You may be aware of the tight connections between salmon, forests and bears. You may have heard that coastal forests safeguard the streams where salmon hatch. And, then, when adult salmon return from the ocean packed with marine nutrients, they spawn and die and are eaten by bears (and many others) that disperse those nutrients into the forest, nourishing the very trees that nurture the salmon’s natal streams. Something like this:
But have you ever thought about the connections between salmon and songbirds?
Each spring billions upon billions of migratory birds flood north in a continental shift of biomass that is nothing short of staggering. In these animations from eBird.org we can actually see data points from the migrations of barn swallows and Swainson’s thrush; the birds flow between hemispheres like sand in a gigantic hourglass. Imagine that hourglass filled with hundreds more species and the magnitude of this bi-annual natural history event really starts to sink in.
In human terms, the legacy of salmon nutrients for coastal songbirds is akin to a personal trust fund bequeathed by people you never met.
Inevitably, some of those birds will return to the west coast to nest in river valleys that are also home to salmon
The marine nutrients carried upstream by salmon enhance the growth rates of plants in the river valleys and increase the complexity of understory vegetation, creating greater habitat diversity and attracting more bird species. All that extra nitrogen also supports greater numbers of aquatic and terrestrial insects – rich sources of protein for nesting birds trying to feed their hungry broods. The result is that salmon producing watersheds have greater bird abundance and diversity than watersheds without salmon.
Of course, the birds don’t know any of this.
The presence of salmon can be heard in birdsong
Birds and salmon alike migrate in mass numbers to coastal river valleys to breed, but they do so at different times of year: salmon in the fall, birds in the spring. They are passing ships in the night. In spring, when the songbirds are raising their young, there are no adult salmon in the creeks at all – only the little juvenile fish that have yet to migrate to the ocean. Unlike bears and wolves, songbirds don’t actually eat salmon – they are thriving on the echo of salmon from previous years. In human terms, the legacy of salmon nutrients for coastal songbirds is akin to a personal trust fund bequeathed by people you never met.
People tend to think of the autumn salmon run as an isolated and time-limited event. But, in reality, salmon are ubiquitous in coastal forests; their presence can be detected at any time and in numerous ways. It can be analyzed in nitrogen isotopes in a lab, measured in the size of growth rings of trees, and tabulated in the abundance of stream invertebrates. It can be counted in the number of pups in a wolf pack and the number of new bear cubs each season. But it can also be heard in the diversity of birdsong in river valleys each spring.