DOI: 10.5061/dryad.547d7wmhp
title: Data from: Urban bird commensals maintain coexistence under extreme food shortages

description: In developed cities, bird communities are typically comprised of a few
 dominant invasive species that can cause considerable social and economic
 costs. While various studies advocate restricting anthropogenic food as a
 suitable management approach, a significant knowledge gap persists
 regarding how these species interact and respond to such an intervention.
 Here, we evaluate if limiting a shared food resource may affect their
 abundances similarly and assess if such limitations influence their niche
 dynamics. In Singapore, open food centres for people, colloquially known
 as hawker centres, serve as key food sources for three highly adapted
 urban birds: feral pigeons, Javan mynas, and house crows. We counted these
 three species across 63 hawker centres and analysed their niche dynamics
 across different phases – before, during COVID-19 social restrictions when
 dining-in was prohibited, and during an enforcement phase mandating the
 return of crockery. We modelled their counts, diet niche widths, and niche
 overlaps, considering predictors which include the sampling phases, food
 availability, structural characteristics of hawker centres, and spatial
 attributes such as distance to public housing. During social restrictions,
 feral pigeon and Javan myna counts showed a significant decline, while the
 count proportions of the three species compared to each other remained
 relatively stable. Hawker centres closer to bridges and public housing,
 and those that structurally more open, attracted more birds. The niche
 widths of feral pigeons and Javan mynas significantly narrowed during
 social restrictions due to reduced food availability. However, their niche
 overlaps remained consistent across sampling phases, indicating resource
 partitioning strategies to cope with extreme food shortages – feral
 pigeons adapted by foraging more on grass verges outside, while Javan
 mynas frequented tray return stations. This resilience in maintaining
 species proportions and the absence of significant niche overlap suggested
 the existence of an ecological balance despite substantial reductions in
 available food. Synthesis and applications. Our study underscores the
 importance of controlling human-provided food to collectively manage
 dominant urban bird commensals. Beyond the two social restriction phases,
 curbing the availability of anthropogenic food through enforcement also
 kept nuisance birds away, validating a cost-effective approach in reducing
 their counts.
description: A total of 63 hawker centres in Singapore were surveyed from 2019
 to 2021. We conducted area searches and recorded the counts of feral
 pigeons, Javan mynas and house crows within the hawker centre premises and
 10m of its perimeter (i.e., outside). To minimise double-counting, we only
 counted individuals that were detected in frontal view and omitted
 individuals that flew in from other sections of the hawker centre. The
 same surveyor conducted the surveys between different sampling occasions.
 We also observed their activity (i.e., feeding, perching and others which
 constitute moving either by flying, hopping or walking), perch location
 (i.e., ground, table, chair, elevated structures, tray return station and
 outside) and foraging substrate (i.e., ground, table, chair, crockery,
 tray return station and others). A tray return station is an open shelving
 unit for returning crockery, utensils and trays.
 Predictors affecting bird commensal counts included food
 availability, structural attributes, spatial factors and weather. Food
 availability metrics included the number of food stalls operating, the
 frequency of feeding incidences, food waste detected (within a
 1m<sup>2</sup> grid), uncleared crockery (i.e., plates and
 bowls that were left unattended), and the proportion of uncleared to total
 tray station levels. Additionally, we quantified the number of active
 cleaners and the extent of deterrent measures used (i.e., bird spikes,
 screens at tray station, nets, dummy predators and wires) by rating them
 from a scale of 0 to 3 (not used, low, medium to high coverage
 respectively). The scales for each measure were summed to obtain an
 overall deterrence metric. We also measured two structural characteristics
 of each hawker centre – the total length of elevated structures (e.g.,
 beams, pipes, stall sign-boards and vents) representing available perches
 for birds to rest, and the openness as a proxy of accessibility, measured
 as a cross-sectional area in m<sup>2</sup> (including access
 points for patrons and ventilated areas) with the aid of a laser
 rangefinder. Spatial predictors included the proportion of public
 residential housing within a 200m radius of a hawker centre, and the
 distances to nearest Mass Rapid Transit viaduct and bridge; structures
 commonly used by urban adapted birds for nesting. Last, we noted if the
 weather was clear (i.e., no rain or otherwise) during our
 surveys. To test the effect of food reduction on our
 bird counts, we surveyed the same 63 hawker centres during two social
 restriction phases and a tray return enforcement phase when mandatory
 return of crockery was enforced in hawker centres (Table 1). These three
 surveys were conducted at 1330 and 1500 as our previous surveys showed the
 highest bird count at these times. Table 1: Effective
 dates and survey dates during pre-COVID-19, social restriction and tray
 return enforcement phases. 
