House sparrows exhibit individual differences in generalization when confronted with different novel stimuli

Generalization refers to the process by which animals assign different stimuli into cognitive categories based on their similarity/dissimilarity to previously experienced stimuli. Stimuli that strongly deviate from previous experience may be avoided due to their novelty (i.e., neophobia), while stimuli that are sufficiently similar to a known/recognized cue may elicit the same response as that cue (i.e., generalization). While generalization has been widely researched, few studies have examined among-individual variance in its expression. Our study quantified among-individual variation in neophobia and generalization in house sparrows (Passer domesticus) by placing a series of different objects next to each subject's food source on successive days and measuring the subject's latency to approach the food source in the presence versus absence of each object. Similar to previous work, we found that house sparrows are neophobic on average and exhibit significant among-individual variance in neophobia. More interestingly, we found that the neophobic response declined across presentations of different novel objects. This implies that the sparrows generalized some aspect(s) of the objects. We also found significant among-individual variance in the rate at which approach latency changed across this series, possibly reflecting individual differences in propensity to generalize. These results raise new questions about how neophobia, habituation, and generalization are linked and about the potential for selection to act on these traits under different ecological conditions.