Suet is particularly important for the insect eaters such as woodpeckers, chickadees and nuthatches. It is a quick source of energy and, therefore, heat during the cold and is sure to provide many birds with the extra "margin".
Of the three essentials for attracting birds, shelter, food and water in winter, food rises to the top of the list. After a long and bitter night, birds are at their lowest ebb and their stored calories are nearly gone.
During the daytime hours, the food consumed fuels a bird's high metabolic rate which produces the heat to keep him alive in the bitter cold of winter. Birds burn a tremendous supply of energy for their size. Keeping their body temperature near normal (100-112°F) requires huge amount of calories, especially on those nights when it drops to below zero. They need to find quick food in the early hours of the morning.