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Estimating Bird Mortality
by Steve Hampton
When assessing injuries to a spill, we typically seek to estimate, using scientific methods, the actual number of birds impacted by the event. For a variety of reasons, the number of birds collected (live and dead) on the beach may be only a small fraction of the total number impacted. The majority of the birds are never found. This page illustrates some of the reasons why. It is also important to note that a relatively small amount of oil on a bird (e.g., the size of a nickel) may result in death (see photo at right, with small amounts of oil on the breast). Like a hole in a wetsuit, the oil destroys the feathers' ability to insulate the bird, thus allowing cold ocean water to spread against the bird's skin. Birds typically die of hypothermia.
Coyotes, gulls, ravens, and other predators and carrion feeders may quickly prey on live beached birds or remove carcasses from the beach. OSPR studies have demonstrated that most birds disappear within 24 hours. This carcass is only hours old.
Removal or Burial by the Public
Beach-goers encountering dead birds may bury them or put them in garbage cans. This bird, shown here exumed from the grave next to its grave marker, was buried by the public within 20 minutes of its death. This is despite the fact that it was early in the morning and there were only four people on the beach.
Experiments have shown that dead birds may float at sea for up to two weeks. However, wind and currents may carry such birds such that they never make landfall. This bird was apparently attacked and swallowed, then regurgitated. It was found floating at sea.
Departure from the Area
Large birds are sometimes able to travel long distances before succumbing to the effects of oiling. These birds may travel well outside the spill response zone and beyond the notice of responders. In some cases, we have documented birds travelling 60 miles from the spill site within two days. This Brown Pelican, heavily oiled on the belly, was found at Port San Luis during the Torch/Platform Irene oil spill. This location is approximately 40 miles from the spill site.
The general assumption that birds come ashore and stay on the beach is not always valid. Common Murres, a seabird not known for walking great distances, have been found many hundreds of yards inland during oil spills. Sixteen sets of fresh murre tracks were counted on this 0.75 mile-long beach, all heading straight out of the water and up into the dunes. However, no birds were found despite intensive searching.
Significant stretches of the coastline often go unsearched because they are inaccessible. While it is sometimes assumed that such reflective coastlines are non-depositional, a recent OSPR study revealed that birds do wash up on such beaches if rocky pockets exist.
OSPR studies have shown that finding beachcast birds is not so easy. In fact, search efficiency plummets with small, dark-bodied birds. In this photo, on a beach free of wrack and other material, note the footprints (far left) of the observer who missed this carcass (far right). Beachcast birds are easily overlooked on cobble shorelines strewn with kelp and debris.
Some beaches have sand at low tide, but are swept clean at high tide. A recent OSPR study shows that some birds are scavenged on land, swept back into the sea, sink, and are further scavenged by crabs.
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Page Last Updated: March 4, 2012