Pine is a bad choice for birds of any type. Many people are not aware of the toxicity of pine and cedar on animals because much of the research is limited to scientific reports that usually only well studied vets and similar are privy to.
Also, another reason is because of the mental reinforcement of animal stores that sell these toxic woods for the use of small animals. Many store owners are unaware of the toxic affects, and those that are aware and still sell it anyway because either their customers continue to request that litter - even though the store employees recommend against it - or more often times the store owners care more about making money than the welfare of their customer's animals. I have worked at many pet stores and have warned against thees toxic litters, and many customers have thanked me for telling them, and some even switched litters after I've had to show them the photocopied research papers. Some customers thought I was out to make a buck because all of the other litters cost more (which is another reason pine and cedar is used so frequently - it is so darn cheap compared to other litters) even though I've shown the customer the research on the woods. Some customers continue to use pine and cedar because they've "used pine/cedar for years on many animals and none of them have ever gotten sick," or the similar excuse "if it was so bad for them, why do these companies still make it, therefore it must be OK for my animals and I've never heard of any animals ever getting sick because of wood."
I was fired once because the store owner was mad because I continued to inform customers about how toxic these litters are, and since those were the only litters that store sold that meant that his customers had to go somewhere else to buy litter. He was afraid that the customers would buy other things at the other stores while they were there and not come back to his shop. Even though he had some of the most loyal customers I've ever seen, and despite his refusal to buy any other litter (because the others cost more, and supposedly he thought his customers would not wan to pay the higher price - even though they were requesting it) he insisted on not buying anything else and thought that relieving me of my position would be a better choice. Well, many of his customers followed me to my next place of employment because of the care that I had for them and their animals and his shop closed down less than a year later.
There are many studies on the toxicity of pine, cedar, and a few others on small animals, birds, reptiles, and insects (for those people that keep insects as pets). It takes some time and effort to find them (which is another reason why a very large majority of people have no idea about it - because even after they are told about it, they tend to not want to take the time to research the facts for themselves.
OK, now that I'm off my soap box, yes Aspen is OK for birds, and you must follow similar safety precautions as you would with any other small animal or rodent when using it. I'm more of a fan of not using any word for litter for birds, but if you are going to use a type of wood, stay away from the soft woods, choose a good hard wood like Aspen.
Thank you very much for bringing up this question.
