Birdie is an English diminutive used historically as a nickname for Bertha (Germanic "bright"), Bridget (Irish "strength"), or simply for any small girl. The literal English meaning — "little bird" — is sweet.
Birdie was popular in the late 19th century, peaking at #232 in 1881. After a long quiet it is returning fast as part of the broader vintage diminutive revival.
Birdie does not currently appear in the US Social Security Administration's top 1,000 girls' names, so we don't publish a US rank or birth count for it. That says nothing about the name's standing elsewhere in the world — only that it sits outside the ranked US data we rely on.
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In Pythagorean numerology the letters of Birdie reduce to 2, The Peacemaker. This is a traditional interpretive system, not a factual claim about the name.