An ordinary thrush gains 1 ranks (1 HD: 2 Animal, -4 int). These ranks are immovably put in
Fly.
Familiars use their master's skill ranks if they are better, but always uses their modifiers.
This familiar own's ranks will be bold.
Familiars do not gain HD (they just count as having more HD), so they don't gain skill ranks with HD.
This also affects the maximum number of ranks that can be put in a skill.
However, they do gain ranks when their int increases.
Familiars always treat
Acrobatics,
Climb,
Fly,
Perception,
Stealth,
Swim as class skills.