Possible Clues:

• Gift in 'The 12 Days of Christmas'
• Trade union wrangling revolted flyer
• Something turned and swooped over the pond — a bird
• This animal presumably spends its entire life in a shell ... correction: just the first part of its life
• One purrs, showing true love — heartlessly told off
• Bird between the partridge and French hen
• Affectionate bird
• Slow type comes out of shell every so often with bird for Christmas, one of two?
• Bird in a holiday song
• One of a carol duo