DOI: 10.5061/dryad.9cnp5hqn1
title: Kin do not always help: testing multiple hypotheses on nest feeding in a cooperatively breeding bird

description: In cooperatively breeding species, group members may derive multiple
 benefits from helping to raise other individuals’ offspring, yet not all
 individuals do so. In this study, we tested several hypotheses to explain
 why group members feed offspring of breeding placid greenbuls
 (Phyllastrephus placidus). In accordance with the kin selection
 hypothesis, all helpers were first-order kin of the breeding female and
 the presence of helpers was associated with increased survival of the
 breeding pair. However, the propensity to help varied widely among group
 members, as 46% of group members related to the breeding female did not
 feed nestlings. Sex, relatedness to the breeding male, and group size did
 not explain additional variation in helping propensity or effort, however,
 younger helpers fed offspring more often than older ones. Overall, our
 results show mixed support for predictions of the group augmentation
 hypothesis, while predictions of the pay-to-stay hypothesis and skills
 hypothesis were not supported. We suggest that costs associated with
 providing food to nestlings, and benefits of other types of helping
 behavior (e.g. anti-predator behavior), may jointly explain why members of
 breeding groups often refrain from helping at the nest.
description: <strong>Study system and
 data collection</strong> Data were collected during the breeding seasons
 of 2012-2018 in the Dabida forest archipelago of the Taita Hills (SE
 Kenya; 30°25'S, 38°20'E). The landscape consists of a
 heterogeneous mixture of indigenous cloud forest remnants (&lt;1 ha to
 120 ha), exotic plantations and small-scale subsistence agriculture. Eight
 of these forest remnants contain subpopulations of the placid greenbul, a
 long-lived insectivorous passerine from East Africa’s moist forests that
 occupies pair- or group territories year-round (DVL & BA personal
 observations). Their breeding season coincides with the onset of the short
 rainy season in November and lasts until March. Typically, breeding
 females lay and incubate two eggs that hatch synchronously, and pairs
 generally re-nest after breeding failure and (occasionally) also after
 successful breeding. Approximately 50% of greenbul offspring in our study
 area consist of extra-pair young sired by neighboring territory owners
 (Cousseau, Van de Loock,
 et al. 2020). Nest failure
 is mostly due to predation (Spanhove et al. 2014) and larger breeding groups are associated with
 lower rates of nest predation and higher post-fledgling survival
 (Van de Loock et al. 2017;
 Van de Loock 2019). Due to
 continued ringing and nest monitoring effort since 1996 and 2007,
 respectively, ca. 75% of the greenbul population is color-banded at any
 time (based on the ratio of color-banded individuals vs. total traps
 during mist-netting). In our study area, the vast majority of
 subordinate group members are offspring from previous breeding season(s)
 that delay their natal dispersal, although non-natal individuals may
 occasionally join breeding groups (Cousseau, Hammers, et al.
 2020). Both subordinate
 males and females show delayed dispersal, but males delay on average
 longer and eventually
 start breeding closer to their natal territory than females
 (Cousseau, Hammers, et al.
 2020).
 While males disperse
 earlier when born in smaller forest patches compared to larger ones
 (Cousseau, Hammers, et al.
 2020), this is not
 reflected in spatial variation in the likelihood that a breeding group
 contains subordinates, neither in the size of these groups
 (Van de Loock
 2019).
 Placid
 greenbul breeding behavior was monitored in all eight known subpopulations
 of the Dabida forests. Upon detection, each nest was visited every 3 to 5
 days until all nestlings had fledged or the nesting attempt failed. At an
 age of ca. nine days, nestlings were individually color-banded, measured
 and a blood sample was taken. We recorded group size, identified breeding and
 subordinate group members, and quantified their helping behavior and
 individual nestling provisioning rates through a combination of focal
 observations, targeted mist-netting, and nest video recordings.
 We
 conducted focal observations during incubation by hiding less than 10m
 from the nest (2012-15 only) as well as opportunistically at each nest
 visit (all years). We erected mist nets around the nest when nestlings
 were ca. 5 days old (range
 3 – 8 days) or after nest depredation. Upon trapping, birds were measured,
 a blood and/or a feather sample was taken and the breeding status (cloacal
 swelling or brood patch) determined (see below). Unringed individuals were
 metal- and color-banded. Between 2012 and 2015, focal observations and
 mist-netting were conducted using playback of greenbul distress calls for
 a maximum duration of 10 min. As group members respond to these playbacks
 by approach and displays, presumably to distract and lead away potential
 predators, this method proved to be a rapid and efficient way to assess
 group membership and trap individuals. Playback was not used when a
 predator was present. When nestlings were ca. 8 days old (range 6 – 10
 days), we video-recorded food provisioning for 5-6 hours continuously
 between 7 a.m. and 2 p.m. using a HD camera (Sony Corp.) installed on a
 tripod about 1.5 m from the nest (one video per nest; on average 25 videos
 taken per year). Besides helping behavior, video recordings allowed us to
 extract hourly provisioning rates (number of feeds per hour per nestling)
 of breeders and helpers. We only extracted provisioning rates when we
 could identify the visiting individuals in at least 70% of all visits over
 the total recording period. Each breeding group consisted of the territorial
 breeding pair (henceforward called ‘breeders’) and all subordinates (if
 any) observed at a particular nest. Breeders were identified based on
 cloacal swellings (males), the presence of a brood patch or observed
 incubation (females). All other individuals present at a nest were
 assigned as subordinates. A subordinate was classified as a ‘helper’ if it
 was observed feeding nestlings at least once during the nest video
 recording. It follows that subordinates that were never observed feeding
 nestling during this period were classified as ‘non-helping subordinates’.
 Whether or not a subordinates helped during a breeding event is called
 ‘helping propensity’. We measured ‘helping effort’ as the number of feeds
 per hour per nestling. The age of each group member was assessed from its
 fledging year (when ringed as nestling) or its feather development at
 first capture (‘juvenile’, ‘immature’ or ‘fully-grown’ based on molt
 patterns of primaries, secondaries and coverts; Jackson 2005). Individuals were sexed using a set of
 sex-linked primers P2/P8 (Griffiths et al. 1998) and relatedness to the breeding pair was based
 on 12 microsatellite loci (see Husemann et al. 2015 and Cousseau, Van de
 Loock, et al. 2020 for
 primer details). We used the exclusion method (Jones and Ardren 2003; by using a self-written
 R script) to identify
 first-order kin (son or daughter). Previous analysis of known
 mother-offspring combinations in our study species showed genetic
 mismatches at a single locus in only 8% of the cases (Cousseau, Van de Loock, et al.
 2020). Group members were
 therefore considered first-order relatives of a male or female breeder if
 they either showed no mismatch or a mismatch at a single locus, and were
 otherwise considered non-relatives.