If you walk around a woodland in the winter you may be forgiven for wondering where all the birds have gone. In fact, there are likely to be plenty of birds about, but instead of being evenly spread throughout the area, several species group together in a loose, mixed feeding flock. Flocking together in winter improves the chances of locating food and huddling together during the critical night-time period helps conserve body heat.

Source: How Do Birds Survive The Cold? | Birds in Winter – The RSPB