Many people think that birds in the northern parts of North America migrate south through places like San Diego because of the cold and snow. In reality, most northern bird species have good insulation and coping techniques for cold weather. Some birds, like ducks, prefer cooler weather over warmer weather due to their insulating down. While many things about bird and animal migration aren't known, scientists have a good idea as to why birds head south in the winter: Food.
Birds migrate mostly because of feeding opportunities. The high arctic has a large insect population during the summer, so many birds travel to that area to take advantage of that and raise their young. On top of the abundant food the arctic provides, it also provides extended daylight, in the summer, for finding that food. Thus, the birds not only have more food, overall, they can spend more time eating. But, the food supplies are infinite and can’t support large populations indefinitely.
When food supplies dwindle or when the weather or daylight length changes, some birds begin to head south where more feeding opportunities are available. Some birds only travel a short distance, some, like the red knot and the possibly extinct Eskimo curlew, travel to the other end of the Earth near the southern parts of South America. There, they find similar conditions as in the arctic: more daylight and more food.
Places like the San Diego and Mission Bay, Famosa Slough as well as other lagoons and bays in San Diego County, often act as resting and feeding stops for these birds as they head south in the winter and north in the spring. These areas have sufficient food supplies, such as crustaceans and small insects, for some birds to take a brief rest. Many of these birds can be seen roosting (resting or sleeping) on public beaches to store up energy for their journey. It is important to not disturb or chase these birds as this rest is critical to their survival.