Because some of them were recordings.
Sarita reports  on a pre-dawn birding count in Jamaica Plain that involved cranking up a loudspeaker playing "screech owl trills" to try to attract the birds:
Today's Screech owl count was 8, which is a very respectable number. The two first I didn't see, but they both were attracted to the sound from our speaker and vocalized through the dark. The third and fourth ones I eventually got to see - they started tussling with each other up a tree and tumbled down to the ground. Number five we heard from a close distance. ...
- Jamaica Plain 
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