Density and success of bird nests relative to grazing on western Montana grasslands

Grassland birds are declining at a faster rate than any other group of North American bird species. Livestock grazing is the primary economic use of grasslands in the western United States, but the effects of this use on distribution and productivity of grassland birds are unclear. We examined nest density and success of ground-nesting birds on grazed and ungrazed grasslands in western Montana. In comparison to grazed plots, ungrazed plots had reduced forb cover, increased litter cover, increased litter depth, and increased visual obstruction readings (VOR) of vegetation. Nest density among 10 of 11 common bird species was most strongly correlated with VOR of plots, and greatest nest density for each species occured where mean VOR of the plot was similar to mean VOR at nests. Additionally, all bird species were relatively consistent in their choice of VOR at nests despite substantial differences in VOR among plots. We suggest that birds selected plots based in part on availability of suitable nest sites and that variation in nest density relative to grazing reflected the effect of grazing on availability of nest sites. Nest success was similar between grazed plots and ungrazed plots for two species but was lower for nests on grazed plots than on ungrazed plots for two other species because of increased rates of predation, trampling, or parasitism by brown-headed cowbirds (Molothrus ater). Other species nested almost exclusively on ungrazed plots (six species) or grazed plots (one species), precluding evaluation of the effects of grazing on nest success. We demonstrate that each species in a diverse suite of ground-nesting birds preferentially used certain habitats for nesting and that grazing altered availability of preferred nesting habitats through changes in vegetation structure and plant species composition. We also show that grazing directly or indirectly predisposed some bird species to increased nesting mortality. Management alternatives that avoid intensive grazing during the breeding season would be expected to benefit many grassland bird species.     

Water depth influences nest predation for a wetland-dependent bird in fragmented bottomland forests

The alteration and fragmentation of natural habitats has resulted in increased rates of nest predation and poor reproductive success for many bird species. The development of effective conservation strategies to reduce elevated rates of nest predation has been hindered by difficulties in fully understanding mechanisms underlying patterns and rates of nest predation in particular habitats. I recorded the outcome for 2726 nesting attempts of prothonotary warblers (Protonotaria citrea) breeding in fragmented bottomland forests and quantified the effect of nest predation on annual fecundity, determined the influence of different nest predators on rates of nest predation, and identified the factor driving rates of nest predation. There was a highly significant negative correlation between rates of nest predation and the actual annual reproductive output of prothonotary warblers. Of 1156 nest predation events, 73% were attributed to raccoons (Procyon lotor), 15% to snakes and 7% to southern flying squirrels (Glaucomys volans). Rates of nest predation decreased with increasing water depth as a result of nest predation by raccoons. Nests that were over water deeper than 60 cm were particularly successful. Habitat fragmentation, the draining of wetlands, and stream channelization may act synergistically to elevate rates of nest predation for those birds breeding in forested wetlands. Conservation actions designed to stop or reverse these processes will be especially beneficial to birds breeding in bottomland forest ecosystems.     

What do artificial nests tells us about nest predation?

Artificial nests are commonly used to evaluate predation, but the assumption that this method mimics predation on natural nests has seldom been tested. Natural and artificial nests of eastern yellow robins (Eopsaltria australis) were monitored in four, 55-ha plots over two breeding seasons. Overall, daily survival rates were higher (P<0.001) for natural (95%/day) than for artificial nests (88%/day). Among plots, daily survival rates for the two types of nests were not correlated with one another (P=0.72) indicating that the spatial pattern of predation on artificial nests did not mimic that for natural nests. Seasonal variation was evident for natural nests in one year, when they were more successful at the beginning and end of the breeding season. No seasonal patterns were observed for artificial nests in either year. Neither natural nor artificial nests showed annual variation in predation. Previous researchers concluded that large birds were important predators on robin nests. In this study, predation by large birds on artificial nests was positively correlated with the numbers of large birds counted on the plots (P=0.04). However, large birds depredated only 16% of artificial nests. Daily survival rates for artificial nests were recalculated using predation by large birds only. These rates were compared with natural nests, but there was still no correspondence in the spatial and temporal patterns of predation for the two types of nests. These results suggest that inferences about predation on natural nests based on artificial nest studies should be avoided.     

