Reduced Breeding Success Research Articles

Evaluation of the vulnerability of wild bird populations during heatwave events: implications for biodiversity conservation

Heatwaves, defined as prolonged periods of excessive heat, are becoming more frequent and severe due to global climate change. These extreme heat events can have significant impacts on ecosystems and wildlife, particularly on avian fauna. Birds, as vital components of ecosystems, are highly susceptible to the adverse effects of heatwaves. Impacts directly attributable to adverse weather and changing climate regimes include a higher risk of mortality, reduced breeding success, compromised body condition and immunocompetence, declining populations, range changes, and potentially maladaptive behavioral adjustments in foraging, parental care, and migration. This vulnerability is exacerbated by the loss and degradation of their natural habitats, which hinders their ability to adapt to constantly changing climatic conditions. Understanding the vulnerability of bird populations during heatwaves is essential for devising effective conservation strategies. Conservation efforts should focus on preserving and restoring critical bird habitats, providing access to freshwater sources, and creating artificial shelters. Additionally, public education plays a significant role in raising awareness about the importance of bird conservation during heatwave events and in the broader context of climate change. In analyzing the subject, the study aims to provide more precise information to support the public decisions to be made, as well as to guide awareness for the implementation of conservation actions.

Low‐ and high‐intensity fire in the riparian savanna: Demographic impacts in an avian model species and implications for ecological fire management

Abstract Climate change is driving changes in fire frequency and intensity, making it more urgent for conservation managers to understand how species and ecosystems respond. In tropical monsoonal savannas—Earth's most fire‐prone landscapes—ecological fire management aims to prevent intense wildfires late in the dry season through prescribed low‐intensity burns early in the dry season. Riparian habitats embedded within tropical savannas represent critical refuges for biodiversity, yet are particularly fire‐sensitive. Better understanding of the impact of fire—including prescribed burns—on riparian habitats is therefore key but requires long‐term detailed post‐fire monitoring of species' demographic rates, as effects may persist and/or be delayed. We analyse impacts of (prescribed) low‐intensity and (prescribed but escaped) high‐intensity fire in northern Australian riparian and adjacent savanna habitat. We quantify multi‐year impacts on density, survival, reproduction and dispersal of an endangered riparian bird, the western purple‐crowned fairy‐wren (Malurus coronatus coronatus), in a well‐studied individually marked population. Following low‐intensity fire, bird density was reduced by >20% in burnt compared to adjacent unburnt riparian habitat for ≥2.5 years. This was a result of reduced breeding success and recruitment for 2 years immediately following fire, rather than mortality or dispersal of adults. In contrast, high‐intensity fire (in a dry year) resulted in a sharp decline in population density by 50% 2–8 months after fire, with no signs of recovery after 2.5 years. The decline in density was due to post‐fire adult mortality, rather than dispersal. Breeding success of the (few) remaining individuals was low but not detectably lower than in unburnt areas, likely because breeding success was poor overall due to prevailing dry conditions. Synthesis and applications. Even if there is no or very low mortality during fire, and no movement of birds away from burnt areas, both low‐ and high‐intensity fire in the riparian zone reduce population density. However, the mechanism by which this occurs, and recovery time, differs with fire intensity. To minimise impacts of fire on riparian zones in tropical savannas, we suggest employing low‐intensity prescribed burns under optimal conditions shortly after the breeding season.

Projected increases in extreme drought frequency and duration by 2040 affect specialist habitats and species in Scotland

Abstract Climate‐driven increases in drought occurrence are an overlooked risk in temperate climates, which may be less resilient to water scarcity than arid habitats. Scotland provides an example of a temperate oceanic climate in which drought risk is understudied, but could have substantial ecological impacts, particularly in combination with additional stressors. We modelled changes to risk of extreme drought occurrence in Scotland, and considered potential impacts of these changes on two key habitats, ombrotrophic (precipitation fed) wetlands and temperate rainforest. Using temperature and precipitation data from the UK Climate Projections 2018 and the drought index Standardized Precipitation‐Evapotranspiration Index, we calculated and mapped likely changes in extreme drought for the near future (2021–2040) in comparison to a baseline (1981–2001). We found likelihood of extreme drought events increased from an average of one event every 20 years in the baseline period, to one event every 3 years by 2040. Typical events were projected to be up to 2–3 months longer, with an average of 11 extra drought months per decade. Increases were projected throughout the country, but the effect was most severe in the east, and during autumn. Ombrotrophic wetlands have some level of adaptive resilience to drought, but are considered at high risk as several key sites are in drought hotspot areas. In contrast, Scotland's temperate rainforest is located in lower risk areas, but is typically less drought resilient. Potential impacts on these habitats include reductions in ecosystem services including carbon capture, reduced breeding success for multiple taxa and increased competition among plant communities, which threatens drought‐sensitive species. These projections suggest that climate‐driven drought risk is a significant and imminent threat, even in wet climates. In Scotland, the results can direct mitigation and management actions to areas likely to be at greatest risk, as well as informing conservation interventions that can cope with drought. Additionally, other areas with a temperate oceanic climate may be at risk of extreme drought and could experience similar consequences. Therefore, these results have implications both for facilitating improved resilience to extreme drought in Scotland and for potentially overlooked impacts of drought in oceanic climates elsewhere.

