Birds Change Their Priorities as Summer Progresses
Nest-building and raising nestlings start in the springtime and continue into the summer for most birds that visit feeders. Although this entire process can be completed in a month's time, the losses due to predators, disease, or weather often require songbirds to make several breeding attempts prior to successfully raising young. Despite these setbacks, many are able to raise two or more broods by mid-summer. It's hard work to find enough food to support the rapid growth of nestlings. Defending the nest against predators takes a lot of energy, too. The wear, tear, and stresses of producing and rearing the young must eventually be repaired. This repair cycle usually takes place in late summer when tattered feathers are replaced, the immune system is rejuvenated, and nutrient stores begin to be replenished.
Nature's Pantry Is Full of a Changing Variety of Foods
Insects abound throughout early summer and their high nutrient content, especially protein, makes them a priority for provisioning nestlings. As the season progresses, seeds and fruits emerge, which are high in energy and are prized by adults and fledglings. The relative ease of procuring seeds and fruits makes them especially valuable for inexperienced fledglings and for adults in need of replacing feathers and building fat reserves.
Birds Can only Do so Much at Once
Nature's foods are most nutritious in the spring and early summer, so birds time their highly demanding reproductive activity to this time of the year. As summer progresses, food quality declines and there are diminishing prospects of rearing another brood to maturity prior to the bad weather and declining food supplies of autumn. Consequently, parents change their priority to rehabilitating themselves before food becomes limited.
The Role of Supplemental Feeding
In general, nutritious foods are readily available in the summer, and some species are less likely to visit bird feeders than they do in the colder seasons. Nevertheless, many people who feed wild birds see more birds visiting their feeders during the summer than at any other time of the year. For example, during the reproductive period, adults of species that specialize in seeds (granivores) continue to be frequent visitors in order to supply their own needs even though they feed their brood a diet of mostly insects. As priorities switch from breeding to self-maintenance, supplemental foods from feeders attract a greater diversity of species. Fledglings also appreciate a readily available source of food as they learn how to procure nature's offerings. The high level of energy in seeds fuels feather replacement and builds fat reserves. Some birds even take seeds to their own private larder to serve as a cache during winter.
Article by Kirk Klasing. Dr. Klasing is a professor of Comparative Avian Nutrition in the Department of Animal Science, University of California, Davis