Yellow-eyed Penguin Megadyptes Antipodes Research Articles

Conservation implications for post-fledging dispersal of yellow-eyed penguins/hoiho

With the extinction of yellow-eyed penguins Megadyptes antipodes on mainland New Zealand predicted within the next few decades, identifying preventable causes of mortality during the juvenile dispersal period is critical. Between 2017 and 2019, we tracked 30 juvenile yellow-eyed penguins during their post-fledging dispersal to determine their dispersal trajectory, marine habitat use, and to identify overlap with commercial gillnet fishing operations, a known cause of yellow-eyed penguin mortality. Using k-nearest neighbour local convex hulls, we identified core foraging hotspots (50% isopleths) and main marine habitats (95% isopleths). We identified environmental covariates that influenced penguin habitat use during separate stages of the dispersal journey. After fledging, juvenile yellow-eyed penguins from the Otago coast dispersed and foraged 140 to 692 km (mean ± SD = 359.9 ± 129.3 km) from their natal locations, north-north-east into the Canterbury Bight, a region where penguin bycatch in commercial gillnet fishing operations has been recorded. There was a 51.6% overlap between core juvenile foraging hotspots and known commercial gillnet fishery locations (from 2017 to 2019). Less than 12.6% of the core foraging hotspots occurred within the 4 nautical mile gillnet prohibition zone implemented in 2008. A recent extension in the size of the gillnet prohibition zone in 2020 increased this spatial overlap to cover 36.9% of core foraging hotspots and 28.9% of main marine habitats. These findings highlight the vulnerability of juvenile yellow-eyed penguins at this critical life-stage, but there is opportunity to amalgamate marine protection measures with other endangered species with overlapping distributions that are also at risk of gillnet-related mortality.

Foraging areas and plasticity of yellow-eyed penguins Megadyptes antipodes in their subantarctic range

Foraging behaviour is crucial to breeding success for marine predators, including seabirds. Yellow-eyed penguins Megadyptes antipodes are central-place, predominantly benthic foragers around mainland New Zealand. The northern (mainland) population of this Endangered species is declining, with changes in the marine environment a suspected cause, particularly warming water and poorer foraging success. We undertook a detailed foraging study of the data-deficient subantarctic population, which is distinct from the northern population. Over 2 breeding seasons, we collected 91 GPS foraging logs from 69 breeding yellow-eyed penguins from Enderby Island, Auckland Islands, New Zealand. The mean foraging distance was 24 km from shore (max 47 km). Foraging area size was greater for females and for pelagic foragers, although benthic foragers travelled further from shore on average. Diving plasticity was evident both in diving behaviour and foraging area use. Foraging area and distance from shore were greater for all birds in a year of greater breeding effort and fledging success (2016). Foraging occurred over continental shelf waters, similar to the mainland, and in areas up to 150 m deep, so any differences in foraging behaviour compared to those reported for the northern population are likely influenced by local bathymetry, environmental conditions, and individual preference. Despite comparable bathymetry in some areas, the southern population showed greater foraging plasticity, with 62% of foraging trips categorised as pelagic, implying that subantarctic foraging conditions may differ from the predominantly benthic mainland foraging. Variable foraging conditions may therefore have implications for future breeding success in the subantarctic.

Diving plasticity in the ancestral range of the yellow-eyed penguin Megadyptes antipodes, an endangered marine predator

In long-lived marine predators such as penguins, foraging behaviour is related to breeding success. Yellow-eyed penguins Megadyptes antipodes are thought to be predominantly benthic foragers around mainland New Zealand, with previous dive studies showing little variation in diving behaviour and minimal change by year or breeding status. This apparent absence of foraging flexibility may be linked to local prey availability and therefore also to foraging success, factors believed to contribute to the current population decline. Here, we undertook the first detailed study of yellow-eyed penguin diving and foraging behaviour in the subantarctic Auckland Islands, part of the original ancestral range of the species that colonised mainland New Zealand. We collected dive logs from 134 foraging trips made by 73 breeders from Enderby Island, Auckland Islands, in the New Zealand subantarctic. Pelagic dives and foraging trips were recorded in the subantarctic population—a foraging strategy not previously published in dive studies on this species. Changes between benthic and pelagic foraging were recorded for individuals within foraging trips, within a breeding season, and between years. Differences in diving behaviour between the subantarctic and that reported for mainland New Zealand are likely influenced by local bathymetry and environmental conditions, and prey availability. However, the subantarctic population shows a greater use of pelagic foraging not evident in the northern population, even in areas that have a similar depth to some mainland foraging areas. This observed foraging flexibility may have implications for breeding success and potentially the long-term population trends of these 2 genetically similar populations.

