Germination occurs when a wild bird seed drops to the ground and the correct moist environment triggers a process known as imbibation, which essentially means 'to drink'. When seeds imbibe they become hydrated, and this in turn influences the seeds' metabolic activity so that more energy is generated for enlarging the cells.
Shortly after imbibation, the root will emerge from the seed casing. This means that the seed is a viable and active fellow, well on his way to becoming a fully-fledged nuisance in your garden. Add a sprinkling of photosynthesis, some carbon dioxide, sunshine and water, and there's no stopping him! Well, perhaps it's not so dramatic - we don't live in a John Wyndham novel.
Statistically speaking, there will be a number of seeds that are viable for germination in a batch of ordinary, untreated bird seed, though fortunately no Triffids or other human-eating flora.