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Field Log
Field Log is a quarterly column that provides Bird Observatory volunteers and staff the opportunity to share the connections with nature formed through their field experiences!
Spring 2008 - Tracking Northern Harriers in Salt Pond Habitats
If you have ever spent a few hours birding along the salt marsh or sloughs near the bay, you’ve most likely seen a Northern Harrier. These amazing birds are often seen hunting low over the marsh, looking for prey. However, harriers have learned that the salt ponds also provide an abundant food source: shorebirds! In the spring and summer, that food source may include Snowy Plovers! This year the Bird Observatory surveyed the salt ponds and adjacent marshes for northern harriers to determine how they utilize different habitats. Harriers are known predators of Snowy Plovers, a threatened species, and this additional knowledge will help with plover management decisions in the future.
If you have ever spent a few hours birding along the salt marsh or sloughs near the bay, you’ve most likely seen a Northern Harrier. These amazing birds are often seen hunting low over the marsh, looking for prey. However, harriers have learned that the salt ponds also provide an abundant food source: shorebirds! In the spring and summer, that food source may include Snowy Plovers! This year the Bird Observatory surveyed the salt ponds and adjacent marshes for northern harriers to determine how they utilize different habitats. Harriers are known predators of Snowy Plovers, a threatened species, and this additional knowledge will help with plover management decisions in the future.
We conducted our study at three salt pond complexes in the south bay that have nesting Snowy Plovers; the Alviso and Warm Springs sections of the Don Edwards San Francisco Bay National Wildlife Refuge and at California Department of Fish and Game’s Eden Landing Ecological Reserve in Hayward. The other field assistants and I surveyed harriers along defined survey routes at three different time periods each week. We looked at the use of habitat types that were classified as tidal marsh, salt ponds, upland habitat and levees.
In April and May, our days were spent locating harrier nests. We watched the harriers for hours, looking for courtship displays, adults carrying nesting material, food drops or any other behaviors that might indicate that the harriers had a nest. We located seven nests during the season. Six of the nests were in bulrush in the slough channels adjacent to the salt ponds. We located one nest in a pickleweed marsh, hidden in a patch of tall vegetation.
Our goal was to band adult harriers and put numbered patagial tags (wing tags) on them to follow individuals’ movement in the salt pond system. To do this we had to trap the harriers, which turned out to be a little more difficult than we anticipated. Harriers are extremely territorial near their nests. We borrowed a mechanical Great-horned Owl and set up a dho-gaza breakaway net between the owl and the nest. The mechanical owl's head moved side to side and when we played recordings of harrier calls, the harriers seemed convinced their nest was in danger. The harriers stooped the owl but quickly figured out that there was a net between their nest and the owl. We managed to catch two adult male harriers and put blue patagial tags on them, numbered "1" and "2".
Six of the seven nests that we found were depredated. This may be due to the high number of predators in the south bay, or to the fact that harriers nest in linear slough channels, which may make it easier for predators to find the nests. The one successful nest was found when the fledglings had left the nest, so we were unable to band them.
Our study found that harriers used the marsh habitat more than the salt ponds. However, harriers did use the levees and the salt ponds for hunting, and were often seen perched in salt ponds. The harriers may use the salt ponds for hunting because of the close proximity to their nesting habitat. If we limit their ability to perch in the ponds, we may be able to reduce the amount of Snowy Plover depredation within the salt ponds.
Have you seen these harriers?
We have received reports that male #1 is being seen in the Alviso area and male # 2 was recently seen near the Palo Alto Flood Control Basin. If anyone sees these tagged harriers please email Caitlin Robinson, at crobinson@sfbbo.org with the date and location in which the birds were seen. Also, please record what type of habitat the birds are in (marsh, slough, salt pond, upland) if possible.
-- Caitlin Robinson
Caitlin is a Biologist with the San Francisco Bay Bird Observatory’s Western Snowy Plover Recovery Program. She is currently in the master’s program in Environmental Studies at San Jose State University. Her master’s thesis focus is on Western Snowy Plover use of salt ponds in the south bay
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