I would both agree and disagree. If a predator is focused on an area, movement will be seen regardless of camouflage. However, a casually scanning predator could easily miss well-camouflaged movement, but it isn't going to miss a color that obviously contrasts with the environmental background. Other than in the snow, a white bird is a great big "here I am" sign, and actively draws in the focus of the predator. There's a reason that advertisers use contrasting colors to draw in your attention, and the military uses camo clothing to hide in plain sight. There's a reason certain color patterns survive in nature and others don't, and a lot of it is predator pressure driving natural selection. If you survive, you have a chance to reproduce. If you're eaten, your color phenotype dies off with you. There are even certain species of birds and mammals that molt/shed out twice yearly to take advantage of this principle, wearing white in the winter snow, then returning to their wild type coloration when the snow melts.