What birds eat and how they get their food can be important identification clues, especially when more obvious field marks may not be easily visible. Birders who take the time to study bird diets will learn more about their favorite species and how subtle diet differences help different bird species exist together without running out of resources.
What Diets Tell Us About Birds
The food a bird eats tells us a lot about the bird, since so many bird diets are specialized. While most birds will at least sample a wide variety of foods and are opportunistic to take advantage of nearly any easily available food source, the main foods that make up the majority of their diet can reveal clues such as:
- Bill Shape: Bird bills are different shapes depending on whether the bird picks, probes, tears, cracks or filters to reach its food. If a bird is eating seeds, for example, the bill shape will be specialized to extract or break into seeds, and the overall bill size will vary depending on the type of preferred seeds. If a birder sees a new bird visiting their Nyjer feeder, it is more likely to be a small finch that can easily eat those tiny seeds, while birds cracking open safflower seed likely have larger, more powerful bills.
- Range: Birds only stay in ranges where their preferred foods are available, both in terms of habitat as well as season. This can help birders know where and when to see birds, particularly target species they are interested in adding to their life list. The bird's diet also determines where they migrate, since they will need to follow suitable food sources throughout the year.
- Habitat: Habitat is a critical component of a bird's diet, and birds only visit habitats that can support their dietary needs. Frugivorous birds are more likely to be found in orchards or areas where fruit trees and berry bushes are abundant, while granivorous birds need weedy fields and meadows with seed-bearing flowers.
- Reproductive State: Bird diets shift when they are mating or nurturing chicks. Some bird species, such as bluebirds, kinglets and swallows, will consume extra calcium early in the mating season, when they need the mineral for strong, healthy eggs. Young chicks require large sources of protein to develop properly, and birds that are feeding hungry hatchlings are likely to have more insects in their diet.
- Migration: Food is fuel for a bird's seasonal migration, and birds' diets can reflect their migrating state. Prior to migration, birds may enter a state of hyperphagia when they feed excessively and gain weight to have a good reserve while they travel. Along their route, birds can appear ravenous at feeders.
Identifying a Bird by What It Eats
In addition to providing more information about the bird, a bird's diet can be an important identification clue. Birders who see birds foraging and eating should watch for specific food-related clues, including…
- Food Type: Exactly what a bird is eating is a good identification clue. Bullock's orioles, for example, enjoy fruit, jelly and nectar at backyard feeding stations, but the similar black-headed grosbeak is more likely to visit seed feeders.
- Food Size: The size of food a bird eats can be an unusual but valuable identification clue. When telling Cooper's hawks and sharp-shinned hawks apart, for example, the size of prey – either a sparrow or a dove – can help clarify which raptor is enjoying the meal by comparing the sizes of predator and prey.
- Foraging Style: How a bird feeds offers great behavioral clues for proper identification. This can be especially valuable for identifying ducks – are they dabblers or divers? Shorebird foraging is also a useful clue – probing or picking behaviors can help distinguish between similar species.
- Caching: A bird doesn't always eat its food immediately, and if the food is cached, the bird's identity can be determined even if the bird itself isn't seen. Evenly spaced, precise granary trees are the work of acorn woodpeckers, for example, while a small bird or mouse impaled on a barbed wire fence belongs to a shrike.
- Feeder Aggression: Some birds share food willingly and may even offer seeds to their mates as part of courtship rituals or continual bonding. Other species, such as pine siskins or rufous hummingbirds, are better known for territorial aggression at feeders and will attack other birds or chase them away from a favored food source.
- Mealtimes: Different birds feed at different times. Some birds, such as northern cardinals, eat early and late in the day and may be both the first and last visitors to feeders. Other birds, such as soaring raptors, must wait until later in the morning when thermal air currents are favorable for their hunting activities.
- Regurgitation: How and why birds may regurgitate food can help identify them. Many birds regurgitate to feed their young, but some vomit a creamy substance called crop milk that is more distinct than partially digested food. Vomiting can be a defense mechanism, such as how vultures vomit to empty their stomachs and escape a threat, or it may be part of the digestive process, like owls regurgitating pellets.
The clues a bird's diet can provide toward its identity may not be enough to provide a confirmed identification, but along with other subtle clues, can be a valuable reference for proper identification. Focusing on diet and foraging behavior can help birders sharpen their identification skills and will provide amazing insights into birds.
Photo – Limpkin With Apple Snail © James Diedrick