Reduced breeding success in Great Black‐backed Gulls (Larus marinus) due to harness‐mountedGPSdevice

Animal‐borne bio‐logging devices are routinely fitted to seabirds to learn about their behaviour and physiology, as well as their interactions with the marine environment. The assessment and reporting of deleterious impacts from such devices on the individuals carrying them is critical to inform future work and improve data quality and animal welfare. We assessed the impacts of thoracic‐harness attachments on the breeding performance and inter‐annual return rates of Great Black‐backed Gulls. We found that tagged individuals hatched fewer eggs per nest (0.67) than two different control groups (handled but not tagged – 2.0, and not handled – 1.9) and had lower hatching success rates per nest (27% compared with 81% and 82% in control groups). Inter‐annual return rates were similar between tagged and control groups, but the harness attachment potentially caused the death of an individual 5 days after deployment. Overall, the harness attachment was a lead driver of nest failure. We urge extreme caution for those wanting to use harness‐mounted devices on Great Black‐backed Gulls.

The Scent Enriched Primate.

Zoos worldwide play an important role in both in situ and ex situ conservation via efforts such as providing breeding programmes and reintroductions into the wild. Zoo populations are crucial as a buffer against extinction. However, a mismatch between the wild and zoo environments can lead to psychological as well as physiological health issues, such as stress, boredom, diabetes, and obesity. These problems, in turn, can impact the reproductive success of individuals. Consequently, some primate species have reduced breeding success when housed in zoos compared to their wild counterparts. To prevent the onset of behavioural, physiological, and cognitive negative effects and to continually improve the welfare of their animals, zoos widely implement different types of environmental enrichment. There are many forms enrichment can take, such as feeding, puzzles and training, but sensory enrichments, including implementing the use of scents, are currently understudied. Scent enrichments are less utilized despite multiple research studies showing that they may have positive effects on welfare for zoo-housed animal species, including non-human primates. Despite being traditionally considered to be microsmatic, various lines of evidence suggest that olfaction plays a larger role in primates than previously thought. This review therefore focuses on scent-based enrichment and the specifics of captive primates.

Oogenesis and in vitro reproduction of the twospot astyanax Astyanax bimaculatus (Linnaeus, 1758) exposed to conspecific alarm substance

Stress situations can be essential to trigger reproduction in fish; however, it may also inhibit it. One of those situations involves the release of the conspecific alarm substance (CAS), a natural stressor, into the water by specific fish epidermal cells after a predator attack. Little is known about the effects of that substance on fish reproduction. This study aimed to evaluate the effects of CAS exposure on the oogenesis and reproduction of the twospot astyanax Astyanax bimaculatus before the hormonal induction for artificial reproduction. No macroscopic or cellular changes in the ovaries were observed for the females exposed to CAS, and the oocyte stages show all females in the same phase of maturation (Spawning Capable). Females exposed to CAS spawned 20 min before the females without exposure. On the other hand, they ovulated only once, whereas the females from the control group ovulated multiple times for approximately two hours after hormonal induction. Moreover, the precocious ovulation of the females submitted to CAS did not generate offspring, since all generated zygotes did not develop. In contrast, the control group females produced more than 11 thousand healthy larvae. Exposing the female fish to CAS during their reproductive management in captivity may reduce breeding success.