Tallgrass prairie management and bird nest success along roadsides

The attributes of roadside vegetation, an important bird habitat in grassland ecosystems, have been shown to affect bird abundance, distribution composition, and diversity, yet there are relatively few works on reproductive success of birds nesting along roadsides. Because roadsides are linear habitats, management at the landscape scale can affect nest success in roadsides through bottom-up and top-down effects. In northeastern Oklahoma tallgrass prairie is subjected annually to prescribed spring fires. In the short term fires can alter both arthropod abundance and predator access to nests. We explored effects of burning on bird nest success with a five-year study along roads that traversed tallgrass prairie habitat. Using data from ∼1400 nests of 23 species, we generated nest survival curves for groups of altricial species defined by nest substrate (ground, shrub, tree, or culvert). We then determined if these curves were affected by management practice (spring burning), food abundance (arthropod biomass), and habitat attributes (tree density and height). Nest substrate had a large effect on nest success: despite their shorter nest exposure period, ground nests were least successful and culvert nests were most successful. An increase in arthropod biomass following burning was possibly the cause for the increased nest success in burned plots, regardless of substrate, suggesting bottom-up control. Tree height and nest height were correlated positively with nest success, whereas tree density had no effect. Conversely, nest predation rates were correlated negatively with nest success, with ground nests experiencing the highest predation, culvert nests the lowest. Our results suggest that burning may increase nest success through bottom-up processes, but some species may not benefit from the increase in food abundance as a result of a concomitant increase in predation.     

Nest predation in a fragmented Afrotropical forest: evidence from natural and artificial nests

Nest predation accounts for a substantial share of nest failure and low reproductive success in most tropical songbirds. Normally, forest fragmentation leads to an increase in nest predation pressure due to reduced cover, fewer (and poorer) nest sites and predator influxes from the surrounding habitats. To test this hypothesis, we studied natural nesting behaviour and nest success of the white-starred robin (Pogonocichla stellata) in seven Afrotropical forest fragments differing in size and level of habitat disturbance. Based on data from 12 nests, we estimate that 29% of all natural nests initiated by the robins survive to produce fledglings across all fragments. We also conducted an experiment using artificial (plasticine) model-eggs to reveal potential predators and compare relative predation rates amongst fragments. This experiment revealed that small mammals might be the major predators on robin nests at the egg-stage. In addition, it showed that the highest incidences of nest disturbance during this stage were in the most heavily disturbed fragment. This was presumably attributable to an influx of mammalian predators from the surrounding habitats as forest degradation created suitable habitats for them. Such an infiltration was recently reported in this study site. Both nest placement and microhabitat did not significantly affect depredation levels in our experiment. This suggests that depredation was predominantly incidental (i.e., predators mainly encountered nests fortuitously while foraging for other food items), where the likelihood of encountering a nest largely depended upon the prevalence of the principal potential predators - the small mammals.     

Numerical and behavioral responses of waterfowl to the invasive American mink: A conservation paradox

A behavioral response is usually the first adaptation of animals to environmental changes such as the introduction of an alien invasive species, and it seems to be the most important factor in reducing the negative impact of new predators on naive prey. This study examined changes in the abundance and behavior of breeding coots (Fulica atra) and great crested grebes (Podiceps cristatus) in response to invasion of the Mazurian Lakeland (NE Poland) by American mink (Neovison vison), first recorded here in 1984. Feral mink densities varied from 1.0 to 14.6 individuals per 10 km of shoreline. Mink were responsible for the losses of 17–58% of experimental clutches (artificial nests containing chicken eggs) that were deployed. Mink destroyed more experimental clutches than other predators; the effects of mink predation were similar in reeds and on the bank. Radio-tracking showed that mink avoided lake banks near human settlements. Mink have markedly affected the breeding distribution of both species and caused a greater than 20-fold decline in coot numbers. For coots and grebes breeding distribution and nesting success were positively associated with built-up areas along lake shores and also with whether the birds were breeding in colonies. Increased brood survival of coots and grebes near human settlements is a paradox in the sense that a man-made habitat change has provided refuge from an alien predator.     

