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Following Sea-eagles

An 8-week old Steller's sea-eagle, recently color banded. Photo credit: E. PotapovIn 2006, the San Diego Zoo, Natural Research Ltd., and Magadan Zapovednik (state nature reserves are called zapovedniks in Russia) partnered to track young Steller’s sea-eagles via satellite and to continue the long-term monitoring of breeding eagles in the zapovednik and surrounding area. Since then we have tagged 15 eagles, and it seems only two have survived beyond their first winter.

Given this apparent high mortality rate in 2009, we have been considering whether to continue the tracking effort. We are returning this month with the hopes of tagging a single bird and continuing the survey of breeding eagles. So, to pick up the thread and remind you of where we were…

By spring 2008, we had fitted 10 nestling eagles and tracked them. All except one either went off the air or were no longer moving (see post, Tracking Steller’s Sea-eagles, Part 2). We recovered one tag that had its harness cut, and others were transmitting from the middle of fishing villages, suggesting that humans were involved in some deaths of eagles.

A typical coastal cliff Steller's sea-eagle nest site near the Inya River east of OkhotskIn early April 2008, the sole surviving Steller’s sea-eagle from our study group tagged in 2006, No. 65632, crossed from its wintering grounds in Hokkaido north to Sakhalin and spent most of the spring and summer on the lagoons that are on the east coast of that island. This is the same area where it summered in 2007 when it was one year old, right in the middle of some of the island’s oil and natural gas development. Around June 14, it moved some 112 miles (180 kilometers) north on the island only to move back south to the central lagoons a few days later. In early August this bird moved north again, reaching the most northerly point of Sakhalin on August 4, then moved back south a bit before going off the air on August 27, 2008.

That same year, we fitted five satellite radio transmitters to nestlings in the Magadan region. Here are their stories:

#82161
This eagle left its natal area around September 12, 2008, and headed inland, looping around and arriving back on Motekley Bay, where it was reared, on September 16, then back to its actual nest area around September 25. Five days later it headed east, against the stream of normal migration that goes west, then south, along the Sea of Okhotsk. It moved around quite a bit in inland areas east of the city of Magadan before getting its bearings and heading back west on October 13. From October 19 to November 1, it stayed on the Inya River east of the village of Okhotsk and then migrated steadily until November 15. It stopped transmitting just north of the town of Chumikan. This is where another bird went missing in 2006 (see post, Tracking Steller’s Sea-eagles). Surprisingly, it started transmitting again on May 5 in the sea east of Sakhalin. The transmitter is probably on an oil rig there, having been brought by a human.

#82162
This eagle left its site on September 13, 2008, and headed north for a day before returning to Motekley Bay only 1 mile (1.75 kilometers) from its natal site. It was last heard on October 16. In the last few days, the tag was moving about 660 feet (200 meters) per day southward along the coast. We don’t know whether the tag was on an ill bird that was walking or on a corpse that was being carried along by the rise and fall of the tide. This is a known area for duck hunting at this time of year, and so we think there should have been plenty of food (and plenty of people with guns). It stopped transmitting about 1.9 miles (3 kilometers) from the place where we found a transmitter cut from a Steller’s sea-eagle tracked in 2006.

#82164
This eagle left its natal site on September 25, 2008, and headed south to the coast. It started migrating on October 4, flying inland parallel to the coast some 12 miles (20 kilometers), making short stopovers at all the major rivers. It settled on the Okhotka River on October 20 (this is still very far north) and stayed there until December 7. This is a place where we have had eagles apparently “trapped” by the weather in the past. On December 7, it started heading south but did not make much progress, as it apparently died on the coast nine days later.

#82165
This eagle moved away from its natal site on September 14, 2008, moving north to the upper bays of Motekley Bay, then started heading slowly south on September 20. It reached the main sea coast on September 28, then moved inland, making stopovers on the larger rivers as it moved south, making steady progress (about 37 miles or 60 kilometers per day) to a remote inland site south of Tugur. It went off the air on November 24. We think this bird died, because the transmitter came to life in May 2009 at the time of snow melt. We will be looking at information from the transmitter to give us indications of the fate of the bird. Did it die naturally or drop its tag? At this remote site we think the bird probably died naturally, although there are trapping activities in the area, and the nearest village appears to be about 19 miles (30 kilometers) away.

#65629
This eagle is the apparent success story of 2008. It left its natal home range on September 18 and headed a short distance north. From September 21 to 29 it was on the Yama River and then moved to the Chelmdze River (part of the Magadan State Reserve where our survey focuses). On October 25, it had made its way to the coast and was migrating. It made good progress south, sometimes covering more than 93 miles (150 kilometers) per day to mid-November. Its pace slowed as it took a shortcut across inland areas, and at the turn of the year it was near the town of Terney on the Russian coast of the Sea of Japan. It made a small move back north, approximately 93 miles (150 kilometers) and settled in for the winter. In 2009, it started its northward migration along the coast. It then headed inland, approximately 155 miles (250 kilometers) near the city of Amursk (population 40,000). On May 4, it continued its northward push across inland areas. By May 15, 2009, it was on a small island northeast of Shantar Island in the Sea of Okhotsk. From there it went out onto the sea ice. Since May 27, it has been on the mainland near Ayan. This bird’s tag was retrieved in 2007 after it was cut off an eagle that had originally been tagged in 2006. We refurbished the tag and put it on this bird in 2008.

We remain surprised that such a large proportion of the birds we tagged have apparently died and that in most cases the culprits in those deaths are humans! Steller’s sea-eagles face many threats. There was the lead poisoning threat of the 1990s, and there is the constant pressure (as yet unmeasured) of purported overfishing. The fossil fuel energy developments of Sakhalin and the wind farms of Hokkaido are a worry. We always knew that there would be some direct human persecution. Although few people live in these areas, most have guns and many are very bored. Because of this, and despite the fact that not everyone out there shoots eagles, if our sample is representative, we should be worried!

Although we will not be fitting many tags this year, let us hope the eagle we do tag in July will have more luck surviving its first year. At least one of the reasons we did not fit more tags this year is that we needed to buy a new outboard motor. With that cost behind us (and a new, reliable outboard), we hope to resume tagging more eagles in 2010. It is important that we accurately estimate eagle mortality as a way of monitoring the conservation status of the eagles and targeting conservation threats.

Mike McGrady, Ph.D., is a researcher with Natural Research Limited, partnered with the San Diego Zoo.

Here’s more information about the Steller’s sea-eagle project