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Stress situations can be essential to trigger reproduction in fish; however, it may also inhibit it. One of those situations involves the release of the conspecific alarm substance (CAS), a natural stressor, into the water by specific fish epidermal cells after a predator attack. Little is known about the effects of that substance on fish reproduction. This study aimed to evaluate the effects of CAS exposure on the oogenesis and reproduction of the twospot astyanax Astyanax bimaculatus before the hormonal induction for artificial reproduction. No macroscopic or cellular changes in the ovaries were observed for the females exposed to CAS, and the oocyte stages show all females in the same phase of maturation (Spawning Capable). Females exposed to CAS spawned 20 min before the females without exposure. On the other hand, they ovulated only once, whereas the females from the control group ovulated multiple times for approximately two hours after hormonal induction. Moreover, the precocious ovulation of the females submitted to CAS did not generate offspring, since all generated zygotes did not develop. In contrast, the control group females produced more than 11 thousand healthy larvae. Exposing the female fish to CAS during their reproductive management in captivity may reduce breeding success.

Estimating capercaillie Tetrao urogallus population size in Scotland from annual leks and counts of broods over the period 2010–2020

The population size of capercaille Tetrao urogallus in Scotland was estimated from annual counts of males at 151 known spring lek sites and 45 counts of males, females and their broods in August during the period 2010–2020, combined with published survival estimates. Population size declined from an estimated 580 birds (95% CL: 462–698) in 2011 to 304 (239–369) in 2020 and the extinction probability was 23% after 25 years, 95% after 50 years and 100% after 100 years. Removing mortality associated with collisions against fences lowered extinction probabilities to 0, 3 and 40% respectively, emphasizing fence removal as a management priority. Greatest declines occurred in Deeside and Donside, Morayshire and Ross‐shire and Perthshire and Trossachs. In the Badenoch and Strathspey stronghold, where an estimated 83% of males remain, numbers of occupied leks and lekking males have decreased in the last five years and are associated with reduced breeding success over the last 10 years. Our estimates of population size are only half that derived from the last official national survey in winter 2015/2016. We discuss possible reasons for this difference including underestimates from lek‐based surveys linked to male detection probabilities. Finally, we recommend improvements to the current annual monitoring scheme, including repeat counts at leks and use of novel remote acoustic devices and restoration of annual brood counts in remaining key forests.

Contrasting impacts of environmental variability on the breeding biology of two sympatric small procellariiform seabirds in south-eastern Australia.

Seabirds play a vital role in marine ecosystems and the long-term study of their responses to environmental variations can be used to monitor the effects of climate change on marine fauna. However, slight differences in similar seabird species result in a range of responses which complicates our understanding of the effects of environmental changes to marine ecosystems. The present study investigated inter-annual differences in the breeding biology (breeding phenology, chick growth rates and breeding success) and environmental conditions (seasonal sea surface temperatures) of important foraging areas in two sympatric small Procellariiform species, the fairy prion (Pachyptila turtur) and the common diving petrel (Pelecanoides urinatrix), over four reproductive seasons (2017–2020) in Bass Strait, south-eastern Australia. Marine heatwaves occurred during the years of 2018/19 and 2019/20 and coincided with years of delayed laying dates, slower chick growth and reduced breeding success, in both species. While fairy prions maintained a relatively high breeding success and broadly constant breeding phenology, common diving petrels delayed the start of the breeding season by up to 50 days and experienced dramatic collapses in breeding success in years of high marine heat wave occurrence. The difference in foraging ecology and physiological capacity (largely in the production of stomach oils and fasting abilities of adults and chicks) between both species are likely to influence the variability and phenology in the observed breeding seasons.

Early Breeding Conditions Followed by Reduced Breeding Success Despite Timely Arrival in an Alpine Migratory Songbird

Timing reproduction to coincide with optimal environmental conditions is key for many organisms living in seasonal habitats. Advance in the onset of spring is a particular challenge to migratory birds that must time their arrival without knowing the conditions on the breeding grounds. This is amplified at high elevations where resource availability, which is linked to snowmelt and vegetation development, shows much annual variation. With the aim of exploring the effects of variability in the onset of local resource availability on reproduction, we compared key life history events in an Alpine population of the Northern Wheatear (Oenanthe oenanthe) between years of contrasting timing of snowmelt. Based on remote sensed images, we identified 2020 as an exceptionally early snowmelt and green-up year compared to the preceding year and the long-term average. Individuals tracked with light-level geolocators arrived well before the snowmelt in 2020 and clutch initiation dates across the population were earlier in 2020 compared to 2019. However, observations from a citizen science database and nest monitoring data showed that the arrival-breeding interval was shorter in 2020, thus the advance in timing lagged behind the environmental conditions. While hatching success was similar in both years, fledging success was significantly reduced in 2020. A trophic mismatch in early 2020 could be a possible explanation for the reduced reproductive success, but alternative explanations cannot be excluded. Our results show that, despite the timely arrival at the breeding grounds and a contraction of the arrival-breeding interval, Wheatears were not able to advance breeding activities in synchrony with environmental conditions in 2020. Earlier reproductive seasons are expected to become more frequent in the future. We show that the negative effects of changing seasons in Alpine migratory birds might be similar to birds breeding at high latitudes, despite their shorter migratory distance.

