Abstract:
Avoiding predation risk and finding food resources are essential for the survival of all organisms. Consequently, many animals encode information about food and predators in their calls. Some species thereby use discrete call types, whereas others encode information by a variation within one call type or by altering the proportion of different call types. It is known for many passerine bird species, including great tits (Parus major), the study species of this thesis, that they encode information about predators in their calls. If great tits, however, are also able to decode such information from conspecific calls remains unknown. Further, if great tits use information encoding also in other, non-predatory, contexts, has so far not been investigated.
Therefore, this thesis focuses on the ways of information encoding used by great tits in various contexts and if conspecifics are able to discriminate between calls of different context. I conducted three experimental studies in the field to address these questions.
In the first chapter, I investigated how great tits call in response to taxidermy mounts of two different predators, a high-threat and a low-threat predator. The calling behaviour of tits varied greatly between the two treatments and great tits used four ways of encoding information about predator threat: call rate, D call duration, D element number and interval between D elements. Tits increased all of the four acoustic parameters when confronted with the high-threat predator. The variation in the interval between elements is so far only known from one North American passerine species, whereas the other three ways of encoding information are commonly found in passerines.
The study of the second chapter, investigated if great tits can discriminate between mobbing calls of two different contexts. I broadcasted calls recorded during the first study to great tits and measured the latency time until they approach the speaker within a radius of six meters and the minimum distance to the speaker. If tits can discriminate between mobbing calls of different contexts, one would most likely observe a difference in behaviour in response to those calls. Indeed, I found birds to have a longer latency time and to keep a greater distance to the speaker when hearing calls of the high-threat context compared to calls of the low-threat context. This indicates that the message encoded in mobbing calls transmits information about predator threat to conspecifics that allows them to alter their behaviour. In addition, there was a difference in behaviour between the two sexes, as males approached the risky situation faster and closer than females. Consequently, mobbing behaviour seems not only to be affected by predation threat, but also by individual factors such as sex.
Lastly, in the third chapter, I compared calls of a mobbing context with calls of a food-associated context. Again, call rate was higher in the mobbing context compared to the food-associated context. Further, I found a variation in the proportion of call types between the two contexts. Great tits produced mainly D calls and a small proportion of other call types in the mobbing context. In the food-associated context, in contrast, they decreased the proportion of D calls and increased the proportion of other call types. Lastly, similar to the first chapter, the acoustic structure of D calls seems to convey contextual information as D calls in the food-associated context had longer elements and shorter intervals between elements than mobbing calls. This indicates that great tits use a set of ways to encode information about different contexts.
predator-prey communication