Salmon Link Songbirds to the Sea

Winter begins this week. Leaves are off the trees. The salmon run is over- leaving salmon carcasses littering the stream banks.  My garden’s summer blooms have degenerated into seed pods, vertical stems and brown leaves. The somewhat stark habitat of my winter garden makes it easier to watch my resident winter wren as she forages for bugs, spiders and other delicious crawling creatures. A bird article recently caught my eye. The subject was how Salmon increase the population of songbirds, like the Winter Wren*.

 Biologists from the University of Victoria, BC looked at two coastal rivers in British Columbia. Both of these rivers have Salmon returning each fall. Both rivers, however, have waterfalls that act as Salmon barriers partway upstream. They sampled the songbird population on the reach each side of the waterfalls on both rivers. They found that on both rivers Winter Wren were more abundant where Salmon were more abundant. 

Two reasons for this phenomenon are:

1. Though Winter Wrens do not eat salmon, the decaying salmon carcasses provide rich food for midge and fly larvae. Insect eating birds, like Winter Wrens, may do better in areas where Salmon cause these particular insects to be abundant.  
2. The second possibility is that the salmon help the birds more indirectly. The nutrients from the dead and dying salmon enrich the entire riparian habitat. These fish are full of minerals and nutrients found only out at sea. The Salmon transport these rich nutrients from the upstream increasing the productivity of plant growth and fruit production. This plant growth attracts more plant eating insects that become a banquet for the Winter Wren.  When biologists studied tissue samples from Winter Wrens living along the salmon streams, they found that these Winter Wrens carry a distinctive molecular signature that is characteristic of nutrients derived from the sea. This tells of the important link salmon play in the relationship between the sea and the Winter Wrens of the forest. 

A note about Winter Wrens
 In 2010 the American Ornithology Union split the species, Winter Wren, into three:
Troglodytes hiemalis – Winter Wren (range from BC to the East Coast)
Troglodytes troglodytes – Eurasian Wren (found outside North America)
Troglodytes pacifica – Pacific Wren (resides on the west coast from BC to California)

*Christie and Reimchen. Presence of salmon increases passerine density on Pacific Northwest streams. Auk 125:51-59 (2008).