Re-evaluating New Zealand’s endemic Pliocene penguin genus

ABSTRACT Fossil penguins from late Neogene sediments are important for understanding the origins of modern penguins. Recent advances in penguin taxonomy and geometric morphometrics allow us to revisit previously described Pliocene fossils and potentially gain new insight into the evolutionary relationships of early crown penguin species. Here we re-evaluate the holotype of Tereingaornis moisleyi, a Pliocene fossil penguin discovered near Te Reinga Falls in the North Island of New Zealand. The holotype (hereafter referred to as the Te Reinga Falls fossil) does not possess any apomorphies that justify recognition as a distinct genus or species, and cannot be distinguished from the living crested penguins (Eudyptes spp.) or yellow-eyed penguin (Megadyptes antipodes) using discrete osteological characters or morphometric analysis. We propose that Tereingaornis moisleyi be considered a nomen dubium. The Te Reinga Falls fossil indicates that a penguin approximately the size of the extant tawaki (Fiordland crested penguin, Eudyptes pachyrhynchus) was present in New Zealand during the middle of the Pliocene (Waipipian Stage, 3.7–3.0 Ma), but a more complete specimen would be needed to confidently resolve the evolutionary relationships between this fossil and other penguins.

Behavioural consequences of human disturbance on subantarctic Yellow-eyed Penguins Megadyptes antipodes

SummaryHuman disturbance can have behavioural, physiological and population-level consequences on wildlife. Unregulated tourism is having a negative effect on the endangered Yellow-eyed Penguin Megadyptes antipodes on mainland New Zealand. Subantarctic Yellow-eyed Penguins are exposed to tourism on Enderby Island in the Auckland Islands group, 450 km south of New Zealand. Restrictions and guidelines for tourism are in place on Enderby Island, but there has been little study on the efficacy of these. We quantified behavioural responses of the Yellow-eyed Penguin on Enderby Island to human presence by documenting movement patterns and behaviour of penguins in the presence and absence of humans, through both controlled approaches and monitoring penguin behaviour in the presence of tourists. We used these data to model the effective approach distances for reducing disturbance. Human presence caused a significant drop in the probability of a successful transit to or from their nest, and significantly increased the time penguins spent alert and decreased the time spent preening. Modelling showed the distance from a human to a penguin is a significant predictor of the likelihood of a bird displaying disturbance behaviour, with the current minimum approach guideline of 5 m not sufficient for preventing disturbance. Our results indicate that the minimum approach guideline needs to be revised if the probability of disturbance is to be reduced. Modelling the appropriateness of minimum approach guidelines by predicting the probability of disturbance is a useful technique that could be applied to other species and systems. Worldwide, management guidelines need to be scientifically evaluated to ensure efficacy and cater for the more sensitive species affected.

Investigation of a mortality cluster in wild adult yellow-eyed penguins (Megadyptes antipodes) at Otago Peninsula, New Zealand

ABSTRACTWe investigated an epidemic mortality cluster of yellow-eyed penguins (Megadyptes antipodes) that involved 67 moribund or dead birds found on various beaches of the Otago Peninsula, New Zealand, between 21 January and 20 March 2013. Twenty-four carcases were examined post-mortem. Histological lesions of pulmonary, hepatic and splenic erythrophagocytosis and haemosiderosis were found in 23 of 24 birds. Fifteen birds also had haemoglobin-like protein droplets within renal tubular epithelial cells. Despite consistent histological lesions, a cause of death could not be established. Virology, bacteriology and molecular tests for avian influenza, avian paramyxovirus-1, avipoxvirus, Chlamydia psittaci, Plasmodium spp., Babesia spp., Leucocytozoon spp. and Toxoplasma gondii were negative. Tissue concentrations of a range of heavy metals (n = 4 birds) were consistent with low level exposure, while examination of proventricular contents and mucus failed to detect any marine biotoxins or Clostridium botulinum toxin. Hepatic concentrations of total polycyclic aromatic hydrocarbons (PAHs) (n = 5 birds) were similar to background concentrations of polycyclic aromatic hydrocarbons previously found in yellow-eyed penguins from the South Island of New Zealand, but there were significantly higher concentrations of 1-methylnapthelene and 2-methylnapthelene in the birds found dead in this mortality cluster. The biological significance of this finding is unclear. A temporal investigation of the epidemic did not indicate either a common source or propagative epidemic pattern. Although our investigation did not definitively implicate a toxic or infectious agent, we could not rule out causes such as toxic marine organisms or mycoplasmosis. Further investigations should therefore by carried out in the event of future mortality clusters.

