We’d like to thank “Sutro Resident” for a comment with a link that led us to an interesting piece of research: That migratory birds may depend heavily on small urban forests as they pass through, and even small forests are useful (though larger ones are better). The link was to a report in the BBC News, headed Urban trees ‘help migrating birds’, by their Science and Environment Reporter, Mark Kinver.
Migratory birds don’t generally fly non-stop from the point of origin to their destination. They break journey along the way, feeding and rebuilding their strength for the onward journey. The ability to do this is critical; without it, a bird cannot successfully make the journey any more than an aircraft with inadequate fuel can reach its destination.
Researchers Steve Mathews and Paul Rodewald of Ohio State University captured Swainson’s Thrushes migrating through Columbus OH, and released them in seven patches of urban forest of various sizes; the two smallest were 1.7 acres and 11 acres, the largest was 94 acres. Even in the smallest ones, 72% of the birds stayed out until they were ready to continue their journey (i.e., they didn’t fly to another nearby forest to break journey).
In the five larger ones, all the birds stayed, indicating that they found the resources they needed to rebuild their strength for the remainder of the migration. In the larger areas, they tended to stay toward the middle of the forest rather than the edges.
Though the study looked at data only from 91 Swainson’s Thrushes, the authors think the same might apply to other forest-dependent species, and suggest this underlines the importance of urban forests.
Mt Sutro Forest is about 80 acres in size (considering the UCSF portion and the contiguous Interior Greenbelt). This would suggest that it is a valuable resource not just for resident birds, but for forest-loving migrators. They are more difficult to spot than in open spaces (for people and hawks alike!) but visitors to the forest will find it full of avian sound, from the treetops, from the understory, and even the ground. Indeed, the evidence suggests that this forest is indeed important to birds.