Step 1: Focus on the Clue - Southern India + Migratory Birds
The question mentions the southern part of India and the arrival of migratory birds in winter — a hint toward coastal Andhra Pradesh.
Step 2: Identify the Festival
The "Flamingo Festival" is organized in Pulicat Lake and Nelapattu Bird Sanctuary in Andhra Pradesh, where flamingos and other migratory birds arrive annually.
Step 3: Eliminate Distractors
Chilika is in Odisha, not southern India.
Dangs festival is in Gujarat, not relevant here.
International bird festival is generic and not linked specifically with the described region.
So, the correct answer is Flamingo Festival.