DOI: 10.5061/dryad.mw6m905x3
title: Carcasses attract invasive species and increase artificial nest predation in a desert environment

description: In addition to feeding on animal remains, many scavengers also function as
 predators. Carcasses may therefore affect local animal communities by
 attracting facultative scavengers and increasing predation risk for other
 species in the vicinity of the carcasses. This risk may be elevated in low
 productivity environments, especially where humans increase carcass
 production and where facultative scavengers include invasive species. In
 June and October 2018, we monitored experimentally placed red kangaroo
 (Osphranter rufus) carcasses and artificial bird nests in two different
 habitats in the Simpson Desert, Australia, to identify the nest predators
 attracted to the carcasses, and to determine how carcasses affect overall
 and predator-specific nest predation. We modelled our nests to approximate
 those of the ground nesting little buttonquail (Turnix velox) and the
 endangered night parrot (Pezoporus occidentalis). Native Corvus spp. and
 then invasive red foxes (Vulpes vulpes) were the top carcass visitors and
 nest egg predators. Carcass presence and open habitat increased overall
 nest predation and fewer artificial parrot nest eggs were depredated
 compared to those of quail. Open habitat and carcass presence only
 increased predator-specific nest predation by foxes, but corvid nest
 predation was highest in June 2018, and for the artificial quail nest
 types. Foxes were the main predator of eggs from night parrot nests. Our
 study shows that carcass provisioning by humans may have indirect,
 deleterious effects on ground nesting birds, and indicates that foxes
 might pose a greater threat to night parrot populations than previously
 recognised.
description: <strong>Site set-up and carcass
 provisioning</strong> We conducted our
 experiments in June and October 2018 (Australia winter and spring,
 respectively), to account for seasonal differences. Eighty sites were
 established, split evenly across the two study periods and generally
 alternating between carcass ‘present’ (n = 40) and carcass ‘absent’
 (n = 40) sites (Fig. 1b). Within each study period, sites were spaced
 ~0.50 km (range: 0.49–1.68 km) apart, with distances of about 1 km (range:
 0.91–2.59 km) between each carcass ‘present’ site. These distances were
 chosen to minimise carcass scent travel between sites, such that
 scavengers would have to actively forage and seek out carcass sites rather
 than being able to move quickly from one site to another. Carcass-present
 and absent sites were also split evenly between dune crest and valley
 habitats. We classed dune crests as “open” and the inter-dune valleys as
 “closed” habitats. Open habitats were devoid of tree cover, while closed
 habitats were sparsely populated by gidgee trees. The centre point of both carcass-present and
 absent sites was marked with a single black 1 m high stake (Fig. 1c). At
 carcass-present sites, we placed a single red kangaroo
 (<em>Osphranter rufus</em>) carcass (mean 25 kg +/- 0.5 se)
 sourced from pre-planned local culls, and as such were not killed for the
 purpose of this study. Any carcass displaying evidence of disease (e.g.
 heavy parasite loads) was not used. Following collection, carcasses were
 placed in the field without freezing within 24 h (June) or 36 h (October)
 of collection. Scientific licences were obtained to relocate and monitor
 the carcasses (SL WA0006737), and all research was approved by the
 University of Sydney Animal Ethics Committee (Project number
 2017/1173). <strong>Artificial
 nests</strong> Artificial
 nests were installed on NW–SE transects in both carcass-present and absent
 sites, following the direction of the sand dunes. Transects were designed
 to intersect either the central carcass subsidy, or the stake used to mark
 each carcass-absent site. In June 2018, 6 quail nests (see design below)
 were positioned at 10 m, 30 m and 50 m along the transect such that each
 distance was sampled in both the NW and SE direction (total of 240 quail
 nests in June 2018). In October 2018, this method was replicated, with the
 addition of two parrot nests (see design below) set up 10 m on either side
 of the centre points in the carcass-present and absent sites. Parrot nests
 were set up only in dune crest sites, as these nests were constructed
 within spinifex hummocks only found consistently in the dune crest habitat
 (total of 240 quail and 40 parrot nests in October 2018). We modelled artificial nests to replicate those of
 two species of ground nesting birds in the region. The first nest model
 mimicked the nest of the little buttonquail (hereafter ‘quail’ nests),
 while the second mimicked a night parrot nest (‘parrot’ nests). Little
 buttonquails are commonly sighted in our study area and build their nests
 on the ground in grassland, usually at the edge of small shrubs and
 overhanging grasses. In contrast, night parrots build their nests in the
 centre of dense shrubs and, in particular, large partially dead spinifex
 hummocks (Murphy et al., 2017). While night parrots have not been sighted
 in our study area, they have been recorded in nearby areas with comparable
 habitats (Murphy et al., 2017); as an endangered species, they
 provide a relevant conservation model to work with (Murphy et
 al., 2018). Artificial quail nests
 were positioned no more than 5 cm from the base of spinifex hummocks,
 small shrubs, low lying gidgee trees or logs, by creating a small indent
 (~10 cm diameter) in the ground with the palm of a hand (Fig. 1d).
