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Can all birds regurgitate food when feeding their young?

Sent in by Craig Rot, Ireland

Not all birds can regurgitate food.

There are several main reasons for regurgitation and it is employed in a number of different ways.

The first example is in gulls and other birds that transport food over long distances. Many of the larger gulls forage over hundreds of kilometres and over several days, so it is impractical to carry fish around because they would be pirated by other gulls and skuas.

Some seabirds are able to spontaneously regurgitate, even as juveniles. Fulmars employ this as a defence when disturbed at the nest.

Terns also regurgitate fish but these are likely to have been locally caught and are not digested to any great extent. This is also a strategy to avoid piracy, although some species will not swallow the catch if foraging close to the nest.

The second example is finches and other seed-eating birds. Each species has a slightly different strategy in terms of how much regurgitated seed is provided to the young.

Surprisingly, many seed-eating species provide soft-bodied invertebrates as a more important element in the diet of the youngsters, but all provide some regurgitated seed as a way of passing on gut bacteria to help digestion of problematic chemicals such as cellulose.

Finally (although there are many more examples), swifts, swallows and martins produce a ball of insects called a bolus. Many of the individual food items are small and, as with the above examples, it would be impractical for the birds to return with these in their beaks, so they are stored until sufficient numbers have been collected.

The food is not always swallowed because the bolus can be formed in the crop and is not true regurgitation given the food is not necessarily mixed with digestive juices in every case.

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