Imprinting in Birds
by Dan Gleason, © 2010
Once free of the confines of a hard-shelled egg, a young bird must quickly learn to cope with its new world if it is to survive. Recognition of your parents is important early in life and recognition of your own species will be important for future development and socialization. This recognition comes as a result of a process known as imprinting. But the process of imprinting that works for one species may not be the best strategy for another.
Precocial chicks (see supplement) will leave the nest soon after hatching. It is imperative that they quickly learn to recognize their own parents and follow them so that they can be protected and learn how to find food. On the other hand, altricial birds have more time to learn who their parents are and to more slowly develop the needed skills to survive. This parental recognition is properly termed filial imprinting. It is filial imprinting that most people are referring to when they speak of “imprinting,” although other types of imprinting will occur as well.
Filial imprinting is strongest in precocial birds, particularly geese, ducks and most grouse. There is a critical time period during which this type of imprinting will occur, but in precocial species, it will always be during the first day, and for many species, within the first hour or two after hatching. Many studies have shown that young ducks and geese will imprint on the first large moving object that they see. Their nest-mates are moving but not large enough to trigger the response and a nearby tree is large enough but not moving. A combination of both attributes are necessary. Fortunately for most chicks, the first large moving object that they see is their mother. Imprinting on their mother assures that they will have the best opportunity to minimize surrounding environmental threats, such as predators, adverse weather or lack of food. If the first large moving object seen is not the parent, the results could be disastrous or fatal for the young bird. Hand-raised waterfowl often imprint on the first human they see, which works well enough if the person is intending to provide the care for these young. The object need not be living for imprinting to occur: there is a record of a Ruffed Grouse imprinting on a farm tractor, following it around the fields and later, doing courtship displays in front of it.
While the exact timing of the critical period for filial imprinting varies between different species of precocial birds, it will usually occur within the first thirty-two hours after hatching. After that time, no filial imprinting will occur and any imprinting that has taken place is fixed and generally cannot be changed for the remainder of the bird’s life. Ducks will eventually lose the bond with their parents as they strike out on their own in the fall, but swans will usually stay together for at least a season and even migrate in small groups that represent an extended family.
Altricial species differ from precocial species in that they lack this immediate filial imprinting. Since altricial birds hatch blind, their only immediate sensory input for possible imprinting is aural or tactile. Most birds have little or no sense of smell so odor is not used for imprinting as it is in some fish or mammals, and a thick coat of feathers, even just down feathers, greatly reduces tactile abilities, making it of less importance. Sound is therefore most important and the hatchlings of many birds learn to recognize their parents’ vocalizations just as the adults learn the sounds of their young. For example, in Wood Ducks, females “talk” softly to the eggs a few days before they hatch, and a day or two before hatching responses can be heard coming from within the eggs. At hatching, the ducklings respond strongly to the sounds of their own mother but become quiet if a female Mallard’s voice is experimentally substituted instead. In some precocial birds, the chicks respond best to a combination of sound and visual cues.
Instead of responding to only the parent, most hatchlings of altricial birds raise their heads and gape, begging to be fed by whoever they hear arrive at the nest, be it parent or possible predator. They remain silent only if they know and hear sounds from their parents that mean to lie still and keep quiet. While there is no immediate filial imprinting in altricial birds, other types of imprinting occur later.
As all chicks grow and develop, they will undergo another type of imprinting, one that provides recognition of their own species. It is important to know what species you are in order to establish proper social interactions with your own kind later in life. The timing of this type of imprinting is variable between species but begins at least several days after hatching. In some birds there is a recognitions of species but no recognition between parent and young. Northern Rough-winged Swallows, for example, will not feed chicks of other species of swallows but will care for other Rough-winged Swallow chicks that are placed in their nests. Bank Swallows, in contrast, will feed only their own young, ignoring all others.
Recognition of your own species is not only important for proper social interactions of an immature or adult bird, but it becomes important for sexual imprinting, or identity, as well. This type of, imprinting helps define not only which species to select for mating, but, often, which color morph is preferred. For most species of birds, mates will most often be selected based on what the bird’s parents looked like. For example, there are two very different color phases of Snow Goose: the all white “Snow Goose” pattern and the darkly mottled “Blue Goose.” Young geese raised by white parents will select white mates and those raised by dark parents will select a “Blue Goose” form as a mate. Those raised by mixed parents may select either type when the time comes to mate. The timing of sexual imprinting is highly variable between different species of birds, but develops well after hatching and occurs in both precocial and altricial species of birds.
A human-raised raptor will have no filial imprinting after hatching because it hatches blind, but if humans are its only role models and source of food and protection, then it will imprint on the human for species recognition and sexual imprinting. As a sexually mature adult, it will reject its own kind for mating and seek the attention of humans.
Imprinting, as discussed thus far, varies in intensity between different species and between precocial and altricial species. That some form of imprinting occurs, is important for a majority of species. But such imprinting is detrimental to a few species, in particular, brood parasites. Cowbirds, cuckoos and other brood parasites must not imprint on their host species in any way if they are to successfully find and breed with their own kind later in life. Their species recognition is instinctual and completely uninfluenced by their foster parents. But even in these birds, there is an additional type of imprinting that must occur and that is habitat imprinting.
Before becoming completely independent, fledglings imprint on their surrounding habitat. This is especially important to migratory species that will need to know what habitat is important to look for when breeding time arrives. Even when a species is found over a wide range of habitats, first-time breeding birds look for habitat similar to that in which they were raised. An American Robin raised near suburban neighborhoods will return there rather than to an Aspen thicket where Robins may also be found. For some birds, this habitat recognition becomes an even more precise geographical recognition. In such cases, birds look for not only the right habitat but return to the same general geographic region in which they were raised, often only a short distance away.
Habitat and geographical imprinting are most important to those species that disperse some distance after nesting or are migratory. It is less important in sedentary birds. Bewick’s Wrens, for instance, typically disperse only one-half to one mile or less from where they were hatched and here they will select a mate and spend the remainder of their lives. Wrentits seldom disperse even one-quarter mile from their hatching site. For these species, dispersal is over such a short distance that they are assured of finding the correct habitat and habitat imprinting is weak.