I am glad you asked! As I have often stated, I try my very best to give to my birds a life in captivity that resembles, as much as possible, their life in the wild. I do firmly believe that it is this (plus the fact that I am EXTREMELY consistent in their routines) that makes it so my birds are never sick, never scream, never bite, eat/poop/sleep well, etc.
Now, in the wild, there are three environmental triggers for their seasons: point of refractoriness (length of daylight hours), food availability and weather. Basically, evolution tweaked and tweaked their endocrine system so their circannual cycle (resting, breeding, raising, molting seasons) would perfectly conform to the environmental conditions that make their reproduction the most successful (survival of the species being the most peremptory drive animals have). This is the reason why we have birds that are short day breeders and some that are long day breeders. One would think that it would be better for ALL birds to breed when the days are longer because they can feed the babies more often and for more hours every day BUT what if the food is not available during the long days? Or there are terrible storms that would prevent them from foraging for food? Hence, the short day breeders - birds that breed when the days are getting shorter instead of longer because this is the time when there is richer and more abundant food in their natural habitat and/or the weather is good - for example, IRNs are short day breeders because during the long days, it's monsoon season and with terrible winds, the birds cannot go out and get enough food for themselves and the babies and, even if they could, the plants do not produce (plants cannot flower and give fruit if there are severe winds).
So, because we know (through scientific studies) that ALL birds revert to being photoperiodic (meaning, when all things are equal, they would all go by the length of the day), I try to also give them the other two triggers by making the winter diet less rich and the temperature cooler and, when breeding season starts in earnest, I make their diet richer by upping the protein and fat levels. I do this by adding sprouts I make myself (*) and dehydrated mealworms (**) to their winter gloop (which is made with quinoa instead of millet) and adding hemp seed to their seed mix.\
Birds are all seasonal eaters and, in captivity, this doesn't really happen - mainly because we would never actually deprive a bird of food (birds often go hungry during the resting season) and also because we tend to think that having a perfectly balanced and/or a richer diet every single day is best. But, for some animals, having the same rich food all year round is not only not good, it's actually bad. When I had the bird rescue, I took in a female Northern Cardinal from a lady in Manhattan (mind you, it's illegal to trap, breed, cage, keep, sell, buy, etc any wild bird in USA but there is a very healthy black market for them and people simply do not care). This bird had something that looked like large thin 'wings' of a translucent, brownish looking something that looked and felt almost like plastic growing out of the sides of her legs. I researched and researched and couldn't find anything on this or anything similar so I risked taking her to my avian vet but she had never seen anything like this either so she took pictures and contacted other avian vets, including her University professors in Avian Medicine. They did not know what these things were either. After a month or so, she got an answer from a zoo vet: it was hyperkeratosis but of a degree that the zoo vet had never seen. So I contacted the lady and asked her what kind of diet the bird had been getting and she told me that she had been feeding her insects for years and years! This diet had caused the abnormal growth of the scales on her legs because of the huge amount of protein the bird had been eating. You see, Northern Cardinals switch their diet 180 degrees from season to season. During the breeding season, they are 90% insectivores and only 10% granivores BUT, during resting season (winter), they are 90% granivores and 10% insectivores. The lady figured that more was better so she gave her insects all year round which caused her liver to become damaged and not able to metabolize protein properly. It took me over two years of a low protein diet and herbal supplements to bring her liver back to a semi-normal level (she would never be completely cured but I found a wildlife rehabilitator 2 hours away from me that had a wintering aviary and agreed to keep her in it and continue her diet and supplements).
(*) The sprouts I use are not the commercial ones for two reasons: 1) there have been incidents of salmonella in the commercial ones and 2) they are too big and the birds pick them out and throw them instead of eating them. I use a soaking and sprouting mix I buy from ABBA - this mix is meant for passerines so the seeds are small and all grass ones so they are not as high in protein as the commercial ones (which are made of beans) and, because they are little, the birds eat them along with the gloop without a problem.
(**) I use dehydrated mealworms because the live ones are too fatty, they often have salmonella and the birds will not eat anything that moves