DOI: 10.5061/dryad.6wwpzgmvw
title: Habitat fragmentation shapes natal dispersal and sociality in an Afrotropical cooperative breeder

description: 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.
description: 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.
description: 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