Resolving the urban nest predator paradox: The role of alternative foods for nest predators

Urbanization is a leading cause of species endangerment in the United States; however, certain species thrive in urban habitats. The loss of key predators or the addition of new predators in urban areas could alter the structure of urban communities. A reduction in nest predation is hypothesized to explain the high density of urban birds, yet urban areas typically have increased populations of avian nest predators. The loss of important nest predators in urban habitats, prey switching of urban predators, or successful nest defense against avian nest predators could explain this urban nest predator paradox. To assess these hypotheses I compared nest predation rates of Northern mockingbirds (Mimus polyglottos) in parking lots and residential neighborhoods to populations in pastures and wildlife preserves during 2007–2009 in Florida, USA and placed video cameras on a subset of nests in 2008–2009. Data do not support the hypothesis that urban nest predation rates are consistently lower than non-urban nest predation rates. Of the 56 nest predation events recorded, cats were the dominant urban predator and Cooper’s hawks (Accipiter cooperii) were the dominant non-urban predator. There was no evidence for a loss of important nest predators in urban habitats; however, prey switching by Cooper’s hawks likely occurred. There was also indirect evidence for the importance of nest defense. Furthermore, some of the cats recorded as nest predators in residential neighborhoods were owned cats and all but one cat predation event occurred at night. To reduce nest predation rates, cat owners should keep their cats indoors at night.     

Corvid response to human settlements and campgrounds: Causes, consequences, and challenges for conservation

Human development often favors species adapted to human conditions with subsequent negative effects on sensitive species. This is occurring throughout the urbanizing world as increases by generalist omnivores, like some crows and ravens (corvids) threaten other birds with increased rates of nest predation. The process of corvid responses and their actual effects on other species is only vaguely understood, so we quantified the population response of radio-tagged American crows (Corvus brachyrhynchos), common ravens (Corvus corax), and Steller’s jays (Cyanocitta stelleri) to human settlements and campgrounds and examined their influence as nest predators on simulated marbled murrelet (Brachyramphus marmoratus) nests on Washington’s Olympic Peninsula from 1995 to 2000. The behavior and demography of crows, ravens, and jays was correlated to varying degrees with proximity to human development. Crows and ravens had smaller home ranges and higher reproduction near human settlements and recreation. Annual survival of crows was positively associated with proximity to human development. Home range and reproduction of Steller’s jays was independent of proximity to human settlements and campgrounds. Local density of crows increased because home ranges of neighboring breeding pairs overlapped extensively (6× more than ravens and 3× more than Steller’s jays) and breeders far from anthropogenic foods traveled 10s of kilometers to access them. Corvids accounted for 32.5% of the predation events (n = 837) we documented on artificial murrelet nests. Small corvids (jays) were common nest predators across our study area but their contribution as predators did not vary with proximity to settlements and campgrounds. In contrast, large corvids (crows and ravens) were rare nest predators across our study area but their contribution varied greatly with proximity to settlements and campgrounds. Managers seeking to reduce the risk of nest predation need to consider the varied impacts and variable behavioral and population responses of potential nest predators. In our situation, removing large corvids may do little to reduce overall rates of nest predation because of the diverse predator assemblage, but reducing anthropogenic food in the landscape may be effective.     

