All sheet music is in public domain, and can be found at the Petrucci Music Library (IMSLP).
Amy Beach, The Hermit Thrush at Eve
John Vance Cheney: uncredited, Public domain, via Wikimedia Commons
Musical score: Holograph manuscript, 1921. Arthur P. Schmidt Company Archives, which is located in the Music Division at the Library of Congress.
Content is available under the Creative Commons Attribution-ShareAlike 4.0 License.
Catharus guttatus GJ: Cephas, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons.
Hermit Thrush in Prospect Park: Rhododendrites, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons.
Amy Beach in 1917: No photographer credited., Public domain, via Wikimedia Commons.
Charles Griffes, Three Preludes
http://www.nndb.com/people/896/000116548/, Public domain, via Wikimedia Commons
John Knowles Paine, In the Country op. 26
Caroline Cranch, Public domain, via Wikimedia Commons