CT AM had this to say:
"Chirpers: If Droyne do not caste, they do continue some development, though much more slowly. Sexual differences eventually emerge, but casteless Droyne tend to remain small and semi-intelligent and continue to resemble immature Droyne in appearance. Those Droyne groups which have lost the ability to caste (and these are fairly common) generally live as primitives.
Chirper. Semi-intelligent minor race native to many worlds in and outside the Imperium. Chirpers are omnivore/gatherers in the 25 kilogram mass
range. They have vestigial wings (chirpers on some small worlds can fly short distances) and opposable thumbs. Living in small groups with limited social organization, they follow age-old patterns of foraging for fruits and berries, supplementing their diet with occasional small animals which
they catch and kill with crude tools.
Chirper intelligence is at the low end of the scale and ranges from a few points above animal levels to a few points below average human.
Chirpers are named for the sharp, bird-like chirp which characterizes their speech. They can learn human speech, often handling a large vocabulary with ease, but their conversation retains the chirping overtones.
Chirpers are recognized by the Imperial authorities as intelligent and, as such, enjoy the protections and responsibilities of intelligent species within the Imperium. Most chirpers live in established reservations with only limited intercourse with humans.
Note that this AM came out long after Research Station Gamma and Secret of the Ancients, so the truth was already known to Traveller players and referees, but in game the Secret of the Ancients is safe and Imperial scientists are still studying Droyne and chirpers.