Managing predation on ground-nesting birds: The effectiveness of nest exclosures

Ground-nesting birds have declined world-wide, probably partly due to high nest predation. A non-lethal method for decreasing predation uses protective cages at nests. Tests have mainly looked at the effect of such nest exclosures on hatching success and adult predation, but several additional aspects need to be explored for a comprehensive evaluation of this conservation technique. Here, we test the effect of nest exclosures in two common European shorebirds: northern lapwing (Vanellus vanellus) and redshank (Tringa totanus), measuring hatching success, incubation length, hatching synchrony, hatchability, partial clutch loss, chick condition, and adult predation. In both species, protected nests had higher hatching success than unprotected nests. Taking into account incubation time, nest abandonment, hatchability and partial clutch loss, protected nests still hatched more young than unprotected controls. In lapwings, but not in redshanks, protected nests were incubated longer, but this did not impair the condition of lapwing chicks. Protected redshanks suffered increased predation on incubating adults, which often sit on the nest until a predator is close by. Our results emphasize the need for caution in the use of nest exclosures, particularly in redshanks and other species with similar incubation behaviour. Exclosures can, however, be a useful management tool in shorebirds that leave their nest early, when an approaching predator is still far away.     

Edge effects on nesting success of cavity-nesting birds in fragmented forests

Forest fragmentation leads to the creation of isolated forest patches and habitat edges with subsequent impact on forest-interior bird species. Although the effects of fragmentation and edge on avian nesting success are well documented for open cup-nesting species in eastern deciduous forests in North America, it is unclear whether these effects are common for all birds nesting in predominantly forested landscapes. In particular, edge effects on nesting success of cavity-nesting birds are poorly understood. Using natural cavity nests, we examined nesting success of four species of cavity-nesting birds (two nonexcavators and two excavators), the yellow-rumped flycatcher (Ficedula zanthopygia), the great tit (Parus major), the great spotted woodpecker (Picoides major), and the grey-faced woodpecker (Picus canus) in relation to forest edges in Zuojia Nature Reserve, Jilin province, northeastern China. Our primary objective was to assess whether distance to the edge of agricultural lands was related to nesting success for cavity-nesting birds in fragmented forests. A total of 439 natural cavity nests of the four species were located and monitored during four breeding seasons. Probability of nest success was influenced by distance to forest edge for nonexcavators, but not for excavators. The rate of nesting success of the two nonexcavators, yellow-rumped flycatcher and the great tit, increased with distance from the edges. For all cavity nests, nesting success was 0.43 at 0-100 m, 0.56 at 101-200 m, 0.68 at 201-300 m, 0.61 at 301-400 m, 0.77 at 401-500 m from the edges. Nesting success ranged from 0.57 for the yellow-rumped flycatcher to 0.89 for the Grey-faced Woodpecker. Failed nests were often occupied by nest-site competitors (accounting for 68%). However, predation only accounted for 20% of all nest failures. Our results suggest that negative edge effects do exist for some cavity-nesting birds, especially for nonexcavator species.     

Effects of landscape composition, habitat features, and nest distribution on predation rates of simulated turtle nests

We investigated predation of simulated turtle nests in an effort to understand how land-use patterns and the availability of nesting habitat may affect turtle recruitment in a region where human populations and associated development are increasing. Simulated nests were patterned after those created by painted turtles (Chrysemys picta), a common aquatic turtle in our study area, and distributed in four patterns (clustered and near pond, scattered and near pond, clustered and far from pond, and scattered and far from pond) around 36 ponds. Landscape composition (500-2000 m from pond perimeters) and habitats surrounding pond edges (an area extending 250 m from the shore of each pond) were then compared with rates of predation at each pond. Nest-site characteristics also were compared to the fate of individual nests. Landscape composition and habitats surrounding ponds apparently had little influence on predation rates. Nest distribution and the immediate habitat features associated with each nest did affect vulnerability to predation. Clumped nests were preyed upon at a higher rate than scattered nests, and nests close to ponds (within 50 m) were more vulnerable to predators than those created far (100-150 m) from a pond. Counter to our expectations, proximity to edge habitats (other than the shore of a pond) reduced the probability that a nest would be detected by predators. Also, nests placed near roads and suburban lawns had a reduced likelihood of predation whereas those placed in agricultural areas or disturbed sites had a greater probability of being preyed upon. Our results suggest that predation of simulated turtle nests may be a consequence of their distribution and location relative to the foraging activities of common nest predators, especially raccoons (Procyon lotor). Efforts to enhance recruitment among declining populations of turtles should consider the abundance and distribution of nesting habitat. Providing additional nesting sites away from predator foraging habitats may reduce nest predation and increase the recruitment of hatchlings into a population.     

