Darcy has number of nicknames. The main one is Little Bird. Phil says it’s what their parents used to call her because even when she was a baby she was bright eyed and attentive. Except when he’s teasing, then he says they called her that because when she was hungry she screeched just like a baby bird. She usually hits him after that.
Phil himself usually calls her Flutterbudget because when she was little she was always fidgeting and in motion. No one else is allowed to call her that, and to be honest, no one would dare since they wouldn’t want either of the Coulsons to come after them in revenge.
Nick and Maria call her Little Bird in private, Darcy when with civilians, and Junior the rest of the time, but there’s always a note of fondness when they use her codename, even when Nick’s growling it across base when she’s done something. Jasper starts calling her Pajarita which is Spanish for Little Bird when he starts teaching her Spanish.
Clint is the first person to come up with something new. Young Darcy had a knack for finding Probationary Agent Barton and would gleefully find her way to him whenever she could escape supervision. (The number of near heartattacks this gave her brother before Barton became Clint are a story for another time.)
One day she found her way to him, and cheerfully started telling him about her day in baby Spanish. She’d found out that he could speak Spanish and proceded to take what she was learning from Jasper and practice on Clint. That day she had been to the labs with Maria and had seen some prototype hovercraft based on hummingbirds. She’d been fascinated by the science and delighted by the pretty pictures of the birds.
She got frustrated because she didn’t know the word for hummingbird in Spanish, and Clint had solemnly told her it was colibri. Then he watched her flutter around the room and he laughed and told her he was calling her Colibri from now on because she was like a little hummingbird herself.
When Natasha was brought in and integrated into Darcy’s selected family she soon started calling her little bird in Russian. On certain special occasions she also called her sister in Russian.
Jane sometimes gets frustrated with Darcy because she’s slow to respond when Jane calls her name, Darcy just smiles because while she’s always Darcy, she’s rarely expected to respond to that name at home.