"Rara Avis" means 'rare bird, strange bird' and it is used with the sense of 'exceptional or hard to find person'. The origin of Rara Avis is from the phrase:
'Amicus verus est rara avis'. What does it means: 'A true friend is a rare bird'.
That is why it appears on the necklace with two swallows, because they symbolize loyalty and fidelity.
The swallows are considered the animals that travel the most kilometers in their migrations (more than 50,000 km per year), but always return home.
Fun fact: Swallows are also birds that remain faithful to the same partner throughout their lives. They are a symbol of absolute loyalty.