Implications of intraguild predation for sea turtle nest protection

In the United States, raccoons Procyon lotor are often removed from sea turtle nesting beaches to decrease egg mortality. However, raccoons also consume ghost crabs Ocypode quadrata, another common egg predator. Reducing predator populations can benefit secondary predators, inflating total predation pressure and leading to a decline in prey species. We used track and burrow counts to compare raccoon and ghost crab abundance at four beaches in Florida, USA, that differ in management activity and determined predation rates on loggerhead Caretta caretta nests by each predator. Mean raccoon abundance (range 0.12-0.46 tracks plot⁻¹ night⁻¹) and ghost crab density (0.09-0.19 burrows m⁻²) were inversely correlated. Ghost crabs were largest at the site with the fewest raccoons. The stable nitrogen isotope ratios of ghost crabs (mean 9.8‰) were positively correlated with body mass, indicating larger ghost crabs feed at a higher trophic level and suggesting large ghost crabs may consume more loggerhead eggs. The highest rates of egg predation by both predators (31%) occurred where raccoon abundance was lowest and ghost crab abundance was highest, suggesting ghost crab burrows may facilitate predation by raccoons. Our data suggest that predation by raccoons limits ghost crabs and that removing raccoons can increase ghost crab abundance and sea turtle egg mortality. Although predator removal can be effective when nest predation rates are quite high, maintaining moderate raccoon densities may be important for controlling ghost crabs. These results highlight the importance of understanding food web connectivity in developing management strategies to achieve conservation goals, especially when the species of concern are threatened or facing extinction.     

Influence of food availability, predator density and forest fragmentation on nest survival of New Zealand robins

The decline of avian populations in fragmented landscapes is often attributed to a decrease in nest survival rates for species breeding within these habitats. We tested whether fragment size and connectivity, livestock grazing, predator density or invertebrate biomass were correlated with nest survival rates for an endemic New Zealand species, the North Island robin (Petroica longipes). Across three breeding seasons (2002-2005) daily nest survival rate for the 203 robin nests monitored in 15 forest fragments was 0.315 (SE 0.003), with nest survival rates increasing with invertebrate biomass (indexed with pitfall traps) and marginally decreasing with fragment size. Footprint tracking rates for exotic ship rats (Rattus rattus), which are likely to be the key nest predator, varied greatly among fragments, but were not a useful predictor of nest survival. We found no relationship between the number of fledglings per successful nesting attempt and invertebrate biomass. We conclude that fragment size and connectivity does not appear to be negatively influencing robin nest survival, potentially because of the already high impact that mammalian nest predators have in this unique system.     

Nesting ecology, current status, and conservation of sea turtles on an uninhabited beach in Florida, USA

Although nesting beaches are critical resources for sea turtles, most beaches in the United States are disturbed by human influence, including human access, artificial lighting, and habitat alteration. In contrast, very few undisturbed beaches remain along the Atlantic coast, and these areas represent unique habitats that can help decipher the impacts of development on various species. We monitored nesting patterns and hatching success of three sea turtle species (Caretta caretta, Chelonia mydas, and Dermochelys coriacea) from 1985 to 2003 on Canaveral National Seashore, an uninhabited 38.3 km stretch of beach in Florida. We monitored the number of nests deposited annually, hatching success, predation rates, and spatial nesting patterns. Clutch sizes varied within, but not among years, most likely due to variation in size of nesting females rather than climate change or resource availability. The number of nests increased over the study period for all three species, indicating growing populations. Higher numbers of nests were deposited on the southern end of our beach than on the northern end. Hatching success did not vary by species, but was dependent on nest-protection effort, which increased during the study period. Protecting more nests with wire screens resulted in lower predation rates. We did not find any evidence suggesting that predators are using nest markers to locate eggs, even after using the same method of marking nests for 19 years. Our conservation efforts have lowered predation levels through increased screening effort, and over time the number of nests laid increased for each species. Collecting baseline data on nesting patterns in undisturbed locations will allow comparisons to be made on nesting trends and patterns at geographically close, but disturbed, localities.     

