APPENDIX S1
METHODS
SABAP data collection protocol
Data collection for SABAP are as follows: checklists are submitted to SABAP by ‘citizen
scientists’ who are registered with the programme. Citizen scientists tend to be keen birders,
and over 5000 individuals have submitted checklists for SABAP to date (Harrison et al.,
2008; http://sabap2.adu.org.za/). There are some differences between SABAP1 and SABAP2
in their spatial and temporal resolution. Spatially, SABAP1 was implemented on quarterdegree-grid cell (QDGC) resolution which, in South Africa, translates to an area ca. 26 km x
27 km. SABAP2 is surveyed at the pentad scale (9 pentads in one QDGC; ca.9 km x 9 km
each), although these data are available from SABAP collated to QDGC level, to facilitate
comparison between SABAP1 and SABAP2, and we use these collated data in this study
(http://sabap2.adu.org.za/). For each checklist card submitted to SABAP, the citizen science
observer spent a minimum of 2 hours intensive observation time in a single pentad
(SABAP2) or QDGC (SABAP1), recording all bird species. Further species could then be
added to the same checklist card for the pentad or QDGC, up to a maximum of one month
(SABAP1) or five days (SABAP2) after the initial 2 hour observation period (Bonnevie,
2011). In both SABAP1 and SABAP2, surveys were conducted year-round. There were no
strong seasonal patterns in the numbers of checklists submitted to SABAP per month in either
survey period, although these tend to be slightly lower in winter and slightly higher in spring
(http://sabap2.adu.org.za/). For further information, see http://sabap2.adu.org.za/.
Centre of gravity shift in the pied crow population within South Africa
We assessed whether the changes in abundance in Pied Crow populations in South Africa
observed between SABAP1 and SABAP2 constituted a centre of abundance (“centre of
gravity”) shift in the population.
In order to do this, we divided the QDGCs into “northern”, “southern”, “eastern” and
“western” depending on latitude and longitude, respectively. Given the location of South
Africa, “northern” QDCGs were those falling between the Botswanan, Zimbabwean or
Namibian border and -28.5° latitude (mid-latitude in South Africa); “southern” QDCGs fell
between -28.5° and the south coast. “Eastern” QDGCs were those with longitude greater than
25° (mid-longitude in South Africa) and “western” QDGCs were those with longitude less
than 25°.
We conducted a series of 4 paired t-tests (two-tailed) to assess the magnitude of differences
in reporting rate in northern, southern, eastern and western QDGCs between SABAP1 and
SABAP2.
We also plotted reporting rates of Pied Crows from SABAP1 and SABAP2 at 1° resolution
latitude and longitude, respectively, in order to visually assess the trends.
RESULTS
Two-tailed paired t-tests showed that reporting rates of Pied Crows decreased significantly in
the North and East of South Africa, but increased significantly in the South and West ,
between SABAP1 and SABAP2 (Table S1).
Visual examination of the data confirms that absolute reporting rates of Pied Crows were
highest overall (suggesting crows were most abundant) in Southern and Western regions of
the country in SABAP2, whereas reporting rates were more evenly distributed in SABAP1
(Figure S1).
The changes in abundance of Pied Crows between SABAP1 and SABAP2 therefore seem to
constitute a centre of abundance shift in the population towards the South and West.
TABLES
Table S1: Difference between SABAP1 and SABAP2 reporting rates of Pied Crows in
eastern, western, northern and southern regions of South Africa. The data show declines in
the east and north, and large increases in the west and south. Reporting rates are presented as
means ± 1 s.d. for each region. Statistics are drawn from two-tailed paired t-tests.
Region of South Africa
Reporting rate
SABAP1
Reporting rate
SABAP2
T –value
df
p-value
East of 25° longitude
West of 25° longitude
North of -28.5° latitude
South of -28.5° latitude
0.35 ± 0.31
0.28 ± 0.28
0.32 ± 0.31
0.33 ± 0.28
0.32 ± 0.29
0.47 ± 0.34
0.27 ± 0.28
0.47 ± 0.32
-2.95
14.92
-4.98
13.21
782
374
580
576
0.003
<0.001
<0.001
<0.001
FIGURES
Figure S1: Comparison of SABAP1 and SABAP2 reporting rates of Pied Crows along (a)
latitudinal and (b) longitudinal gradients within South Africa. SABAP1: open circles and
dashed line; SABAP2: filled circles and solid line. Note the higher reporting rates of Pied
Crows in the south and west of the country during SABAP2. Reporting rates are presented as
means ± 1 se per each 1° increment in latitude and longitude, respectively.