During periods of less light, many plants lose their leaves. does this mean that photosynthesis stops?

No, because photosynthesis is when plants make food from the sunlight. If that whole process was to stop, then the plant dies. I read that leaves are lost to deal with harsh winter conditions. Plants also lose leaves, to conserve water.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.