The intense growth period of a baby bird, known as the nestling stage, requires an immense amount of energy and fuel. From hatching to fledging, the young bird must increase its body mass rapidly, sometimes doubling or tripling its weight in a matter of days. This rapid development demands a near-constant supply of high-protein food to build feathers, bone, and muscle tissue. This energy transfer from parent to offspring sets the stage for the extraordinary feeding demands observed at the nest.
The Staggering Frequency of Feeding
The question of “how many” is best answered by considering frequency, as the volume of food delivered is nearly continuous during daylight hours. For many small songbirds, parent birds visit the nest with food anywhere from 50 to over 100 times per day. This astonishing rate emphasizes the high-octane metabolism of the nestlings, which burn energy at a prodigious pace.
Feeding rates are often measured in visits per hour, with some species averaging 15 to 20 trips an hour, especially during the peak growth period. Newly hatched birds require feeding every 15 to 20 minutes from sunrise to sunset because their digestive systems are small and food passes through quickly. This provisioning is driven by the nestlings’ high metabolic rate, necessary to sustain their exponential growth and generate body heat.
The parents are pushed to their physiological limits to maintain this feeding schedule, sometimes flying cumulative distances that exceed 100 kilometers in a single day. As the nestlings grow, the parents often increase the size of the food item they deliver rather than increasing the frequency of visits. This strategy allows the adult birds to maximize the energy delivered per trip, helping to manage their own sustained metabolic effort.
Beyond Worms: The Real Nestling Diet
The popular image of a parent bird delivering an earthworm to its young is largely a misconception for many species. While some birds, like robins, do feed worms, the primary diet of most insectivorous nestlings is high-protein, soft-bodied arthropods. This includes prey items such as caterpillars, spiders, and various insect larvae.
Caterpillars, in particular, are a favored food source because they are rich in both protein and carotenoids, which are vital for immune function and healthy development. Studies of certain songbirds have shown that larval Lepidoptera (butterflies and moths) can comprise up to 87% of the estimated prey biomass delivered to the nest. The food delivered must also contain a high moisture content, as young nestlings cannot leave the nest to drink water.
Parent birds select prey that is easy for the young to digest, often avoiding insects with hard exoskeletons, like many adult beetles. This soft-bodied diet ensures that the nestlings receive the necessary nutrients quickly and efficiently for their accelerated growth. The nutritional quality of the arthropods plays a direct role in the prey selection made by the adult birds.
Biological Factors Determining Feeding Volume
The precise amount of food a nestling consumes is not a fixed number, as it is influenced by several biological and environmental variables. The age and developmental stage of the young is the most significant factor, with food demand increasing dramatically as the chicks approach fledging. A bird that is only a few days old requires far less food volume than a bird nearing the point of leaving the nest.
Species differences also play a large role, as the feeding requirements of a small finch are vastly different from those of a larger corvid. Birds with a larger clutch size (more mouths to feed) necessitate a higher total number of parental visits to the nest. However, the food volume provided per individual chick may be lower in larger broods.
Environmental factors, such as ambient temperature, can also alter a nestling’s energy needs. In colder weather, young chicks must burn more energy for thermoregulation, which directly increases their caloric demand and the required feeding volume. Conversely, a prolonged rainy period that reduces the availability of invertebrate prey can constrain the parents’ ability to meet the high demand, leading to slower growth rates for the nestlings.