EXTREME RAIN AFFECTS MAINE SEABIRD NESTING
by Stephen W. Kress
Expanded from Egg Rock Update, 2009
Extreme rains are a predicted outcome of global warming1. Observations from this past summer are consistent with a pattern of wetter-than-usual summers on Maine islands. Yet even given this trend, summer 2009 was memorable for record-breaking amounts of rain. High rainfall usually results in poor seabird nesting, but this year the negative effects were countered on some islands by the exceptional amount of Atlantic herring that parent seabirds brought to their chicks.
Photo by Derrick Z. Jackson - Boston Globe
While it’s usually difficult to separate the effects of weather, food, and predators on chick survival, this year the extreme weather and abundant herring were clearly dominant forces. The timing of storms, and features such as topography and nesting substrate, are also important to seabird nesting success. Only during extreme weather does the role of such factors become apparent. Long term studies like those conducted by Project Puffin provide rare opportunities for insight into how these forces interplay.
For example, last summer Eastern Egg Rock seemed to be sitting under more rain clouds than any of our islands. Interns recorded nearly four times the usual rainfall, with 27.9 inches of rain from late May to early August. This compares to an average of 7.4 inches during the same period for the previous 20 years. The rain and cool temperatures (averaging just 60 degrees F.) particularly affected the terns, whose surface nests are easily impacted by weather. Despite the extreme rain, most terns faired well through incubation and the first week following hatching. During this period, adults could shelter their eggs and small chicks under their bodies and wings. Rain that fell after the chicks were about a week old did the greatest damage, as the larger chicks were more exposed and their food demands were greater. At this age, chilling rains often lead to chick death through exposure.
Also, parent seabirds can afford fewer long distance foraging ventures during sustained rainy weather; such trips keep them away from their chicks longer, resulting in more exposure to chilling rain and opportunistic predators such as Herring and Great Black-backed Gulls. For this reason, the appearance of a consistent and abundant supply of high-calorie herring was especially important this summer. Atlantic herring typically makes up about 20% of the Common Tern chick diet, but last summer, herring comprised more than 50% of the chicks’ diets across all islands.
At Eastern Egg Rock, herring, while more common than usual, still made up just 20% of the food delivered to Common Tern chicks. Only 0.7 chicks fledged per nest. Likewise, Arctic and Roseate Terns had the lowest productivity in eight years. Even puffins, which nest beneath a protective cover of boulders, demonstrated the effects of the weather by producing 0.79 chicks per nest, notably below the 21-year-average of 0.9 chicks per nest. Egg Rock’s geology may have also contributed to chick mortality because rain collects on the impermeable granite bedrock, flooding many tern nests. Likewise, no Leach’s Storm-Petrel chicks were observed this year, as most of their burrows were saturated by rain.
In contrast, islands with higher proportions of herring and less rain had much higher nesting success rates than Egg Rock. At Stratton Island, for example, Common Terns fledged 1.74 chicks per pair, far better than the island’s typical average of just one chick per pair. This excellent productivity occurred despite 23.7 inches of rainfall from early May to early August. Here the diet of tern chicks contained 42% herring—more than twice that received by Egg Rock chicks. Also, the sandy nest substrate at Stratton Island allows water to drain and prevents nest flooding.
In 2009, herring increased to record high amounts in tern chick diets.
In late August, Hurricane Bill roared up the Atlantic coast. It missed a direct hit to Maine, but the storm was predicted to create 20-foot waves that would roll onto the Maine coast on August 23rd. By this date all Project Puffin field stations were closed for the season, except at Seal Island National Wildlife Refuge where Matt Klosterman and Lauren Scopel had just returned to set up a banding station to monitor the fall bird migration.
On receiving news of the impending storm, Brian Benedict, deputy refuge manager for the Maine Coastal Islands NWR, raced to Seal Island ahead of the storm to evacuate Matt and Lauren. They returned on August 25th to find that waves had crashed over the island, nearly flooding their research cabin. They also found most of the puffin nesting habitat flooded, even though it is typically high and dry this time of year. The thriving puffin colony here was especially fortunate as most of the puffin chicks had fledged just before the storm. Apparently, the abundant herring which nurtured this year’s puffins helped the chicks fledge earlier than usual. This near-miss was another reminder of how vulnerable Maine’s puffin nesting islands are to flooding from storms and predicated ocean level rise from melting Arctic ice.
|Photo by Kevin Colton
A Common Tern Chick peeks out from under the protective cover of its parent during a summer rain shower at Eastern Egg Rock.
1Richard P. Allan and Brian J. Soden 2008. Atmospheric Warming and the Amplification of Precipitation Extremes. Science. Vol. 321, 5895:1481-1484.