Selection for protection from insolation results in the visual isolation of Yellow-eyed Penguin Megadyptes antipodes nests

SummaryThe concealed and widely dispersed nests of the rare and endangered Yellow-eyed Penguin Megadyptes antipodes, or “hoiho”, have been considered to reflect an essential requirement for the visual isolation of nest sites from conspecifics. However, this may be a consequence of selection for habitat features that provide protection from insolation, thereby minimising the risk of heat stress. To help improve the understanding of hoiho nesting requirements and the effectiveness of habitat restoration, we aimed to determine whether visual isolation from conspecifics or protection from insolation is the primary driver of hoiho nest site selection. We compared the mean maximum distance of visibility and the mean percentage insolation cover of active nests with randomly sampled unused sites in flax Phormium tenax and Hebe elliptica coastal scrub at Boulder Beach, and in coastal forest at Hinahina Cove, New Zealand, 2006–2007. Results of univariate tests and the evaluation of logistic regression models suggested that the amount of insolation cover was more important than visibility for hoiho nest site selection, particularly in flax and scrub. In addition, Spearman's correlations indicated that decreasing insolation cover significantly increased the visibility of nests in the forest habitat, and had a similar effect on inter-nest distance in flax. We infer that hoiho nest site selection and distribution are influenced primarily by the location and density of micro-habitat features (particularly within 1 m of the ground) that provide optimal protection from insolation, possibly along with other important features such as a firm backing structure. Strong selection for these features results in the typical but non-essential visual isolation of nest sites from conspecifics. Restoration of nesting habitats with a relatively high density and diversity of vegetation and solid structures within 1 m of the ground may eventually provide an optimal availability and quality of suitable nest sites.

Investigation of an outbreak of craniofacial deformity in yellow-eyed penguin (Megadyptes antipodes) chicks

AIM: To investigate an outbreak of severe craniofacial deformity in yellow-eyed penguin (Megadyptes antipodes, hōiho) chicks at a single breeding site on the Otago Peninsula in the South Island of New Zealand.METHODS: Morbidity and mortality of yellow-eyed penguins breeding on the coastal regions of Otago was monitored from November 2008 to March 2009. Dead chicks and unhatched eggs were recovered and examined. Between October and December 2008 32 eggs were recorded at 17 nests in the Okia Reserve. Eleven chicks survived to about 90 days of age, of which eight were found to have moderate to severe craniofacial deformity. The six most severe chicks were subject to euthanasia and examined in detail at necropsy, and the remaining two affected chicks were released to the wild after a period of care in a rehabilitation centre. Post-mortem samples were analysed for inorganic and organic toxins.RESULTS: The six deformed chicks all had severe shortening of the mandible and maxilla by 20−50 mm. The rostral and caudal regions of the skull were approximately 40 and 80% of normal length, respectively. Other, more variable lesions included crossbill deformity, malformed bill keratin, microphthalmia with misshapen scleral ossicles and oral soft tissue excess thought to be secondary to bony malformations. During the same year, mild sporadic bill deformities were also reported in 10 unrelated chicks from >167 chicks at other breeding sites on the southern Otago coast. Concentrations of organic toxins and heavy metals in body tissues from affected chicks were apparently similar to those in unaffected chicks on other beaches.CONCLUSIONS: No cause of this outbreak of craniofacial deformity could be established although the high prevalence at a single site suggests that it was due to an unidentified local teratogen.

