Birds migrate great distances each year and the primary cue for migration are changes in day length. these changes in day length in turn trigger changes in the birds. behavior and body changes follow as the birds prepare for their seasonal migrations.
a) genetic
b) hormonal
c) physical
d) reproductive

i'm pretty sure it's spores

volcanoes (an event in the geosphere) release a large amount of particulate matter into the atmosphere. these particles serve as nuclei for the formation of water droplets (hydrosphere). rainfall (hydrosphere) often increases following an eruption, stimulating plant growth (biosphere).

the answer you are looking for is the united states forest service.

hope this !