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4.3 Methods
4.3.4 Nest-site selection
We collected habitat data for each Spotted Thick-knee nest site to determine whether certain habitat characteristics were preferred by breeding pairs when selecting locations for their nests.
Measures of fine-scale microhabitat characteristics were recorded within a 10 m radius sample site with the nest as the central point. The circular plot was further divided into four equal sections (sections A to D) that intersected at the nest as the central point (Supplementary Fig.
S4.1). We averaged the data from each of the four sections to represent the sample site, and which was known as a ‘nest-plot’ in the study. We recorded information on the following nest- plot characteristics: land cover, vegetation height, slope, altitude and nest-cover structure.
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For each Spotted Thick-knee nest site, we recorded percentage estimates of land cover composition for bare ground surface cover, man-made (i.e., concrete), grass and vegetation (inclusive of leaf litter). Vegetation structures, other than species of grasses, were expressed as proportions of the vegetation cover within the nest-plot according to categories of the structure’s max height level in metres, whereby they either belonged to height level 1 (0.15 m
< height < 2 m), level 2 (2 m ≤ height ≤ 5 m), or level 3 (height > 5 m) (Supplementary Fig.
S4.1). Height level 1 had a minimum value 0.15 m because structures below that level were not found at any site, but this could have been because of obstruction from tall grass height in some nest-plots. Grass height in metres (m) was estimated and recorded separately from vegetation data.
We found Spotted Thick-knee nest sites either had an absence of a nest-cover structure, or a shrub-like species or a tree species as a nest-cover structure. Nest-cover structure height was measured using a sectional pole and recorded the same way for all nest-cover structures.
However, the method for width of shrub-like structures differed from that of nest-cover structures which were trees, although both measurements were taken using a measuring tape.
Width for shrub-like structures was measured from the side of the structure that had the nest under it. Tree widths were measured by the commonly used dendrometric measurement of
‘diameter at breast height’ (DBH), which was always from 1.65 m above ground because the same investigator carried out all measurements. Patterns of nest construction were not investigated in this study because all nests were made from scrapes in the ground, and only one nest was lined with material such as dry leaves (pers. obs.). We used a GPS to record the nest- plot altitude or height above sea level in metres (m. a.s.l), with the nest as the reference point.
Slope degree (°) of the nest-plot was measured using an optical clinometer (Brunton Lensatic, F-OMNISLOPE) for the most discernible slope of a 5 m line length with the nest at the midpoint (i.e., 2.5 m) (Supplementary information Fig. S4.1). Slope angle/gradient was
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considered at a fine-scale level (< 5 m) of topography as it has been suggested to be influential in nest-site selection, but other factors may overshadow its importance if slope is measured at a larger scale beyond this range (Korne et al. 2020).
Estimates of surface land cover percentages, average grass height and measurements specific to the nest-cover structure were taken on the first observation day of the Spotted Thick- knee nest site. They were considered as important factors in nest-site selection and measuring them at a later time in the nesting period could have altered results in the study. For example, short grass height may have been a factor of preferential site selection for a nesting attempt, therefore, measuring the grass height at a later stage where it could have grown, would have biased findings of the study. Site measurements of variables such as vegetation heights, altitude and slope were taken once a nesting attempt had concluded, or the nest site was deemed inactive because the breeding pair was not observed and there were no eggs in the nest. These variables either had: no support in existing literature as important factors of selection for ground-nesting species; the investigator assumed them to have a slight change in a measure during the nesting period which would not impact study findings; they would have taken too long of a time to measure during the early incubation period, therefore, increasing investigator disturbance and risk of failure on the nesting attempt.
We selected 26 random site locations to examine the differences between actual nest sites and potential nest sites available for breeding pairs. The coordinates for these random sites were computer-generated projected into Google Earth version 7.3. Random sites that were inaccessible were replaced with locations that had suitable accessibility to allow for appropriate data collection of ‘hypothetical’ nest-plots. Permission was granted by homeowners where a site was within their property. Identical approaches to data collection were undertaken at both actual and random nest sites. These sites were visited, and the closest vegetation structure (tree or shrub-like) within a 2 m distance from the initial random location was selected as one of the
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26 random site locations. If there was no vegetation structure within 2 m, then that site was considered as one with no nest-cover structure.