Site-specific reproductive failure and decline of a population of the Endangered yellow-eyed penguin: a case for foraging habitat quality

Sub-populations within species can exhibit differing population growth trajectories in relation to one another depending on various environmental factors. In threatened species, neg- ative population growth in some sub-populations can ultimately cause the demise of the species; therefore, understanding causal factors of population change is critical to inform management aimed at reversing population declines. Feral house cats Felis catus are potential predators of Endangered yellow-eyed penguins Megadyptes antipodes and were considered the principal causal factor in the species' decline on Stewart Island/Rakiura, New Zealand. The number of yellow-eyed penguins breeding on Stewart Island is low relative to the number on close outlying islands, where cats are absent, and a census had recorded few juveniles on Stewart Island, sug- gesting poor reproductive success. Yellow-eyed penguin breeding attempts on the northern coast of Stewart Island and outlying islands were monitored for 5 yr, but predation by cats was not evi- dent. Instead, disease, probably aggravated by starvation and poor dietary provisioning, was found to be a significant cause of chick mortality on Stewart Island. Reproductive success was con- sistently low there (0�33%), in contrast to outlying islands (27�76%). Little recruitment was recorded on Stewart Island, and the number of breeding pairs on the northern coast of Stewart Island declined by 27% between 1999 and 2008. Factors unique to the north coast of Stewart Island are believed to be adversely affecting nesting yellow-eyed penguins, as a similar decline was not recorded elsewhere on the island or on outlying islands.

Yellow-eyed Penguin (Megadyptes antipodes) as a case study to assess the reliability of nest counts

Population monitoring of seabirds plays an important role in conservation since it provides the information required to evaluate conservation programmes of endangered species, to guide harvest management and to monitor indicators of marine ecosystem health. Annual nest counts are often used for the long-term monitoring of breeding seabird populations. While such counts provide a direct and cost-effective survey method, single nest counts will almost always yield an underestimate of the true number of nests and provide no means of expressing uncertainty. In this study, we used double counts and the Lincoln–Petersen estimator to assess the reliability of nest counts for the Yellow-Eyed Penguin (Megadyptes antipodes) in the South Island of New Zealand. We estimated the detection rate of single nest counts at 88%, and we calculated that double counts are sufficiently precise to detect changes in the size of the breeding population as small as 3.3% between years. We failed to detect observer bias or any effects of search effort or habitat on the detection rate, the only significant factor being the steepness of the terrain. Given the high detection rate and the susceptibility of Yellow-Eyed Penguins to human disturbance, we conclude that the established protocol for single nest counts is adequate for the regular monitoring of the population. We suggest that double counts could be conducted once every 5 years to obtain precise estimates for the purpose of long-term population monitoring. We also highlight the importance of repeating similar studies for a variety of species and habitats.

The use of approximate Bayesian computation in conservation genetics and its application in a case study on yellow-eyed penguins

The inference of demographic parameters from genetic data has become an integral part of conservation studies. A group of Bayesian methods developed originally in population genetics, known as approximate Bayesian computation (ABC), has been shown to be particularly useful for the estimation of such parameters. These methods do not need to evaluate likelihood functions analytically and can therefore be used even while assuming complex models. In this paper we describe the ABC approach and identify specific parts of its algorithm that are being the subject of intensive studies in order to further expand its usability. Furthermore, we discuss applications of this Bayesian algorithm in conservation studies, providing insights on the potentialities of these tools. Finally, we present a case study in which we use a simple Isolation-Migration model to estimate a number of demographic parameters of two populations of yellow-eyed penguins (Megadyptes antipodes) in New Zealand. The resulting estimates confirm our current understanding of M. antipodes dynamic, demographic history and provide new insights into the expansion this species has undergone during the last centuries.

Consistent foraging routes and benthic foraging behaviour in yellow-eyed penguins

Yellow-eyed penguins Megadyptes antipodes seemingly forage at discrete marine loca- tions over the continental shelf, where they are believed to feed predominantly at the seafloor. Such behaviour would distinguish them from most other penguin species that generally employ pelagic foraging strategies. From 2003 to 2005 we studied the foraging behaviour of yellow-eyed penguins breeding near Oamaru, New Zealand. We used 2 types of data loggers: GPS loggers recorded geo- graphical position and dive depth at set intervals, while time-depth recorders (TDRs) recorded only dive depth. The penguins performed day trips (range: 12 to 20 km from the coast) or shorter evening trips (range: <7 km). Consecutive foraging trips of individuals revealed remarkably consistent forag- ing routes. Birds travelled along similar — at times congruent — paths, markedly changed course at distinct locations, and revisited certain locations on separate trips, indicating skilful navigation. Three trip stages could be distinguished on one-day trips. During the outgoing (seaward travelling) and incoming (shoreward travelling) stages the birds followed linear courses. These stages were sep- arated by a period of midday activity in which birds exhibited higher dive effort and often tended to stay within confined areas. The diving behaviour revealed an exclusive bottom-foraging strategy, with 87% of all dives being benthic dives; the majority of non-benthic dives occurred during the last 2 to 3 h of the trip, indicating primarily travelling behaviour. Furthermore, yellow-eyed penguins employ benthic dives not only when feeding but also frequently when travelling. We suggest that benthic dives might facilitate navigation and, consequently, account for the consistent foraging patterns of yellow-eyed penguins.

