10.5061/DRYAD.KD51C5B30
Downs, Colleen
0000-0001-8334-1510
University of KwaZulu-Natal
McPherson, Shane
University of KwaZulu-Natal
Muller, Rebecca
University of KwaZulu-Natal
Sumasgutner, Petra
University of Vienna
Amar, Arjun
University of Cape Town
Data from: Urbanization is associated with increased breeding rate, but
decreased breeding success in an urban population of near-threatened
African Crowned Eagles (Stephanoaetus coronatus)
Dryad
dataset
2020
Stephanoaetus coronatus
urban birds
Urban areas
National Research Foundation
https://ror.org/05s0g1g46
Downs - various
D’RAP Ethekwini Municipality*
various
University of KwaZulu-Natal
https://ror.org/04qzfn040
various
FitzPatrick Institute of African Ornithology
https://ror.org/05rgme147
various
Claude Leon Foundation
https://ror.org/00xtpbs68
Sumasgutner - various
Ford Wildlife Foundation*
various
D’RAP Ethekwini Municipality
various
Ford Wildlife Foundation
various
2021-04-01T00:00:00Z
2021-04-01T00:00:00Z
en
https://doi.org/10.3356/JRR-17-83
https://doi.org/10.2989/00306525.2018.1449027
https://doi.org/10.2989/00306525.2016.1259185
https://doi.org/10.1016/j.landurbplan.2015.10.004
https://doi.org/10.1007/s11252-015-0500-6
31765 bytes
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CC0 1.0 Universal (CC0 1.0) Public Domain Dedication
Urban areas can be attractive to certain species because of increased food
abundance and nesting availability which in turn may increase productivity
or breeding rates. However, there are also potential costs associated with
urban living such as higher nest failure, poorer body condition or
increased prevalence of disease. These costs may result in species trading
off the number of young produced against the condition of their young.
African Crowned Eagles (Stephanoaetus coronatus) are a rare example of
large, powerful apex predators that breed in some urban areas in Africa.
In this study, we explored the breeding performance of these eagles across
an urbanization gradient in KwaZulu-Natal Province, South Africa, over
seven breeding seasons. We predicted that living in an urban environment
would increase productivity through an increase in breeding rate (shifting
from typically biennial breeding to annual breeding). We then explored if
there were any hidden costs associated with such a change in breeding
strategy by examining the body condition of chicks from pairs which had
successfully bred in the previous year. We found that pairs in more urban
areas were more likely to breed annually, resulting in higher breeding
rates, but were also less likely to successfully fledge a chick (i.e.,
lower breeding success). These two contrasting responses counteracted each
other and resulted in similar productivity across the urbanization
gradient. For those eagles that bred in consecutive years, annual breeding
did not appear to have a negative cost on chick condition. The switch to
annual breeding is thought to be a response to improved or more constant
food sources in urban areas, while higher failure rates might be because
of increased nest disturbances from anthropogenic sources (e.g.,
vegetation clearing, development of industrial areas, human and car
traffic). However, although urbanization negatively affected the breeding
success of African Crowned Eagles, they are able to persist and thrive in
this highly transformed environment likely through an increased breeding
rate.
Study Area and Data Collection The study area covered ~20,000 km2 in
southern KwaZulu-Natal Province, South Africa, centered on the metropoles
of Durban and Pietermaritzburg, and extended to several coastal towns both
north and south of Durban (Figure 1; McPherson et al. 2016a, b, 2019).
Crowned Eagle nesting sites were initially found by networking with
interested individuals/groups (local birding experts, Birdlife and
Falconry club members, and online community groups), and accessing
unpublished databases from Durban Natural History Museum, eThekwini
Municipality, and SABAP2, as well as by direct searching in suitable
habitat or where territorial displays were observed. Crowned Eagle nest
monitoring was conducted from August through January the following year
for the years 2011 to 2017 (i.e., 7 breeding seasons), which covers their
peak annual breeding period in the area (McPherson et al. 2016a, b).
Territories were visited regularly, at least twice in the first month, in
order to assess occupation (e.g., nest building, incubation or brooding
behavior). A nest was classified as active if nest building or fresh green
leaves were seen on the nest or if the adults were present in either of
these first two nest visits. A nest was classified as having a breeding
attempt if incubation or brooding behavior was seen. Nests with a breeding
attempt were then monitored during 2-3 nest site visits until conclusion
of the breeding event (i.e., until the chicks were around 70±5 days old).
Breeding success was defined as having a chick survive until banding age
(70 ± 5 days old). After this age, failures in this species and most other
large raptors are relatively low (Brown, 1976). Nests were observed from
vantage points generally 50 – 200 m away from the nest (see details in
McPherson et al. 2016a). Crowned Eagle chicks were banded when
their estimated age was 65 - 75 days, a time window recommended by experts
(S. Thomsett and B. Hoffman pers. comm.). The age estimates used in this
study were ascertained by photo reference material of pulli of known age
(McPherson et al. 2017) and were based on size and plumage development.
