It will come as no surprise to learn that birds use energy in the metabolic processes to maintain life - the most obvious of these being the act of flying. It should be obvious that a bird first needs to have obtained enough energy from its food in order to take flight and that a bird's energy expenditure is at its lowest when it's inactive. The balance is everything - being idle means a bird requires fewer calories, being too idle would mean it couldn't forage optimally, leading to its eventual demise. Hence, birds are not idle.
We humans tend to think of our day as segments of time: the morning, midday, the afternoon, the evening, etc. At the weekends and on holiday, our day may be broken into events like breakfast, lunch and dinner. In the bird world, birds allocate the day and its activities based on energy out/energy in - with around fifty percent of their energy expended on foraging for food (one of the most demanding of energy draining activities). The other fifty percent is wisely invested in bird maintenance. Consider how much easier a wild bird's day becomes when supplementary food of the highest, cleanest quality is made available, without the loss of energy associated usually on the daily forage? (Less time foraging means more time for bird maintenance).
Continuing with the holiday theme for a moment... generally, when we return from holiday, the last thing we want to see is the bathroom scales. Would you agree with me that we humans can sometimes be accused of overeating? I know I can! Well, how often have you seen an overweight garden bird? In fact, have you ever seen an overweight garden bird? Consider that a Blue Tit may need to eat its 11g body weight in food each day, because of its high metabolic rate. This gain/loss happens real-time, though, minute by minute, day by day.
The Blue Tit and other birds' energy requirements will of course vary throughout the year. However, let's get specific and talk about a bird's demands during the season of winter...
High-calorie foods such as sunflower hearts and oilseed mixes provide the extra energy birds require to stay warm, forage for food and essential self-maintenance. The fat stored from a bird's diet helps them make it through the winter nights or when food is scarce. If we consider, once again, our 11g Blue Tit, it should be easy for us to accept that the storage of fat she can lay down for such occasions is so tiny that it may only buy her a single night or two. Birds, unlike mammals, do not put on a lot of fat during winter, when they most require it, as their metabolic demands are such that it makes the storage of fat incredibly hard.
What you do for your garden birds is important.