Hormonal correlates of parental behavior in yellow-eyed penguins (Megadyptes antipodes)

Penguins show varying degrees of brood reduction behavior, from obligate brood reducers to brood maximizers, and we hypothesize that this is associated with differences in prolactin secretion. To address this hypothesis, we determined the breeding season prolactin profile of the yellow-eyed penguin (Megadyptes antipodes) for comparison with those of other penguin species found in the literature. We also measured sex steroid plasma concentrations to better characterize the reproductive cycle of the species. Plasma concentrations of prolactin increased from early in the season, reaching a peak during late incubation, and remained elevated up to the guard period. This general pattern was similar to that of other penguins for which we have corresponding data. However, we found that throughout the laying period, prolactin titers in yellow-eyed penguins remained elevated while they fell to basal levels after the laying of the first egg in macaroni penguins, which corresponds to differences in incubation behavior during this time. We conclude, therefore, that differences in the brood reduction behavior in penguins, may be reflected in the pattern of PRL concentrations around the time of egg laying.

THE INFLUENCE OF LAYING DATE AND MATERNAL AGE ON EGGSHELL THICKNESS AND PORE DENSITY IN YELLOW-EYED PENGUINS

Abstract In many bird species, eggs laid late in the breeding season hatch after a shorter incubation period than eggs that were laid early. The proximate mechanisms that cause these seasonal declines in incubation length remain poorly understood. We tested in Yellow-eyed Penguins (Megadyptes antipodes), Hipfner's hypothesis that late-laid eggs have thinner eggshells with a higher pore density that allow embryos to develop more rapidly than in early-laid eggs. In this species incubation length declines with increasing female age, so we also investigated whether eggshell thickness and pore density varies with female age. In addition, differences in shell thickness and pore density between first- and second-laid eggs of the same clutch were examined. Eggshell thickness did not change with laying date and was not related to length of incubation. In contrast, pore density increased with laying date and was negatively related to incubation length in first-laid eggs. Eggshell thickness and pore density increase...

Nature-based tourism impacts on yellow-eyed penguins Megadyptes antipodes: does unregulated visitor access affect fledging weight and juvenile survival?

The Otago Peninsula, South Island, New Zealand offers tourists opportunities for unregulated access to breeding sites of the yellow-eyed penguin ( Megadyptes antipodes). The presence of people on beaches delays post-foraging landing by penguins, which in turn may affect the amount of food delivered by parents to their chicks, with consequences for chick growth and fledging mass. This study explored the relationship between human disturbance and yellow-eyed penguin chick fledging weight and survival by comparing five yellow-eyed penguin breeding areas with different levels of visitor frequency. We investigated whether chick fledging weights vary between breeding areas, and whether fledging weight is a predictor of juvenile survival. In 2002, chicks at Sandfly Bay, an area with high numbers of tourists, had significantly lower fledging weights than chicks at Highcliff, an area with no tourist visitors. An analysis of sightings of 2125 yellow-eyed penguin chicks banded between 1981 and 2000 indicated probability of survival was positively associated with mass at fledging. Thus lower fledging weights may have long-term population consequences. Fledging weight is influenced by many factors, however the results suggest the possibility of an effect of tourist numbers on chick fledging weight. This is noteworthy, especially in light of the rapid rate at which wildlife tourism is increasing in coastal areas of southern New Zealand.

