Sewall K and Hahn T. (2009) Social experience modifies behavioral responsiveness to a preferred vocal signal in red crossbills, Loxia curvirostra. Animal Behavior. 77: 123-8

Vocalizations of birds often vary among individuals and groups, but more functionally, they vary depending on responses to different conditions or situations. For example, one call variation could indicate the quality of food found by a scout, the threat of a predator, or the importance of a conspecific nearby. The relevance of call variations can be affected by social experience. Contact calls, in particular, function to coordinate movement and sustain contact between members of the group. Responses to these contact calls may also vary and the plastic nature of these calls could support changes in interaction between conspecifics and potentially, movement among different social groups.

Red crossbills, loxia curvirostra observed in this study, are an example of an ecologically diverged species, termed ecomorph. The different ecomorphs have adapted a variety of changes in body size, beak shape, and vocalization according to their environment. Ecomorphs 3 and 4 and calls from ecomorph 2 were used in this study. Two main experiments were performed to test these call features. The first tested the relative response of one ecomorph to a conspecific before and after being paired with a conspecific for a long period of time. The second tested the response to callbacks from crossbills of the same ecomorph and then to playbacks of a different ecomorph.

Results showed that the crossbills responded more strongly to calls of their own morph. Birds that were paired with a different ecomorph responded equally to both the other type with which it was paired and to its own type of crossbill. Control birds that were paired with their own type continued calling most strongly to birds of their own type. These results are consistent with previous research on response specificity to contact calls in other birds.

The findings of this study support the proposed plastic nature of contact call responses of crossbills and suggest that the function of these calls can apply to cross-ecomorph communication when the birds are in the significant presence of different subtypes. Contact calls allow birds to keep in touch, determine where others of their clan are, and indicate future movement.

Usually, the different subtypes of birds would not be in such close contact because of their ecological differences; however, if global warming, for example, had a significant effect on their territory, they may end up moving closer to another ecomorphs region. If these birds are in close enough contact with another clan that they begin hearing the other call on a regular basis, they will start to recognize and respond to that call as well as their own, perhaps representing a pseudo-merging event between the two groups. I say “pseudo” because the study indicates that though these birds will respond to a different call, it is unlikely that any birds of that different ecomorph would be welcomed or accepted into their inner circle community, but rather simply recognized as another member of that regional area. Perhaps these contact responses could even turn into territorial calls to let the nearby group know that getting any closer would be trespassing. With these possibilities in mind, we can imagine how the different subspecies might react to environmental and/or social changes through the contact call.