The results indicate that season does not influence the response of birds to allopatric calls. Great tits approached the sound source during both seasons and there was no significant difference in response between seasons. It is important to note, however, that a much bigger sample size is required to definitively support the null hypothesis (failure to reject the null hypothesis in this case is a long way from supporting the null) and to gather enough certainty about what the data is showing. An approach response can be a result of either (i) great tits perceiving the chickadee call as a general heterospecific mobbing call, and so approach to determine if a predator is present, or (ii) approach may be due to the novelty of the call as it is produced by a species that is unfamiliar to the great tit. I found that great tits responded more strongly towards chickadee mobbing calls than towards the chickadee territorial song, suggesting that great tits perceive chickadee calls as mobbing calls and that their response is not due to novelty. These results corroborate findings in previous studies (Randler 2012; Salis et al. 2021a) and indicated that great tits view chickadee call as a mobbing call. It would be of interest to compare the response of great tits to mobbing calls from conspecifics versus mobbing calls of allopatric species. I previously found that the responses of great tits to conspecific mobbing calls were similar to those expressed in response to mobbing calls emitted by chickadees, indicating that great tits extract information equivalent to their own mobbing calls (Dutour et al. 2017, 2020).
There is one main possible explanation why the response of great tits to allopatric calls is not influenced by the season: great tits could generalize responses from conspecific calls to allopatric calls that are acoustically similar (Ghirlanda and Enquist 2003; Sturdy et al. 2007). Such a hypothesis might explain these results as there is suggestion of acoustic similarity between the mobbing calls of great tits and black-capped chickadees calls (Dutour et al. 2017), and because season does not influence the response of great tits to conspecifics mobbing calls (respectively, 50% and 65% of great tits approached the loudspeaker during playbacks of mobbing calls in spring and winter; Salis et al. 2021b). In contrast, however, results of a recent study indicated that great tit and black-capped chickadee mobbing calls are indeed dissimilar (Dutour et al. 2020). Future studies are needed to gain further insight into the perception of allopatric calls and the generalization process in great tits.
My findings are in line with previous studies looking at the approaching behaviour of great tits in response to black-capped chickadee mobbing calls during spring (Randler 2012; Dutour et al. 2017, 2020; Salis et al. 2021a). This study’s result does, however, contradict the findings of a previous study which found an increased mobbing intensity in autumn at the heterospecific communication level (Dutour et al. 2019b). Together, these results suggest that great tits may respond more to sympatric heterospecific calls during the non-breeding season because they have an increased number of opportunities to learn to associate these calls with predatory threats when they form mixed-species flocks at this time of year.
In the present study, I investigated the mobbing behaviour of great tits during only two seasons of the year. Consequently, the study is somewhat limited as I was not able to take into account the responses of the great tits during winter, when predatory pressure is higher. In addition, I investigated variation in response between seasons; however, it would be of interest to investigate variation within season since some studies have documented a temporal intensification in mobbing behaviour during the breeding period (Montgomerie and Weatherhead 1988; Redondo 1989).
It has been suggested that great tits may be more effective in discriminating unfamiliar calls in winter, consequently leading to high scanning behaviour and low approaching behaviour toward unfamiliar calls (Salis et al. 2021b). My results show that this is not the case in autumn, although future work should measure other behavioural variables such as scanning, wing flicking, tail flit or calling in order to provide a more reliable overview of the response of the birds (Cully and Ligon 1976; Salis et al. 2021a).