 
 Phases Phase start
 Phase end Survey start
 Survey end 
 Pre-COVID-19 -
 - 4-Nov-19
 27-Dec-19 Circuit
 Breaker 7-Apr-20
 1-Jun-20 4-May-20
 18-May-20 Phase 2
 Heightened Alert 16-May-21
 13-Jun-21 16-May-21
 13-Jun-21 Tray
 return enforcement 1-Sep-21
 - 3-Sep-21
 30-Sep-21 
 We used aggregated counts of feral pigeons, Javan mynas and house
 crows separately at different perches for each sampling phase and hawker
 centre to quantify their niche widths. The niche widths were calculated
 using the Shannon-Weiner index since our data were discrete. For niche
 overlaps between the feral pigeon and Javan myna, we used Morisita’s index
 of similarity since it showed the least bias with changing sample size,
 number of resources, and resource evenness. We used
 general linear mixed models (GLMM) with negative binomial error
 distribution and log link function to determine the factors affecting
 their counts. To determine the effect of food availability on feral pigeon
 and Javan myna niche widths at our hawker centres, zero inflated gamma
 GLMM with log link function was used since our response included zero and
 positive values. Zero-one inflated beta regression (ZOIB) with random
 effects was used to determine the factors that influence the proportion of
 niche overlap between the feral pigeon and Javan myna.
description: ## Urban bird commensal maintain coexistence under extreme food shortages
 --- **Bird count data** 56 out of 63 hawker centres initially surveyed
 were used for bird count analysis 7 hawker centres were omitted to reduce
 autospatial correlation; all retained hawker centres are at least 400m
 apart Occasion - period when the surveys were conducted: Before CB
 (Circuit Breaker), CB, P2HA (Phase 2 Heightened Alert), and Enforcement
 (i.e., after the enforcement of the tray return policy) Hawker_centre -
 name of hawker centre R.pig - rock dove or feral pigeon count J.myn -
 Javan myna count H.cro - house crow count Stalls_open - number of hawker
 stalls opened at 1330 and 1500 divided by number of repeat surveys. In
 Before CB phase, the surveys were repeated twice at 1330 and same for
 1500. Hence, Stalls_open = total stalls open / 4 *Next five variables
 below reflect the average values as described above for Stalls_open*
 Feeding_incidences - number of bird feeding incidences within hawker
 centre Total_FW - total food waste Uncleared_crockery - number of crockery
 left uncleared (i.e., not returned to the tray return stations)
 Sqrt_Perc_Uncleared_levels - proportion of tray levels that remained
 uncleared (square-root transformed) Active_cleaners - number of active
 cleaners working Total_deterrence - sum of the extent of all bird
 deterrences used Total_length (m) - total length of the elevated
 structures (e.g., beams and pipes) in hawker centres that can be used for
 perching Total_openness (square m)- total area available to birds to enter
 and exit the hawker centre Sqrt_Perc_HDB - Percent of area covered by
 Housing Development Board (HDB) housing with 200m radius Distance_to_MRT
 (m) - distance to nearest Mass Rapid Transit (MRT) train station
 Distance_to_bridge (m) - distance to nearest bridge in m Weather - clear
 or rain --- **Niche data** 56 out of 63 hawker centres initially surveyed
 were used for niche analysis 7 hawker centres were omitted to reduce
 autospatial correlation; all retained hawker centres are at least 400m
 apart Occasion - period when the surveys were conducted: Before CB
 (Circuit Breaker), CB, P2HA (Phase 2 Heightened Alert), and Enforcement
 (i.e., after the enforcement of the tray return policy) Hawker_centre -
 name of hawker centre Time - time when the surveys were conducted nw_pig -
 Shannon-Weiner niche width for feral pigeon nw_myn - Shannon-Weiner niche
 width for Javan myna nw_crow - Shannon-Weiner niche width for house crow
 pigNmyn_m - Morisita niche overlap between feral pigeon and Javan myna
 Stalls_open - number of hawker stalls opened Feeding_incidences - number
 of bird feeding incidences within hawker centre Total_FW - total food
 waste Uncleared_crockery - number of crockery left uncleared (i.e., not
 returned to the tray return stations) Sqrt_Perc_Uncleared_levels -
 proportion of tray levels that remained uncleared (square-root
 transformed) Active_cleaners - number of active cleaners working
 Total_deterrence - sum of the extent of all bird deterrences used
 Total_length (m) - total length of the elevated structures (e.g., beams
 and pipes) in hawker centres that can be used for perching Total_openness
 (square m) - total area available to birds to enter and exit the hawker
 centre Sqrt_Perc_HDB - Percent of area covered by Housing Development
 Board (HDB) housing with 200m radius Distance_to_MRT (m) - distance to
 nearest Mass Rapid Transit (MRT) train station Distance_to_bridge (m) -
 distance to nearest bridge Weather - clear or rain Missing data code: NA