Early Breeding Conditions Followed by Reduced Breeding Success Despite Timely Arrival in an Alpine Migratory Songbird

Early Breeding Conditions Followed by Reduced Breeding Success Despite Timely Arrival in an Alpine Migratory Songbird

Hemispheric asymmetry in ocean change and the productivity of ecosystem sentinels.

Climate change and other human activities are causing profound effects on marine ecosystem productivity. We show that the breeding success of seabirds is tracking hemispheric differences in ocean warming and human impacts, with the strongest effects on fish-eating, surface-foraging species in the north. Hemispheric asymmetry suggests the need for ocean management at hemispheric scales. For the north, tactical, climate-based recovery plans for forage fish resources are needed to recover seabird breeding productivity. In the south, lower-magnitude change in seabird productivity presents opportunities for strategic management approaches such as large marine protected areas to sustain food webs and maintain predator productivity. Global monitoring of seabird productivity enables the detection of ecosystem change in remote regions and contributes to our understanding of marine climate impacts on ecosystems.

The effects of four decades of climate change on the breeding ecology of an avian sentinel species across a 1,500-km latitudinal gradient are stronger at high latitudes.

Global warming affects breeding phenology of birds differentially with latitude, but there is contrasting evidence about how the changing climate influences the breeding of migrating songbirds at their northern breeding range. We investigate the effect of climate warming on breeding time and breeding success of European pied flycatchers Ficedula hypoleuca in Sweden during a period of 36 years using nest reports from bird ringing. To account for the latitudinal variation, we divided Sweden into three latitudinal bands (northern, intermediate, and southern). We applied a sliding window approach to find the most influential period and environment characteristics (temperature, vegetation greenness, and precipitation), using linear mixed models and model averaging. Our results show a long‐term advancement of breeding time related to increasing spring temperature and vegetation greenness during a period before hatching. Northern breeders revealed a larger advancement over the years (8.3 days) compared with southern breeders (3.6 days). We observed a relatively stronger effect of temperature and greenness on breeding time in the north. Furthermore, northern birds showed an increase in breeding success over time, while birds breeding at southern and intermediate latitudes showed reduced breeding success in years with higher prehatching temperatures. Our findings with stronger environment effects on breeding time advancement in the north suggest that pied flycatchers are more responsive to weather cues at higher latitudes. Breeding time adjustment and, potentially, low competition help explain the higher long‐term success observed in the north. Reduced breeding success at more southerly latitudes suggests an inability to match breeding time to very early and warm springs, a fate that with continued climate change could also be expected for pied flycatchers and other long‐distance migrants at their very northern breeding range.

Homozygosity of BACHD rats not only causes strong behavioral deficits in young female rats but also a reduced breeding success

Huntington’s disease is known to be a purely genetic disease based on an expansion of a CAG base triplet repeat in the coding region of the Huntingtin gene. Some years ago, researchers were able to introduce the extensive full-length gene sequence of the mutant huntingtin gene into a rodent model. The resulting BACHD rat is already well characterized for behavioral deficits. So far, all analyses in this preclinical rat model were performed in male hemizygous animals. As homozygosity of transgenic models often causes an amplification of the phenotype and female HD patients present a stronger phenotype compared to men, we established a homozygous breeding colony and tested 2 and 5 months old homozygous male and female BACHD rats in a behavioral test battery. The tests included the grip strength test, Rota Rod, elevated plus maze, passive avoidance, and Barnes maze test. Our results show strong deficits in young female homozygous BACHD rats including increased body weight, motor deficits, muscle weakness, reduced anxiety and hypoactivity, as well as learning and memory deficits. Analysis of male homozygous BACHD rats showed only weak disease symptoms, similar compared to male hemizygous BACHD rats of already published studies. Evaluation of the breeding success showed that homozygous BACHD have a reduced number of pups at the time of birth that even decreases until weaning. Our results suggest that the phenotype of homozygous male BACHD rats barely differs from already published results of hemizygous BACHD rats while female homozygous BACHD rats display strong and early alterations.

