Birds are important in the life cycle of many plants, because they distribute plant seeds in a number of ways. Plants advertise their seeds to birds by providing colourful, energy-filled fruits or large nuts as a food source. Plants such as the bird cherry have evolved along with birds, reaching a point where their seeds require a pass through a bird's digestive system to help them prepare for germination.
On Their Bodies
Seeds may attach to the feathers of birds through structures present on the seed, such as small hooks or barbs. Birds may disperse seeds, such as those of mistletoe, by carrying them on their beaks after feeding. Mistletoe attaches to other plants; it's not rooted in soil. According to the University of Nevada, Las Vegas, its seeds are covered in a sticky substance; birds wipe their beaks on a branch to rid themselves of the seeds, allowing mistletoe to reach the environment it prefers.
In Beaks or Claws
Birds may carry seeds to a new location in their beaks or claws. A bird may be planning a meal or building a nest, and fly off with a seed or fruit to a safe area. Along the way, the bird may drop whole fruits or a few seeds, allowing them to reach new spots, far from the parent plants. Birds may also drop seeds in or near water, helping them to travel further or reach a moist location. Because birds can fly around barriers and slip into small spaces, they may carry seed to places where larger animals, the action of the wind or water may not.
The seeds of some plants, including the nuts of some large trees, serve as a food source for birds. Feeding birds may knock seeds from stems, cones and flower heads as they eat. While this may not spread the seeds far from the parent plants, it can bring the seeds into contact with the soil, where they can overwinter and germinate into a new generation of plants. Birds may also partially crack or nick the seed coat trying to open a seed, allowing it to more easily germinate.
Other plants depend on birds to forget where they have hidden or buried seeds, effectively distributing the seeds and allowing them to colonise a new area.
In some cases, birds do not digest and receive energy from the seeds they consume. Instead, they seek the fruits and berries surrounding the seeds, and the seeds come along with the bargain. Birds ingest the flavourful pulp; the seeds inside the pulp then move through the birds' digestive systems and are passed out along with their droppings. The fecal material gives seeds a small dose of fertiliser. High in nitrogen, the excrement can help fuel the development of foliage in the young plants.