Nest site restoration increases the breeding density of blue-tailed bee-eaters

For birds that excavate their own nest burrows, the availability of suitable nest sites and substrates may influence the number and density of breeding birds. However, few studies of burrow-nesting birds have experimentally manipulated nest site or substrate availability. The blue-tailed bee-eater (Merops philippinus) is a colony breeding, summer migrant that excavates nest burrows in sandy banks on Kinmen Island, 5 km east of mainland China. To test whether substrate availability influenced the density or reproductive success of breeding pairs of bee-eaters, we removed all vegetation and old nest holes on treated slopes and left control slopes unmanipulated in 2003 and 2004. Plant cover on control slopes was 37.7% (11.9-67.7%). Slope gradient, soil penetration resistance and vegetation height in front of slopes did not differ between treated and control slopes in either year. Combining data from both years, the density of active nests was significantly higher (3.1-fold) on treated slopes than on control slopes. However, the reproductive performance of bee-eaters nesting on treated and control slopes did not differ in either year. Thus, removing vegetation and old nest holes from slopes with sandy loam soil improved the breeding habitat and increased the number of breeding blue-tailed bee-eaters. This technique could be used to support and manage populations of this species and other burrow-nestling species with similar habitat requirements.     

Effects of feral mink removal on seabirds, waders and passerines on small islands in the Baltic Sea

We studied the effects of removing introduced American mink (Mustela vison) on the number of birds breeding on small islands in the Baltic Sea. During autumn 1992-2001 mink were removed from a 72 km² area, while mink were not removed from a 35 km² control area. Second removal (125 km²) and control areas (130 km²) were established during 1998-2001. The breeding densities of ringed plover (Charadrius hiaticula), arctic skua (Stercorarius parasiticus), arctic tern (Sterna paradisaea) and rock pipit (Anthus petrosus) increased markedly in the removal areas in comparison to the control areas. Turnstone (Arenaria interpres), common gull (Larus canus) and wheatear (Oenanthe oenanthe) also appeared to increase. Two species already extinct in one of the removal areas, razorbill (Alca torda) and black guillemot (Cepphus grylle), returned to breed in the area. Breeding densities of great black-backed gull (Larus marinus), oystercatcher (Haematopus ostralegus) and white wagtail (Motacilla alba) were unaffected. We conclude that it is possible to remove feral mink from large archipelagos with many small islands, and that mink removal increases the breeding densities of many bird species in this habitat.     

Predation of artificial nests in a fragmented landscape in the tropical region of Los Tuxtlas, Mexico

Predation rates of artificial nests were investigated in a fragmented landscape in the lowlands of Los Tuxtlas in southern Mexico. Hen and plasticine eggs were used to assess predation pressure in four habitats: the interior of forest fragments, the forest-pasture edge, corridors of residual forest vegetation and linear strips of live fences across pastures. Three sites per habitat were used in three experimental trials. Hen and plasticine ground nests with three eggs each were alternated every 50 m along transects at each site. Predation rates on each type of nest were monitored for 9 days. Survey of potential avian and mammalian potential nest predators were conducted at each site prior to the experimental trails. Readings of amount of light illuminating the ground were taken by each nest at each site to assess exposure of nests. In general, average predation rates were significantly higher for both hen and plasticine nests in the forest-pasture edge and in the corridors than in the interior of the forest fragments. While birds and mammals were the principal predators on hen eggs in the forests, mammals were responsible for the majority (?70%) of eggs damaged at the other habitats. Surveys of potential nest predators showed that avian and mammalian potential nest predators were significantly more common at the forest-pasture edges and at the other habitats than in the forests. Readings of light reaching the ground suggest that concealment of nests by the vegetation may play an important role in predation risk. Our results are consistent with reports from other Neotropical rainforests indicating an increase of artificial nest predation pressures from forest interior to open habitats. Restoration of forest fragments, allowing the vegetation to grow along the forest-pasture edge and the planting of arboreal crops at the forest-pasture edges may be measures that could increase cover and nest protection.     

