Final answer:
Following an abundance of rain in 1983, the bird population in 1984 would likely have diverse beak sizes for varied food sources, with both large beaks for fruits and seeds and specialized beaks for insects being common due to increased availability of different food sources.
Step-by-step explanation:
In 1983, the abundance of rain would likely result in a greater variety of plant species and therefore an increase in the variety of seed sizes. This suggests that in 1984, the bird population would be diverse in terms of beak sizes due to the availability of different food sources.
Birds with large beaks for fruits and seeds might be more common as they can handle a wider range of seed sizes, including larger and harder ones that are produced in greater abundance during rainy years.
Similarly, birds with small beaks for seeds may also thrive if the rain promotes the growth of plants that produce small seeds. Specialized beaks for insects could be common if the rain leads to an increase in insect populations.
Thus, we can predict that diverse beak sizes for varied food sources would be the most common, as the varied plant life would support a range of food sources for birds with different beak adaptations.