Bottom‐up effect of eradications: The unintended consequences for top‐order predators when eradicating invasive prey

Abstract The eradication of invasive species from islands yields significant conservation returns. However, novel challenges continue to arise as projects expand in their scope, complexity and scale. Prey‐loss and secondary poisoning were historically considered to have limited impact on native top‐order predators when planning eradications, but this has rarely been tested quantitatively. We used a 10‐year timeseries of Brown Skua Stercorarius antarcticus lonnbergi breeding surveys and isotopic dietary analysis on Macquarie Island to investigate how prey‐loss and secondary poisoning deaths resulting from the eradication of an abundant invasive prey species, European rabbits Oryctolagus cuniculus, affected a top‐order predator. Skua nest density declined from 7.14 nests/km2 (95% CI: 6.01–8.27) in the presence of rabbits (pre‐eradication) to 3.73 nests/km2 (95% CI: 2.96–4.51) in the first 3 years after the eradication of rabbits, before showing signs of recovery in the 4 years thereafter. However, breeding success dropped from 1.01 chicks/nest (95% CI: 0.76–1.26) to as low as 0.38 chicks/nest (95% CI: 0.23–0.53) with little evidence of recovery. Secondary poisoning affected a greater number of skuas than anticipated prior to the eradication, including skuas nesting in areas where rabbits were not typically hunted as prey. We highlight that invasive prey often replace native prey in the diet of native predators rather than provide an additional source of food, and rapid eradication of non‐native prey can have long‐term impacts for predators, particularly when recovery of native prey is slow. Synthesis and applications. Monitoring programmes that complement large‐scale eradication projects and address (a) trophic‐driven declines in predator populations and (b) population‐level impacts of secondary poisoning are integral to ensuring bottom‐up effects of eradications are anticipated and adequately quantified. If prey deficits caused by eradication of invasive prey are expected to be severe but short‐lived, supplementary feeding programmes may buffer against increased predation pressure on native prey and reduced breeding success of native predators. Alternatively, if the rapid recovery of native prey is not expected to occur naturally, breeding programmes and translocation of native prey prior to assist recovery of native predators should be considered to support ecosystem restoration.

Introduced species and extreme weather as key drivers of reproductive output in three sympatric albatrosses

Invasive species present a major conservation threat globally and nowhere are their affects more pronounced than in island ecosystems. Determining how native island populations respond demographically to invasive species can provide information to mitigate the negative effects of invasive species. Using 20 years of mark-recapture data from three sympatric species of albatrosses (black-browed Thalassarche melanophris, grey-headed T. chrysostoma, and light-mantled albatrosses Phoebetria palpebrata), we quantified the influence of invasive European rabbits Oryctolagus cuniculus and extreme weather patterns on breeding probability and success. Temporal variability in rabbit density explained 33–76% of the variability in breeding probability for all three species, with severe decreases in breeding probability observed after a lag period following highest rabbit numbers. For black-browed albatrosses, the combination of extreme rainfall and high rabbit density explained 33% of total trait variability and dramatically reduced breeding success. We showed that invasive rabbits and extreme weather events reduce reproductive output in albatrosses and that eliminating rabbits had a positive effect on albatross reproduction. This illustrates how active animal management at a local breeding site can result in positive population outcomes even for wide ranging animals like albatrosses where influencing vital rates during their at-sea migrations is more challenging.

Tourism and little penguins: a comparative study of survival and reproductive parameters

Abstract ContextTourism operations that provide the opportunity for wildlife viewing can support conservation management through public education, habitat protection, population management, research activities and revenue generation. However, alongside these potential benefits there can be negative effects on the species that include the possibility of reduced breeding success, survival and, subsequently, population growth. AimWe sought to establish whether a tourism operation affected little penguins Eudyptula minor by comparing reproductive and survival parameters of two managed populations in Oamaru, New Zealand. One population was the focus of a tourism operation, whereas the other (located nearby) did not provide any public access. The tourism operation implemented strategies to reduce human disturbance, including the reduction of noise, light and movement from visitors during viewing, and by ensuring that visitors were restricted to designated viewing areas. MethodsNesting boxes were installed at both colonies that facilitated weekly monitoring of the populations. The monitoring program (1993–2017) included weekly checks of every nesting box in the colony for the presence of eggs, chicks and adults. Chicks were weighed before fledging, and both fledglings and breeding adults were individually marked. Key resultsLay dates, breeding success, chick fledge mass and survival rates were not measurably different between the two populations, indicating that there was little evidence of negative effects on the penguins at the tourist site. ConclusionsThe slight differences observed between the colonies during the time series were attributed to environmental damage sustained at the non-tourist site and a difference in the proportion of un-monitored nest sites available to the penguins when the study was initiated. ImplicationsNotwithstanding the present findings, long-term and consistent monitoring of wildlife populations facing increasing pressure from tourism is essential to ensure effects are detected and that management procedures implemented to reduce potential effects.

