Dr Halsey, a biologist and reader at the University of Roehampton, recently reviewed more than a 100 studies to try and gain a better understanding of how our garden birds remain a relatively stable weight.
It was found that birds exercise by singing, shivering and fidgeting. When a bird has eaten a certain amount of food it can unconsciously balance the amount of exercise needed.
It is understood that further research is required for different species in order to fully understand the different nutritional makeups.
One experiment that has been suggested is to present birds which normally stay slim with a feeder full of easier to digest seed, to see if they are then able to preserve a stable weight when faced with temptation.
*The review was published in the journal Trends in Ecology & Evolution