and possibly other gulls. Caspian Terns were seen nesting at two different islands in late May, but by early June only predated egg fragments were found. Black Oystercatchers were present in all colonies but most nests contained few eggs, although at least two chicks were observed hatching! Glaucous-winged Gulls built abundant nests but between predation and inundation of nests from recent high tides, most nests were empty. Researchers returned to one gull nesting island this week to check up on nesting progress and found a black bear actively predating eggs in the colony. This bear was also taking advantage of the ample strawberries on the island as evidenced by multiple mounds of jam-looking scats!

A black bear forages at a seabird nesting colony.
This black bear was observed eating eggs at a gull colony in Glacier Bay, June 22, 2016. Although many islands offer refuge to nesting birds from terrestrial predators, bears are willing and able to swim to islands close to the mainland, especially for a bounty of eggs and berries. NPS photo

Nesting success of gulls and terns may be influenced by many factors including food availability, which in turn may be influenced by air temperature, precipitation, ocean temperature, and more. Low food availability limits the amount of energy gulls can put into nesting effort and can increase levels of egg predation from other birds. Recent anomalies in sea surface temperature in the North Pacific may have contributed to seabird die-offs observed during the winter of 2015-2016 in Alaska and may also be influencing seabird breeding success this summer in Glacier Bay. The good news is that many seabirds live up to 20 years and so each bird only needs to successfully reproduce once to maintain their populations. 

Mew gull nest
Three eggs is a complete clutch for many species of gulls and terns. If birds lose eggs to predators or flooding, many will continue to lay eggs until a complete clutch is achieved. This Mew Gull nest contained one the few complete clutches observed during bird nest surveys in Glacier Bay in early June, 2016. NPS photo

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Last updated: July 6, 2016

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