Rare mutations in the complement regulatory gene CSMD1 are associated with male and female infertility

Infertility in men and women is a complex genetic trait with shared biological bases between the sexes. Here, we perform a series of rare variant analyses across 73,185 women and men to identify genes that contribute to primary gonadal dysfunction. We report CSMD1, a complement regulatory protein on chromosome 8p23, as a strong candidate locus in both sexes. We show that CSMD1 is enriched at the germ-cell/somatic-cell interface in both male and female gonads. Csmd1-knockout males show increased rates of infertility with significantly increased complement C3 protein deposition in the testes, accompanied by severe histological degeneration. Knockout females show significant reduction in ovarian quality and breeding success, as well as mammary branching impairment. Double knockout of Csmd1 and C3 causes non-additive reduction in breeding success, suggesting that CSMD1 and the complement pathway play an important role in the normal postnatal development of the gonads in both sexes.

Assessing the effectiveness of bird rehabilitation: temporarily captive-reared Little Owls (Athene noctua) experience a similar recruitment rate as wild birds

A large number of young birds are caught each year shortly after having left the nest and subsequently brought to bird care centres. These birds are temporarily hand-raised before release. To date, the effectiveness of this action has remained largely unassessed. Here we monitored the fate of 119 rehabilitated Little Owls (Athene noctua) and found that the recruitment rate of the rehabilitated birds was similar to that of wild birds (11.8% of 119 rehabilitated birds vs. 10.7% of 382 wild fledglings). The timing of release, i.e. whether rehabilitated birds were released in the autumn or in the following spring, did not appear to affect recruitment probabilities, although birds released the following spring showed a tendency for reduced breeding success and dispersal compared to wild birds, suggesting that autumn releases may be more favourable.

Avian and Skunk Predation of Ashy Storm-Petrels at Santa Cruz Island, California

Avian and mammalian predation of Ashy Storm-Petrels (Oceanodroma homochroa) were documented during monthly nest monitoring in 1995–1998 and 2005–2015 and once-per-year monitoring in 1999–2004 at 5 colonies at Santa Cruz Island, California. Throughout the study period, predation by Barn Owls (Tyto alba) was documented at the sea cave colonies, and at Orizaba Rock, and likely contributed to reduced breeding success at Bat Cave in 1995–1997. Recent predation by Common Ravens (Corvus corax) caused reduced breeding success each year in 2012–2015 at Orizaba Rock (35%–48%) and in 2013 and 2015 at Bat Cave (49%–52%). Raven predation appears to be developing into a larger long-term problem that may require additional management actions, but efforts have focused on deploying artificial nests at Bat Cave to provide more protected sites. Two sea cave colonies where skunks previously had not been detected experienced major adult mortality due to unusual predation events by island spotted skunks (Spilogale gracilis amphiala) in 2005 and 2008. At Bat Cave in 2005, 2 skunks were trapped after they had killed at least 76 Ashy Storm-Petrel adults. Prior to this mortality event, the colony contained 64–97 nests per year in 1995–1997, with low breeding success (49%–59%) related largely to organochlorine pollutants. After the skunks were caught, nest numbers increased from 19 in 2006 to 100 in 2015, with variable but generally higher breeding success (49%–90%). At Cavern Point Cove Caves in 2008, 2 skunks were trapped after killing at least 32 adults. Prior to the mortality event, this colony contained 11–17 nests per year in 1995–1997, with a lower breeding success rate of 20%–47%. Nest numbers increased from 2 in 2009 to 10 in 2015, with a variable but higher rate of breeding success (50%–80%). Skunk predation events may have been related to temporarily higher skunk populations at Santa Cruz due to low numbers of island foxes (Urocyon littoralis santacruzae). Deer mice (Peromyscus maniculatus santacruzae) were not significant predators of Ashy Storm-Petrels at Santa Cruz Island.