This post is part of a series on the collection of ornithologist William Brewster (1851-1919) at the Ernst Mayr Library, written by Elizabeth Meyer, library project assistant.
“The conditions which govern the singing of birds are a constant puzzle to me,” Brewster wrote in his journal after decades of ornithological study.  We have some solid information now about how and why birds make certain sounds , but Brewster didn’t know the difference between songs and calls; that sounds can serve to attract mates and declare a territory; or why songbirds would chorus at some times but not others.
Even with today’s apps and audio libraries, figuring out bird calls can be a challenge. Equipped with just pencil, paper, field glasses and ears, studying bird vocalizations is much trickier, as Brewster’s notebooks can attest. Still, he recorded and pondered over data as best as he could. Here are some interesting passages from his journals that illustrate that process.
Today, we can represent birdsong visually using sonograms. In order to translate sounds to text, Brewster listened diligently to invent mnemonics, describe patterns, and make comparisons to other more familiar sounds.
For example, he captured this unusual vocal performance by a crow: “First there would be a short, full whistle, next a gurgling sound as of water escaping from the neck of a bottle, next a succession of clucks or of short choking or gasping cries, finally one or more rich, liquid & really musical notes.” 
Image from one of Brewster's journals showing his written renditions of an unknown bird's vocalizations. He compares the voice to a Blackbird's. June 1, 1902. Journals of William Brewster, 1871-1919 (inclusive). http://www.biodiversitylibrary.org/item/160597#page/5
In his 1893 journal, Brewster described the complex vocalizations of a mysterious owl that he’d been hearing while camping in Maine:
“...the Owl which I had heard imperfectly and only a few times last evening began making the most unearthly sounds in the birch grove just west of the tents[…] The bird made three distinct sounds which may be roughly characterized as a yell, a whistle and a hoot. The yell was repeated from four to six times a minute and was often continued for several minutes in succession. Then the bird would hoot from one to three times and immediately afterwards begin yelling again. It whistled only twice or rather there were only two whistling periods. The yell varied greatly in tone and expression and somewhat in form, on variation usually running gradually into another through intermediate forms. The three typical or extreme forms were haink, very similar to the cry of Ardea herodias [Great Blue Heron], an ouk exceedingly like the honk of the Canada Goose, and a snarling cat-like scream. The haink was not louder than that of the Heron; the other cries could probably have been heard a mile away.” 
Brewster would spend the next decade wondering about the species of this owl. In 1902, after becoming familiar with a wider range of Great-horned Owl (Bubo viginianus) sounds, he began to suspect that it was “after all, nothing but a Bubo.” 
Complicating the matter, we also know today that a song isn’t necessarily the same throughout the whole geographic range of that species. Birds of the same species may have different ‘dialects’ in different places.  Brewster made this interesting observation while listening to Bobolinks (Dolichonyx oryzivorus) in Lancaster, Mass.:
“... at least three out of every four of those [Bobolinks] which inhabit the fields near the house and bordering the lane utter at the very beginning of their songs (or rather just before beginning to sing) a note which, at a distance, sounds exactly like the word queer, but which nearer at hand is more like quee-ah. This is so loud or, at least, penetrating that it can be heard at distances at which all the other notes of the song (which is in other respects quite normal) are inaudible… It is, I believe, peculiar to [‘unique to’] the birds of this region (since I have never noted it at Concord which is only twenty miles distant) and the fact that it is quite as generally used this year as it was last is interesting evidence that I am now hearing the same individuals (with, no doubt, some of their offspring) that I listened to last season.” 
Brewster could have been mis-hearing (the songs of Bobolinks are complex), but he also could have been picking up on a regional dialect. His suggestion that the dialect would be passed on from parent to offspring is on the mark. 
Song of the Bobolink.
Building up a comprehensive knowledge of bird sounds was an enormously time-consuming challenge. At least up to his 1903 journal, I’ve noticed more effort devoted to writing down recordings of bird sounds and identifying their sources, rather than to conjectures about the function of these behaviors. Anthropomorphism (attributing human characteristics to animals) makes its way into many of his observations, so that when Brewster does venture into the ‘why’, he sometimes appears to be framing this question more along the lines of, “What are they saying?”
With some folkloric humor, he interpreted the calls of a crow initiating a group mobbing an owl:
“ ‘An Owl! an Owl! Wake up you sleepy, murderous, yellow-eyed villain. You mule-eared knave! Come on, friends, and help me drive this thief from his stronghold! Let us pluck out his cat ears and gouge out his big eyes and pummel & peck him to death!’ All this and much more to the same purpose, if I understood the Crow rightly.” 
Or, if you'd prefer to end on a sweeter note:
"As she [an oriole] came flying back I was struck by the tone of mingled anxiety and interrogation of her low call. 'Where? Where?' she seemed to say. 'Here-we-are' (Here we are falling inflection) both young would promptly draw in around and then, as she alighted near them, would retreat and extend this to: 'Here-we-are mam-ma. Here-we-are-mam-ma.' It really required almost no imagination to fit these words to the calls in question and now that they have occurred to me the calling of young Orioles will no longer be to my ears, as it always has been, a disagreeable sound." 
 May 18. Journals of William Brewster, 1871-1919 (inclusive). 1902. http://www.biodiversitylibrary.org/item/160704#page/93
 Cornell Lab of Ornithology. 2014. Bird Song. All About Bird Biology <birdbiology.org>. Cornell Lab of Ornithology, Ithaca, New York. https://academy.allaboutbirds.org/birdsong/
 June 2. Journals of William Brewster, 1871-1919 (inclusive). 1902. http://www.biodiversitylibrary.org/item/160597#page/7
 October 7. Journals of William Brewster, 1871-1919 (inclusive). 1893.
 November 8. Journals of William Brewster, 1871-1919 (inclusive). 1902. http://www.biodiversitylibrary.org/item/160597#page/118
 May 20. Journals of William Brewster, 1871-1919 (inclusive). 1902. http://www.biodiversitylibrary.org/item/160704#page/97
 October 8. Journals of William Brewster, 1871-1919 (inclusive).1892.
 July 5. Journals of William Brewster, 1871-1919 (inclusive).1892. http://www.biodiversitylibrary.org/item/156490#page/204