Can an introduced predator trigger an evolutionary trap in a colonial seabird?

Animals use environmental cues, social information and behavioural decision-making rules moulded by natural selection to decide where to breed. We assessed whether the presence of an alien nest predator, the Black Rat (Rattus rattus) is used by a colonial seabird, the Cory’s shearwater (Calonectris diomedea), as an environmental cue in the selection of breeding sites. We compared potential habitat preference using quality of breeders and nest fidelity as response to nest predation between two sub-colonies with different habitat characteristics, breeding success and predation pressure. Quality of individuals was better in the predated habitat and birds did not perceive the presence of predators signalling differences in predation risk and in turn of breeding success. This failure of perception could be at two levels: in the selection of habitat for first breeding and in the breeding dispersal following a reproductive failure. Preference for the sub-colony with higher predation risk suggests the presence of an evolutionary trap. In fact, the introduction of alien predators probably transformed the behaviour of shearwaters in a maladaptive response due to a mismatch between the new environmental factors and their behavioural and evolutionary algorithms. This can be a common pattern in other species with little behavioural plasticity, evolved in stable environments free of predators.     

Conservation and the scarecrow effect: Can human activity benefit threatened species by displacing predators?

Most wild animals show direct negative responses to human disturbance; however, disturbance may also have positive indirect effects by altering species interactions. In the Caribbean, introduced mongooses (Herpestes javanicus) are an important diurnal predator of the nests of critically endangered hawksbill sea turtles (Eretmochelys imbricata). We asked whether daytime visitor activity could benefit hawksbills by reducing mongoose activity in nesting habitat. We used track monitoring sites to measure how mongooses alter their space use in response to fluctuating human beach use and availability of hawksbill nests, and we estimated the indirect impact of human disturbance on nest predation for 5 years of hawksbill nesting. Human activity substantially decreased mongoose use of nesting habitat. Mongoose activity increased with abundance of nests and was lower at the edge than the interior of patches of beach vegetation, but neither factor altered relative sensitivity to disturbance. We estimated that, in addition to deterring mongooses from accessing nests on the open beach, human activity has the potential to reduce predation of nests in vegetation by up to 56% at our site, with the greatest per capita impact at low (<50) visitor numbers. Many forms of human activity negatively affect sea turtles, but our study indicates that there are positive aspects of daytime use of nesting beaches. Optimizing the trade-off between positive and negative impacts of disturbance could provide a useful approach for regulating tourist activity in critical habitat for threatened species.     

Causes of mortality at nests of ground-nesting birds in the Upper Waitaki Basin, South Island, New Zealand: a 5-year video study

We used video cameras over 5 years to quantify causes of mortality at 172 nests of three species of ground-nesting birds that nest on braided riverbeds of the Upper Waitaki Basin, South Island, New Zealand. The species were banded dotterels Charadrius bicinctus (n=114), black stilts Himantopus novaezelandiae (n=23), and black-fronted terns Sterna albostriata (n=35). Of 77 recorded lethal events (excluding four desertions caused by us), 66 involved deaths of only eggs, and 11 involved deaths of adults and/or chicks, and/or eggs. The main predators were cats Felis catus, hedgehogs Erinaceus europaeus, and ferrets Mustela furo, which were responsible for 43, 20, and 18% of lethal events, respectively. Cats were the only predator species to take adult birds. We recorded only two avian predations: a harrier Circus approximans took a chick and a hatching egg from one nest, and an Australian magpie Gymnorhina tibicen ate chicks at one nest. Other causes of mortality were incubating adult birds, floods, and sheep Ovis aries. Each accounted for <4% of lethal events. Ninety percent of visits (151 of 168) by predators or potential predators happened between sunset and sunrise. We found no evidence that video cameras or infra-red lighting influenced predation rates during 2 years of testing for such effects.