Birds are not mammals and do not possess the enzyme necessary to break down lactose that is in milk and its diary byproducts, such as cheese. Therefore birds will get some degree of indigestion when being fed dairy products!
Bird embryos are enclosed in an egg during incubation. This means that when the egg is laid, it does contain all the nutrients and fluids necessary to sustain the bird until it hatches. Once it hatches, the parent birds regurgitate foods that they have eaten to feed the chicks, as birds are unable to produce milk. It makes sense then that milk and products containing milk would be totally foreign food items to a bird. Nowhere in its natural environment would a bird ever be exposed to milk, cheese, yogurt or other products containing milk. It should also make sense that birds would not have developed the enzymes necessary to digest milk sugar.
What happens if a bird ingests milk, or products containing lactose, the milk sugar? Since it does not have the enzymes necessary to digest lactose, milk sugar will often pass through the bird’s digestive tract unchanged. Because it is a foreign sugar, it will draw fluids into the intestinal tract, resulting in diarrhea, in particular if ingested in larger amounts. So, what about commercially available products developed for lactose intolerant humans? The answer to that question is again a resounding NO. The two compounds that lactose is broken down into by these products are toxic to birds!
The evidence seems pretty convincing, and still, opinions are divided on this subject. One of the leading British conservation organisations is actually promoting cheese to be fed to birds. In our view, this kind of advice is potentially dangerous. Although it is also proven that lactose and salt contents of cheese varies, and that there are also some cheese brands, which do not contain lactose, scientific evidence has also revealed that cheese carries aflatoxins and aspergillosis, by degree, depending on its age and type. Soft cheeses are worse contaminants. Therefore there is always the risk of intoxication when feeding dairy products to birds, as they are far smaller than humans, and may not cope easily with these byproducts, which humans usually cope well with. Salt is of course another ingredient to consider, which when ingested in larger amounts can potentially cause kidney disease, and will subsequently result in raised blood pressure and heart damage.
Having said all that, as usual, it all depends on the dose or the amount of dairy products ingested. And it also depends on the age and species of the bird being fed with dairy products. Some bird species may recover, when surviving their nestling and fledgling phase, others will not. Over the many years of being involved in wildlife rescue, we had to care for many baby birds with severe health problems after being raised on dairy products by well meaning but inexperienced or occasionally simply ignorant carer. Sadly, quite a large number of these poorly birds died sooner or later, despite intensive care provided, or became unreleasable due to chronic internal organ failure and secondary plumage problems. This might not be such a big problem with adult birds, but only when small amounts of dairy products are being given occasionally. Adult carrion crows seem to tolerate cheese relatively well, and in fact, they quite often actually enjoy a cheese snack. This is most likely due to the fact that as carrion eaters these birds are better adapted to the broad variety of man made food sources, and are therefore more tolerant than other bird species.
All in all it is better to remain on the safe side. Therefore we feel that the mammalian product ‘milk’ should only be given to the species it is being designed for – cows milk to cows and human milk to humans.