Symptoms of poisoning may be delayed for several days following the ingestion. Oak poisoning is more common in cattle and less so in sheep and horses. The toxicity of the oak varies with the growing season, climatic and geographic factors. In humans, ingestion of ripe acorns in young children seldom causes problems apart from the occasional gastrointestinal upset. It appears that raw acorns are potentially more toxic.
Acorns are still a cause of serious poisoning in livestock.
I was unable to locate reports of toxicity in the avian species. Since there is the potential for causing toxicity, and because the nutritional value is questionable, it is probably best to avoid giving acorns to birds to eat. I realize that oak branches have been used as perches and have not caused problems. There is the potential for toxicity so are probably best avoided.