The expression means to make the most of one's opportunities while you have the chance. And each winter day the birds in our yard must "make hay while the sun shines": they must eat enough food in the short daylight hours in order to survive the upcoming night.
Bitter winter nights require little birds to use more fat just to produce enough heat to stay warm. So every day, they have to eat enough high-calorie food to provide energy to last 24 hours. A little bird that doesn't take in enough calories during the day can deplete all its fat and die of hypothermia in a single frigid night.
And birds expend precious energy just gathering the calories they'll need. Looking for food expends energy and even opening or manipulating the food they find expends energy.
Your birdfeeders filled with high fat foods like suet, seed and nuts can help birds make the most of their daylight hours. They recognize an easy source of quality high-calorie food and will take advantage of it, especially when temperatures drop. They'll be there first thing in the morning until the last bit of daylight, packing on the weight to make it through the night...then start all over again the next morning!!