Which of the following is an example of gene flow?

a. A forest fire kills 50% of red-spotted chipmunks in a forest community.
b. Pollen from a yellow daisy travels across a stream and pollinates an orange daisy.
c. A bird with blue feathers is less camouflaged in the rainforest than a bird with green feathers.
d. Mice with heavy coats are able to survive the winter better than mice with thin coats.