Unfortunately, the willingness to pay for each group was $80, $78 and $88 respectively. That means participants who were told 10 or 100 times as many birds could be saved failed to adjust for that huge difference in the amount they were willing to contribute.
This is known as scope insensitivity. Together with the collapse of compassion, it makes it difficult for us to feel as much empathy as we should for large groups of animals.
Tell a story before describing the larger issue.
How can we overcome this bias and help ourselves, and others, feel more compassion for large groups of animals?
One way might be by telling stories. We usually shouldn't introduce a large issue by describing its scale, even though this might be its most compelling feature from a rational perspective. Instead, it's probably more compelling to begin with a story or description of the individuals involved to establish empathy and a moral connection.
This works especially well when you know of an animal who has been rescued from abuse and now lives at a sanctuary or in a loving person's home, because it shows animals aren't inherently miserable and one-dimensional beings only capable of suffering about which we can do nothing, but complex individuals with a desire for happiness and a good life.