Next batch cut short, because some people can't behave. These results are clean though:
THX (assumed TH3)
Lesser Colibri
Name Drop rate Drop rate (%)
Colibri Feather 109/188 58.0%
Colibri Beak 97/188 51.6%
TH4
Lesser Colibri
Name Drop rate Drop rate (%)
Colibri Feather 31/46 67.4%
Colibri Beak 23/46 50.0%
TH5
Lesser Colibri
Name Drop rate Drop rate (%)
Colibri Feather 2/2 100.0%
Colibri Beak 1/2 50.0%
No gearswaps, no weapon skills, all TH procs accounted for. THF90/NIN45, no TH gear (TH3 base), all upgrades from melee hits. Another interesting tidbit... 4845 melee hits, 50 TH upgrades. I don't know if there was any testing done for this yet (there may have been, I'm a lazy researcher), but it seems the chance from melee procs to increase TH is 1%.