During banding, chicks were weighed (with an electronic hanging scale to
the nearest 5 g) and the total length and unfurled length of the 8th
primary feather was taken (with a straight ruler to the nearest 1 mm). All
measurements were done in accordance with the SAFRING user manual (de Beer
et al. 2001). Urbanization Score In order to establish the percentage of
urbanization around each nest site, we used the LandCover 2014 raster
(GEOTERRAIMAGE, 2015), which classifies land use into 72 different
categories. We chose a circular buffer area of 10 km2 (radius = 1784.1 m)
based on the mean home range size of the species during the breeding
season from four telemetered adults in the study area (McPherson et al.
2019). Once the percentage of each land class around each nest site had
been calculated, the values for all land classes containing sealed surface
(see Rose et al. 2017) were used to calculate an urban score (%) for each
nest. Examples of what land classes constituted sealed surface are urban
residential, industrial, townships, and mines. In territories where there
was more than one nest, the mean urban score was taken to represent the
territory. Statistical Analyses All analyses were conducted in R version
3.5.1 (R Core Team 2018) with the packages ‘lme4’ (Bates et al., 2015),
‘car’ (Fox and Weisberg, 2018), and ‘effects’ (Fox, 2003). All means are
presented with standard deviations. Generalized Linear Models (GLMs) or
Linear Mixed Models (LMMs) were used to analyze the data. An initial model
selection for the GLMs considered both the linear or quadratic
relationship between urbanization and our response variables, as a
quadratic relationship could reveal changed breeding demography at
intermediate levels of urbanization. In all cases, the linear relationship
had the best model fit (lowest AIC) and thus only linear relationships
were considered in the final analyses. We explored how
urbanization affected several Crowned Eagle breeding parameters over seven
breeding seasons. GLMs with a binomial distribution were used to
investigate the effect of urbanization on three key breeding parameters
using the cbind function. These three variables were i) breeding rates:
modelled as the total number of attempts and number of non-attempts (i.e.,
no nest building activity (nest lining, mating behaviour, incubation etc.)
at a previously occupied nest) across the years a territory was monitored;
ii) breeding success: modelled as the total number of successful breeding
attempts and number of failures across the years in which a territory was
active; and iii) breeding continuity: modelled as the number of continuous
breeding attempts (i.e., no gap between breeding attempts) and the number
of non-continuous breeding attempts (i.e., with at least 1 year gap
between breeding attempts) for the total number of years monitored. This
binomial approach also accounted for differences in the number of years of
data for each territory, by effectively weighting each sample according to
the total number of years monitored (models i and iii) or total number of
active years (model ii). Additionally, a different GLM was used to
investigate Crowned Eagle productivity in relation to urbanization. Here
the response variable was the total number of young fledged across all the
years each territory was monitored. Models were fitted with a Poisson
distribution, with an offset specified as the log of the number of years
monitored. An LMM was used to explore whether a Crowned Eagle
breeding attempt or, more importantly, a breeding success in the previous
year, had an influence on the body condition of chicks. For this LMM, the
response variable was the condition of each chick (n = 72), where chick
condition was the residual from a linear regression of weight against the
length of the 8th primary feather. The explanatory variable was either
attempt (t-1), where 0 = no attempt previous year, and 1 = attempt the
previous year; we also ran the same model but specifying success (t-1),
where 0 = no successful chick produced in the previous year, and 1 = chick
successfully produced in the previous year. ‘Year’ and ‘Territory
Identity’ were included as random terms to account for the repeated
measures from the same territory and from different territories in the
same year. As Crowned Eagles only fledge 1 chick per breeding attempt we
did not need to control for the number of chicks in a nest.
Legend for data file The headings for each data sheet included in the
data file are described below. Units are included where required. Nest
productivity data sheet: Site ID The nest site ID which describes the
location of each nest. No.yrs monitored The number of years the
individual nest site has been monitored. No.attempt The number of
breeding attempts made at the individual nest site. No.young The number of
chicks that have hatched at each the individual nest site. Breeding
success The number of breeding attempts divided by the number of young.
Breeding rate The number of breeding attempts divided by the number of
years monitored. Continuity The number of consecutive breeding attempts
across all the years monitored. Urbanization The percentage of
urbanization at each nest site. No.chicks The number of chicks that
fledged at a nest site that were ringed and measured. Average chick
condition The average body condition for the no of chicks fledged and
measured at the nest site. Chick body condition data
sheet: Site ID The nest site ID in which the chick was born.
Year
Year the chick was ringed and measurements taken. Incubation(attempt)
Incubation was observed at the nest and so this is classified as a
breeding attempt. Ringing age (success) The chick from the nest reached
ringing age (70±5 days) and so this is classified as successful breeding.
t-1(attempt) 0= no breeding attempt the previous year, 1= breeding attempt
the previous year. t-1(success) 0= no successful breeding the year before,
1= successful breeding the year before. Sex 1= Male, 2= Female. P8tot
Primary feather number 8 - total length in (mm) Mass Mass of chick in (g)
Chick body condition The residuals of a linear regression of P8tot against
Mass. Urbanization The percentage of urbanization at each nest site.