 Artificial parrot nests were placed in spinifex hummocks (1–4 m diameter)
 using a broom handle to force a 15 cm wide hole in the hummock. A broom
 pole was then used to create a small chamber (&lt; 30 cm deep and
 wide) at the base of the plant where the roots enter the sand (Fig.
 1e). Each nest contained 1 artificial egg, made from
 plasticine modelling clay and coated with Plasti-dip™, and 1 commercial
 quail egg, collected fresh and then refrigerated until use. Nests were
 also scented with ~5 g of feathers and droppings collected from
 domestically reared quails (Game Farm Pty Ltd., Galston NSW). The quail
 egg, droppings and feathers were used as attractants. Quail
 droppings were replenished at any sites where eggs remained 6 days after
 placement. We used latex gloves smeared in quail droppings to reduce human
 scent on nests and, when possible, human footprints were smoothed using a
 rake to minimise any obvious pathways in the sand that might lead
 predators to the nests. A GPS unit (Garmin GPSMAP 64sc) was used to mark
 each nest site. All nests were set out between ~11 am and 8 pm,
 1 day after carcass placement to ensure that scents associated with
 transporting the carcasses were kept separate from the nest sites. When
 setting the nests, we ensured that any nearby scavengers and predators,
 especially corvids, were flushed before starting, to ensure that they
 would not be able to watch nest placement. As nest placement took ~15 min
 per site, it was not expected that this had any significant impact on
 scavengers using the carcass sites. <strong>Carcass
 and nest site monitoring</strong> To allow ongoing monitoring and detection of
 scavengers visiting the carcasses, a remote camera (Reconyx Hyperfire
 PC800) was positioned on the stake marking the centre of each site, 3–4 m
 from the kangaroo carcass (Fig. 1c). The camera was programmed
 to take continuous photographs when triggered by thermal movement around
 the carcass (rapidfire, no wait period). To prevent removal of carcasses
 from the camera monitoring frame, carcasses were secured to the ground by
 wire attaching the neck and Achilles tendon of the animal to two metal
 stakes spaced ~0.6 m apart. We monitored carcasses over the same period we
 monitored nests (14 days). We examined all photographs collected by the
 cameras and tagged them by the animal species present in the frame and
 whether it was engaged in scavenging or not. We then examined the tagged
 images and compiled a list of all vertebrate species observed feeding on
 at least one carcass. Species that we suspected to be feeding on insects
 on the carcass, but not the carcass itself, were included on this
 list. Nests were
 monitored in the landscape for 14 days, as little buttonquails and other
 ground nesting birds in the region generally hatch in an equivalent time
 (Higgins et al., 1990). Every 2 days, we visited each nest and recorded
 evidence of predation. Sites where either clay or quail eggs had been
 bitten into or removed were considered depredated. The plastic-coated clay
 enabled identification of species attempting to depredate the eggs (i.e.
 from the tooth or beak marks in the clay) and helped to minimise the scent
 of the clay, which has been linked to higher rates of predation (Purger et
 al., 2012). In addition to the plastic-coated clay eggs,
 identification of nest predators was aided by smoothing sand around the
 nest site and through the use of remote cameras (Reconyx Hyperfire PC800),
 which were placed 3 m away from select nests at carcass and control sites
 and programmed to take pictures continuously when any animal motion was
 detected (79 nests were monitored by cameras). As camera placement may
 influence nest predation rates, cameras were hidden in adjacent vegetation
 and set on nests at random sites (Richardson et al., 2009).
 No cameras were placed on artificial parrot nests. When reviewing the carcass and nest site camera
 images, differentiating corvid species was challenging. In most cases, we
 could identify Australian ravens by their larger size and stature, smaller
 group numbers and long throat hackles that form a layered beard. We
 identified crows by their thinner appearance, smaller throat hackles,
 white feather bases and generally larger group numbers. While both little
 crows and Torresian crows occur in our study area and are difficult to
 differentiate without hearing their calls, we are fairly confident that
 all crows appearing in our camera images were little crows. This
 assumption is based on the fact that no Torresian crows were detected in
 the field during the two 2-week periods of fieldwork carried out as part
 of this study (i.e. based on calls), or by concurrent bird surveys carried
 out in the study area and surrounding region (pers. comm. Ayesha Tulloch).
 Further, continuous bird surveys in the region typically only place
 Torresian crows in the area following high rainfall events and we
 conducted our study during a dry period. At nest sites, corvid predation
 events could only be specified to species when using camera images (and
 not by prints in the sand or beak marks in the clay eggs).