Birds come in all shapes and sizes, and some species can seem downright pudgy – but do birds really get fat? There are many reasons why birds may look overweight or obese, but birders who understand more about birds' appearances and metabolism are better prepared to help them avoid any possible weight problems.

Why Wild Birds Don't Gain Weight

Birds are constantly foraging, and for many species, a good portion of their diet consists of high calorie, high fat or high sugar foods, such as sunflower seeds, suet, nectar, nuts and peanut butter.

But despite how much birds may eat, their biology and lifestyle keeps them from gaining excessive weight.

• High Metabolism: Birds have a naturally high metabolism and require large amounts of food to convert to energy for flight, generating body heat and maintaining their biological processes. Because of that, even with what might seem like an unhealthy diet to humans, wild birds rarely store fat – all the food they consume is used for energy.
   
• High Activity Levels: Birds are active creatures that require large amounts of energy as fuel. They must forage not only for themselves, but also to feed chicks or brooding mates, and they must constantly be in the lookout for predators. During migration, birds will also use much more fuel to sustain them on their lengthy journeys.
   
• Caching: Even if a bird visits a feeder repeatedly, the food isn't always consumed immediately. Many birds cache food for future use, particularly in late summer and autumn when they are storing food in anticipation of winter's scarcity. Not all of those cached seeds or nuts are eaten, and cached seeds are responsible for many new trees and shrubs each year.
   

• Mistaken Identity: Birders may think that one or two birds are doing all the eating at their feeders, but because many birds look identical (at least to humans), different birds may be visiting without birders realizing it. If one feeder is being emptied every day, the birds aren't overeating if a few dozen birds are doing the snacking.

Birds Can Look Fat

Many birders and non-birders alike are certain they've occasionally seen fat birds, but birds can give the appearance of being fat without actually having a weight problem. Different ways birds may seem fat include:

• Shape and Proportion: Some birds have naturally round body shapes, deep chests, short necks and other features that can make them seem pudgy. Thrushes, for example, generally have broad chests for their large lungs that support their stunning voices and complex songs, but they are not fat birds. Other birds that are shaped in ways that can give the appearance of fat include pigeons, game birds and penguins.
   
• Fluffy Feathers: Birds fluff out their feathers for many reasons, including creating better insulation to keep warm in winter, spreading out while sunning or rearranging their plumage while preening. When the feathers are puffy, the birds can seem much rounder and fatter than normal, but the change is size is due to feather orientation and air pockets, not fat.
   
• Posture: Birds can dramatically change their appearance when they change their posture. The American bittern, for example, can seem long and lanky when its neck is stretched out to mimic surrounding reeds as camouflage, but when it retracts its neck while stalking prey, the bird can appear much pudgier. The angle a birder views a bird from can also make posture and weight differences seem extreme.
   
• Crop Storage: Raptors may seem exceptionally fat after they have eaten and food is stored in their crop. Because it can be difficult to capture prey, raptors gorge when they are successful hunting, and the excess food is stored in their crop to be digested more slowly. As the crop bulges, it gives the appearance of a heavier, fatter bird.
   
• Natural Weight Fluctuations: While birds don't become obese, they do have natural weight fluctuations. Many migratory birds, for example, enter hyperphagia before migration, and during that period they may nearly double their weight while they store fat to fuel their journeys. By the end of migration, however, they will be back to their more typical weight. Many penguins also have extraordinary weight fluctuations relating to their nesting cycle, when they will incubate eggs for weeks without feeding.

Bird Obesity

Though most birds are never notably fat, there are unique circumstances where birds can gain too much weight and could be considered dangerously obese, such as…

• Pet Birds: Because pet birds are not as active as their wild cousins, many pet birds are susceptible to weight gain. Pet bird owners must monitor their birds' diets very carefully to avoid overfeeding that can lead to fat birds.
   
• Domestic Poultry: Ducks, geese, turkeys and chickens that are raised for meat are often much fatter than wild birds, but that weight gain is deliberately managed to increase farmers' profits by creating more meat to sell.
   
• Urban Waterfowl: When birds have unhealthy diets of bread and other human handouts, they do not need to expend as much energy foraging for natural foods, and they may gain unnecessary weight. This often happens with feral waterfowl colonies in urban areas.

Help Birds Control Their Weight

While weight gain is not a concern for most wild birds, backyard birders can help their favorite avian visitors maintain healthy weights by offering a range of good foods and keeping less nutritious items, such as kitchen scraps, reserved for rare treats. Providing natural foods such as berry bushes or seed-bearing flowers is an easy way to encourage birds to have a healthy diet, and feeders can also be carefully monitored to portion foods out in healthy quantities, rather than providing an unending smorgasbord.

By understanding how birds could look fat – and why they're not – backyard birders are better prepared for noticing unusual bird behavior and knowing when a bird's weight might truly be a problem.

Photo – European Robin © Alan Cleaver