Investigation of Interacting Effects of Female Age, Laying Dates, and Egg Size in Yellow-eyed Penguins (Megadyptes antipodes)

Abstract Increased experience with age may allow long-lived seabirds to forage more efficiently and ultimately to invest more into reproduction. In many seabirds, egg size increases with female age, but declines with laying date. Declines in laying date might be due either to quality of individual females or females may adjust their egg size in response to poorer conditions typically found late in the season. We investigated the influence of female age, date of laying, and annual variation on egg size in Yellow-eyed Penguins (Megadyptes antipodes). Further, we tested whether there was an effect of individual females or female age on date of laying. Whereas female age influenced egg volume, mass, and width, laying date had no effect on egg size or mass in Yellow-eyed Penguins. Laying dates differed significantly among females but were not affected by their ages. These results indicate that in Yellow-eyed Penguins, some individual females lay consistently early or late, regardless of age, but that egg size generally increases with age.

Investigation of Interacting Effects of Female Age, Laying Dates, and Egg Size in Yellow-eyed Penguins (Megadyptes antipodes)

Abstract Increased experience with age may allow long-lived seabirds to forage more efficiently and ultimately to invest more into reproduction. In many seabirds, egg size increases with female age, but declines with laying date. Declines in laying date might be due either to quality of individual females or females may adjust their egg size in response to poorer conditions typically found late in the season. We investigated the influence of female age, date of laying, and annual variation on egg size in Yellow-eyed Penguins (Megadyptes antipodes). Further, we tested whether there was an effect of individual females or female age on date of laying. Whereas female age influenced egg volume, mass, and width, laying date had no effect on egg size or mass in Yellow-eyed Penguins. Laying dates differed significantly among females but were not affected by their ages. These results indicate that in Yellow-eyed Penguins, some individual females lay consistently early or late, regardless of age, but that egg size ...

Effects of habitat and introduced mammalian predators on the breeding success of Yellow-eyed Penguins Megadyptes antipodes, South Island, New Zealand

The endemic Yellow-eyed Penguin Megadyptes antipodes is threatened by habitat loss and introduced predators on mainland New Zealand. Nine colonies in the Catlins (south-east coast of South Island) were studied to measure breeding success, penguin abundance, and predator abundance in three successive breeding seasons (1991/92 to 1993/94). Nest numbers increased in all nine colonies in the three years despite predation (probably by Stoats Mustefa erminea) being the most important cause of breeding failure. Larger colonies with higher breeding success were in small gullies with limited shrubs and bushes rather than in the most intact mature forest colonies hitherto assumed to be optimal habitat for the birds. Penguin nests were concentrated near the forest edge, but predators were not, so the predation risk was not elevated near the forest edge. Fragmentation of the original forest habitat had no observable adverse effect on breeding success. Stoats dominated the predator guild, while Ferrets M. furo and Feral Cats Felis catus were rare. Trapping to kill predators early in the season had no marked effect on subsequent predation losses, but trapping intervention when a predation outbreak occurred curtailed further chick deaths. A simple population model predicts that Yellow-eyed Penguins populations will grow provided the average total chicks loss is less than 43% per season, or at least 0.85 chick per nest fledges each year. This requires predation losses to be less than 34%.

Can artificial brood reduction assist the conservation of yellow-eyed penguins (Megadyptes antipodes)?

Can artificial brood reduction assist the conservation of yellow-eyed penguins (Megadyptes antipodes)?

Movements and habitat use of feral house cats Felis catus, stoats Mustela erminea and ferrets Mustela furo, in grassland surrounding Yellow-eyed penguin Megadyptes antipodes breeding areas in spring

This paper reports a radio-tracking study of movements of feral house cats Felis catus, stoats Mustela erminea and ferrets Mustela furo during spring around Yellow-eyed penguin Megadyptes antipodes (hoiho) breeding areas in coastal Otago, South Island, New Zealand. Grassland around these breeding areas has been retired from grazing by domestic stock to establish ‘vegetation buffers’ intended to reduce predation of hoiho chicks by introduced mammalian predators. The ‘vegetation buffers’ did not exclude or deter predators. In fact, the opposite effect was evident: ‘vegetation buffers’ attracted cats, stoats and may have attracted ferrets to the area. Attraction of predators to ‘vegetation buffers’ could increase encounter rates between predators and penguin chicks, especially by stoats, which are difficult to trap. ‘Vegetation buffers’ may therefore increase predation rates, but this is unmeasured. This research highlights the need for rigorous testing of proposed biological controls such as habitat modifications before widely implementing them as ways of protecting native wildlife.