The easiest way may be to add something to their water. I use dish soap every so often. I put a squirt in a gallon of water or 3 squirts in our 3 gallon water containers. I will make a guess that my squirt is between a teaspoon and 2 teaspoons. I keep the soap in the water for at least a week, sometimes up to a month. I tend to do it in cooler weather so fall and spring. I don't want to risk a bird not liking the soap and overheating in the summer.