Kidnapping intergroup young: an alternative strategy to maintain group size in the group-living pied babbler (Turdoides bicolor)

Abstract

Both inter- and intragroup interactions can be important influences on behaviour, yet to date most research focuses on intragroup interactions. Here, we describe a hitherto relatively unknown behaviour that results from intergroup interaction in the cooperative breeding pied babbler: kidnapping. Kidnapping can result in the permanent removal of young from their natal group. Since raising young requires energetic investment and abductees are usually unrelated to their kidnappers, there appears no apparent evolutionary advantage to kidnapping. However, kidnapping may be beneficial in species where group size is a critically limiting factor (e.g. for reproductive success or territory defence). We found kidnapping was a highly predictable event in pied babblers: primarily groups that fail to raise their own young kidnap the young of others, and we show this to be the theoretical expectation in a model that predicts kidnapping to be facultative, only occurring in those cases where an additional group member has sufficient positive impact on group survival to compensate for the increase in reproductive competition. In babblers, groups that failed to raise young were also more likely to accept extragroup adults (hereafter rovers). Groups that fail to breed may either (i) kidnap intergroup young or (ii) accept rovers as an alternative strategy to maintain or increase group size. This article is part of the theme issue 'Intergroup conflict across taxa'.

Keywords: Turdoides bicolor; cooperation; extinction; intergroup interactions; kidnapping; pied babbler.

Figure 1.

The relationship between adult group size and territory size. The line of best fit is generated from the output of the LMM: shaded areas represent s.e.; grey dots are raw data. (Online version in colour.)

Figure 2.

The relationship between the probability of group extinction and (a) number of chicks recruited during the breeding season and (b) adult group size. Curves are generated from the output of the model presented in table 1: shaded areas represent s.e.; grey dots are raw data. (Online version in colour.)

Figure 3.

The relationship between the probability of kidnapping behaviour and chick recruitment within the focal group during each breeding season. The curve of best fit is generated from the output of the model presented in table 2: the shaded areas represent s.e.; grey dots represent raw data. (Online version in colour.)

Figure 4.

The relationship between the probability of a group accepting a rover and (a) the number of chicks recruited during the breeding season and (b) relative group size. Curves are generated from the output of the model presented in table 3: shaded areas represent s.e.; grey dots represent raw data. (Online version in colour.)

Figure 5.

Values of the right-hand side of equation (4.1b) as a function of relatedness and group size. If the survival odds improvement is of at least the indicated magnitude (e.g. 1.2 = 20% improvement) when group size increases by one individual, then the benefits of kidnapping exceed the costs in the present situation. (Online version in colour.)