Hey Henry, I'll reply as best I can according to my experience. If others want to chime in that would be great too...
1. What is the source of your bird songs?
I've just switched to using the app called BirdTunes on my iPhone. I used to use iBird but BirdTunes has more songs for each species.
I use the Stokes recordings as their songs are the longest ones I know of (30-45 second clips, and some over 1 minute rather than 8-10 seconds with many others)
2. Do you have any experience with wireless speakers?
I've just started using wireless (bluetooth) speakers. Much faster setup and no tangles. I'm using the speaker by Jawbone called Jambox. It is expensive and small but it is elegant and it plays loud.
I have no experience with wireless speakers, but that is an option I'd toy with for sure. I like the idea of one less thing to carry in the field...
3. What is the optimal placement for the speaker in relation to the desired perch?
What height? What direction should it be pointing? Up at the sky?
Close enough. Some birds will actually land on the speakers. I like to place them directly underneath the area I want the birds to perch on. Sometimes just a few inches below. For rocks and logs just place the speakers on the ground below the perch. Experiment as not all individual birds will do the same.
4. Is it important that the speaker be camouflaged?
5. Do you know of a list, blog or website that gives specific notes on how different species respond?
Warblers and sparrows may respond well in general, but what about specific warblers? Is one species always a dud and one almost sure to succeed?
I realize that results will probably vary, but if a Bay-breasted Warbler never responds it would be good to know.
I don't know of any such list. I find it is easier to assess by "families" of birds rather than species. For example, one year I had zero success with Ovenbirds, then the next year they were all posing like crazy. Same with Blackburnian Warbler. One year nothing, another lots. Even a species that does not work well in one spot may in another. For years I tried Eastern Wood-pewee in vain...last summer I tried for the heck of it and BAM with 5 seconds one perched beautifully inches from the speakers.
In my experience sparrows and warblers in general work better than other families.
6. Does it help to vary the song if you have the choice? Does it help to pause the song from time to time?
I don't think it helps to vary it, but you can try. What I did with my audio though is to eliminate the parts where it is the "call" rather than the "song". Some recordings even have the "alarm call". Get rid of that for sure as that tends to spook the birds or stress them. Defenitely do pause though, even if it is working. If after a few minutes nothing happens either shut it off, or switch species.
7. I plan to use this mostly with the smaller birds (passerines or songbirds). Are there any other species that are especially responsive?
Most passerine species will have some individuals respond at some point in time. Locally the rail family responds very well (Virginia Rail and Sora). Owls can respond, but not usually well enough for good photos. Woodpeckers - sapsuckers especially so in my experience. Some waterfowl such as grebes and some species of ducks approach. There may be others that I am not thinking of right now, or haven't tried with yet.
Note that a bird "responding" may not actually be responding well for photography. MANY birds will respond but stay well back. Far fewer will "respond" by coming right in and perch for beautiful photos. Still well worth it though!!
Thanks for any help!
You are welcome!