In a story I'm writing, there is a species of extremely rare cave dwelling bird. A scientist and a plummer have to find a specimen of this bird before it goes extinct. When they finally find it, the plummer asks why it is white, and the scientist explains why (most) creatures in caves are albino, but the problem is, I don't know why animals in caves are albino. Can you guys help me?
Cave creatures aren't white as much as they are "I don't care what color I am." It just happens that when you look at all the different compounds required to keep a creature alive, they tend to roughly reflect all of the spectrum equally.
Consider this as an example: the Olm
In this picture we can see that the whole creature is white. However, its gills are an exception. They are red because the hemoglobin in its blood drastically adjusts the absorptivity of the blood to be more red. In fact, if you look at the whole body, it's not actually "white" as in Titanium White. It actually has a reddish hue throughout due to the hemoglobin in its body. However, this is such a slight effect compared to what you or I are used to seeing that our brains register it as "white" or even "unusually white."
To have a color, creatures would need a predominance of one chemical or another, like we see with the hemoglobin in the blood. The dark pigments in our own skin are an example of this. Melanin makes our skin dark, and the quantity of melanin we have is enough to have it play a substantial effect in the color of our skin. This melanin is not cheap. Current science believes it has a half life on the order of weeks, meaning we have to be constantly replacing it. Melanin requires the oxidization of tyrosine, an amino acid. A species which forgoes its production can use that tyrosine for other purposes (it's a major player in signal transduction). Over an evolutionary period of time, this can make enough of a difference in survival to cause a species to discard its ability to produce such pigments all together.
Best use of available light
Caves are naturally dark. But sometimes there is a little light - either reflected from the entrance to the back of the cave, phosphorescent minerals, or perhaps artificial light in some form. But the light is limited, so the best visibility is provided by the highest contrast between animals and the background - and that means white animals.