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Habitat fragmentation shapes natal dispersal and sociality in an Afrotropical cooperative breeder

Citation

Cousseau, Laurence et al. (2020), Habitat fragmentation shapes natal dispersal and sociality in an Afrotropical cooperative breeder, Dryad, Dataset, https://doi.org/10.5061/dryad.6wwpzgmvw

Abstract

It remains poorly understood how effects of anthropogenic activity, such as large-scale habitat fragmentation, impact sociality in animals. In cooperatively breeding species, groups are mostly formed through delayed offspring dispersal, and habitat fragmentation can affect this process in two opposite directions. Increased habitat isolation may increase dispersal costs, promoting delayed dispersal. Alternatively, reduced patch size and quality may decrease benefits of philopatry, promoting dispersal. Here, we test both predictions in a cooperatively breeding bird (placid greenbul, Phyllastrephus placidus) from an Afrotropical cloud forest archipelago. Males born in fragmented forest dispersed about one year earlier than those born in continuous forest. Contrary to females, males also started to reproduce earlier and mostly settled within their natal patch. Females only rarely delayed their dispersal for more than one year, both in fragmented and continuous forests. Our results suggest that early male dispersal and reproduction is jointly driven by a decrease in the value of the natal territory and an increase in local breeding opportunities in fragmented forest. While plasticity in dispersal strategies of cooperative breeders in response to anthropogenic change is believed to optimize reproduction-survival trade-offs, to what extent it shapes the ability of species to respond to rapid environmental change remains to be studied.

Methods

Data were collected during the breeding season (from November till March) in the indigenous cloud forest archipelago of the Dabida massif of the Taita Hills (30°25'S, 38°1250 20'E). During eleven breeding seasons (2007-2019; no data available in 2010 and 2011), placid greenbul nests were monitored in eight subpopulations. Upon detection, each nest was visited every fourth day until all nestlings had fledged or the nesting attempt failed. Prior to fledging, at around nine days old, nestlings were individually colour-banded and a blood sample was taken. During nest visits, a combination of focal observations, video-recordings, and targeted trapping sessions were used to identify the members of a breeding group and determine their breeding status. Upon trapping, each adult was metal- and colour-banded, aged, and a blood and/or a feather sample was taken. Each ’breeding group’ consisted of the territorial adult breeding pair (henceforward called ‘dominant breeders’) and all subordinates (if any) observed at a particular nest. Dominant breeders were identified based on cloacal swellings (males), or on the presence of a brood patch or observed incubation (females). All other individuals present at a nest were assigned as subordinates. Individuals were sexed molecularly using a set of sex-linked primers P2/P8.We constructed resighting histories of 481 individuals that fledged between 2007 and 2017. During each breeding season (until 2019), birds that were resighted in a breeding group were classified as breeder, philopatric subordinate or as non-natal subordinate. A subordinate was considered philopatric if it was resighted together with its presumed mother or father or if it was never observed farther than 100 m away from its natal nest. Otherwise it was considered a non-natal subordinate. Individuals that were not resighted were given the status ‘unknown fate’. This ‘unknown fate’ category combined three different cases: (i) individuals that were dead; (ii) individuals that were alive and part of a breeding group as subordinate or breeder, but that were not resighted; (iii) individuals that were alive and had dispersed from a breeding group and (temporarily) employed a ‘floating’ strategy, and that were therefore not resighted in a breeding group (Supporting Information S1). Since only five individuals were resighted as non-natal subordinates, these were lumped with birds of ‘unknown fate’ to avoid over-complexity of the model.

Usage Notes

At every resighting occasion (i.e. a breeding season, columns X2007 to X2019)

- 0 is when a bird was not yet born or not resigted

- 1 is when an individual was resighted as philopatric subordinate (exept first occurence: fledging year)

- 2 is when an individual was resighted as non-natal subordinate

- 3 is when an individual was resighted as breeder