Birds measuring space

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“It is difficult to form an idea of the horrible noise occasioned by thousands of these birds in the dark part of the cavern, {...}. The shrill and piercing cries of the guacharoes strike upon the vaults of the rocks, and are repeated by the echo in the depth of the cavern. The Indians showed us the nests of these birds,by fixing torches to the end of a long pole. These nests were fifty or sixty feet high above our heads, in holes in the shape of funnels, with which the roof of the grotto is pierced like a sieve.” Alexander Von Humboldt (Von Humboldt's diary)

Background :
During a 2016 trip to Colombia, I was very fortunate to be welcomed by the staff of the Rio Claro Natural Reserve, located in the central Cordillera. After several days of recording in the caves –the ones open to the public– inhabited by small colonies of cave-dwelling Guacharos (Steatornis caripensis), Juan Guillermo Garcés, director and founder of the Rio Claro sanctuary, allowed me to access a secret location outside of the reserve, home to what ornithologists reckon is one of the largest known colonies of these birds in Colombia.