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Raihani & Ridley: Variable fledging age in a cooperative bird
Variable fledging age according to group size: trade-offs in a
cooperatively breeding bird: Electronic supplementary material
Study species
Pied babbler groups typically comprise a dominant breeding pair plus 1 – 10
subordinate adult helpers (mean 3.2 ± 0.2) adults (any individual over 12 months old)
per group. Individuals were identified using colour rings and sexed using a DNA test
(described in Radford & Ridley 2006). All group members feed the young produced
by the dominant pair. Young continue to receive food from adult group members for
58.9 ±1.9 days post-fledging (Ridley & Raihani 2007). Nests are open-cup and
usually situated 5 - 6 m high in camelthorn trees (Acacia erioloba). Brood size ranges
from 1 – 4 (2.3 ± 0.2) nestlings.
Habituation
All babblers in the population were habituated to the close presence (< 2m) of a
human observer. This meant that observers could walk freely among the foraging
group without causing individuals to stop foraging or to emit alarm calls. See Ridley
& Raihani (2007) for a detailed description of the habituation process. As is the case
for many altricial species (Griffin 2004), newly fledged babblers take their cues
regarding predators from adults and newly fledged young were therefore not alarmed
by human observers. Instead, newly fledged young seemed impervious to our
presence, to the extent that young occasionally ran over our feet as they moved
between trees.
Raihani & Ridley: Variable fledging age in a cooperative bird
Statistical methods
We checked that data conformed to the assumptions of the tests used and transformed
data where necessary. All statistical tests were two tailed. Where appropriate, we used
(Generalized) Linear Mixed Models ((G)LMMs). These are similar to general linear
models, but allow repeated measures to be included as random terms, thus controlling
for their effects on the distribution of the data. LMMs had a normal distribution of
errors and an identity-link function. GLMMs had a binomial distribution of errors and
a logit-link function. All statistical analyses were conducted using Genstat 8.1 (Lawes
Agricultural Trust 2005).
Fledging age
Accurate fledging ages (days post-hatching) were calculated by checking nests daily
for hatching and fledging. Same-brood nestlings typically fledged on the same day.
Where they fledged on consecutive days (2 / 40 broods), a mean fledging age was
calculated for the brood. All nestlings were ringed and weighed on day 11 posthatching. The mean weight (g) of all nestlings in each brood was calculated. Fledging
age was set as the response term in a LMM. The effects of group size and mean
nestling weight (g) on fledging age were included as potential explanatory terms.
Group identity was included as a random term.
Feeding rate
All feeds by all adults were recorded and categorized according to food item size.
Using pre-determined biomass values for food size categories (see Raihani & Ridley
in press) we calculated the total biomass provisioned to each chick (by all adults) per
observation hour (g / chick / h). This value was log transformed and set as the
Raihani & Ridley: Variable fledging age in a cooperative bird
response term in a LMM. The number of days post-fledging (-1 / 0 / +1) and group
size were included as potential explanatory terms. Data on fledging day (day 0) were
collected only while young were still in the nest. Data were restricted to 55
observation sessions where groups were observed for at least 60 minutes. Group (N =
9) and brood (N = 35) identities were included as random terms in the model.
Nestling & fledgling predation
Babblers are primarily terrestrial foragers and use their bills to dig in the substrate to
uncover food (Ridley & Raihani 2007). As such, individuals are vulnerable to both
aerial and terrestrial predators. Common aerial predators at this study site included
pale chanting goshawk (Melierax canorus), lanner falcon (Falco biarmicus), spotted
eagle owl (Bubo africanus) and giant eagle owl (Bubo lacteus). Most terrestrial
predators at this site can climb trees and may thus predate nestlings as well as
fledglings. Terrestrial predators included cape cobra (Maja nivea), yellow mongoose
(Cynictis penicillata), slender mongoose (Galerella sanguinea), small spotted genet
(Genetta genetta) and African wild cat (Felis siverstris).
To investigate whether fledging young early reduced the risk of predation, we
determined whether nestlings or fledglings were more vulnerable to predators, and
whether there was an effect of group size on predation rates. All instances of offspring
predation during the variable fledging period (13-19 days post-hatching) were
recorded, as well as whether individuals were predated before or after fledging.
Nestling predation was inferred if nests were deserted during this period. We checked
deserted nests for bodies to rule out the possibility that nestlings starved. Fledgling
predation was inferred if fledglings disappeared during this period. Dispersal was
Raihani & Ridley: Variable fledging age in a cooperative bird
ruled out since fledglings were still nutritionally dependent on adults at this stage
(Ridley & Raihani 2007) and individuals never dispersed in the first four months postfledging (Raihani & Ridley in review).
We used a GLMM to investigate the effect of group size and food availability
on predation during the variable fledging period (13-19 days post-hatching). As
fledgling predation was extremely rare during this period (only 4 / 120 fledglings
predated), we restricted the analysis to nestling predation. For each brood, we
recorded how many nestlings were predated during the variable fledging period. This
was set as the response term in the model. The total number of nestlings in the brood
was set as the binomial total. Rainfall was used as a proxy for food availability (sensu
Ridley & Raihani 2007) as there is often a protracted period of time between rainfall
and increased insect abundance (Cumming & Bernard 1997). The total rainfall (mm)
in the 60 days prior to hatching was measured and categorized as low, medium or
high, according to the lower and upper quartiles of the data. Data from 60 broods
from 15 groups were available. Group identity was specified as a random term in the
model.
Distance moved by fledglings
On the day of fledging we took waypoints every 15 minutes using handheld GPS
trackers (Garmin, UK). The total distance moved (m) by fledglings was measured by
calculating the distance between consecutive waypoints using the programme
MapSource Version 6 (Garmin, UK). This was set as the response term in a LMM,
with group size and fledging age (days post-hatching) included as explanatory terms.
Raihani & Ridley: Variable fledging age in a cooperative bird
Data come from 33 broods from 11 groups. Group identity was included as a random
term.
Cumming, G. S. & Bernard, R. T. F. 1997 Rainfall, food abundance and timing of
parturition in African bats. Oecologia. 111, 309-317.
Griffin, A. S. 2004 Social learning about predators: a review and prospectus. Learning
& Behaviour. 32, 131-140.
Radford, A. N. & Ridley, A. R. 2006 Recruitment calling: a novel form of extended
parental care in an altricial species. Curr. Biol. 16, 1700-1704
Raihani, N. J. & Ridley, A. R. In press. Adult vocalizations during provisioning:
offspring response and post-fledging benefits in wild pied babblers. Anim. Behav. doi
10.1016/j.anbehav.2007.02.025
Ridley, A. R. & Raihani, N. J. 2007 Facultative response to a kleptoparasite by the
cooperatively breeding pied babbler. Behav. Ecol. 18, 324-330.
Ridley, A. R. & Raihani, N. J. 2007 Variable post-fledging care in a cooperative bird:
causes and consequences. Behav. Ecol. doi 10.1093/beheco/arm074