REPORT TO THE U.S. FISH AND WILDLIFE SERVICE FOR HAWAIIAN STILT ENDANGERED SPECIES PERMIT: TE-25955C-1 REPORTING PERIOD JANUARY 01, 2019 – DECEMBER 31, 2019 SUBMITTED BY: Melissa R. Price Assistant Professor University of Hawaii-Manoa 1910 East-West Road Sherman Hall Rm 118 Honolulu, HI 96822 Phone: 808-956-7774 Email: [email protected] January 31, 2020 1 2 EXECUTIVE SUMMARY 3 The Hawaiian Stilt (Himantopus mexicanus knudseni) is an endangered subspecies of the 4 Black-necked stilt (Himantopus mexicanus) that inhabits wetlands throughout the Hawaiian 5 Islands. Depredation of eggs and chicks by introduced predators is a major threat to Hawaiian 6 Stilt populations. Where and when a bird decides to nest may impact the likelihood of egg or 7 chick depredation. Nesting in close proximity to water may decrease depredation rates by 8 mammals, as water can act as a barrier to mammalian predators, does not hold scent, and 9 provides an obstacle-free escape route for chicks. Alternatively, some mammalian predators may 10 be attracted to water, and a number of aquatic species have been identified as predators of 11 Hawaiian Stilt chicks, including the American Bullfrog (Lithobates catesbeianus). Vegetation 12 height is also an important factor for egg and chick survival, as taller vegetation may help 13 conceal nests and chicks from predators, particularly aerial species. Additionally, depredation is 14 often not constant across the breeding season due to changes in parental activity, nest and chick 15 abundance, or habitat characteristics. The Hawaiian Stilt nests from February to September 16 across the Hawaiian Islands. The nesting season coincides with a seasonal decline in 17 precipitation, which may alter habitat characteristics and thus impact depredation rates. Further, 18 management tools, such as mammal-exclusion fencing, are currently in use and may greatly 19 increase egg and chick survival. The objectives of this project were to: 1) identify habitat 20 characteristics important for nest-site selection and chick habitat use; 2) identify factors that 21 impact hatching and fledging success. 22 We found that stilts preferred to nest in shorter vegetation than what was available and 23 preferred Pickleweed (Batis maritima) rather than other available plant species. However, nest- 24 site characteristics, such as vegetation height and distance to water, did not have an impact on 25 egg depredation risk. Early nests had a higher chance of hatching than late nests. The number of 26 depredated nests peaked later in the nesting season, following a peak in nest initiation. 27 Introduced mammals were the primary egg predators and included rats (Rattus spp.), feral cats 28 (Felis catus), and Small Indian Mongooses (Herpestes auropunctatus). The number of eggs laid, 29 as well as hatching success, was greater inside the mammal exclusion fence at Honouliuli 30 Wetland, compared to a nearby site without a fence, Waiawa Wetland, where mammalian 31 predators are only excluded via trapping. The average home range size for 12 tracked pre- 32 fledglings was 0.94 ± 1.42 acres, and most chicks were observed using vegetated mudflats near 33 open water. Of the 20 chicks that were tracked in this study, 7 fledged (35%), 6 had unknown 34 fates (30%), 4 died due to unknown causes (20%), 2 were depredated by a feral cat (10%), and 1 35 died due to emaciation (5%). 36 Our results suggest that management of predators, particularly mammals, is key to 37 improving stilt hatching success, as preferred nest-site characteristics do not reduce the 38 likelihood of egg depredation. Tall, invasive vegetation, such as California Grass (Brachiaria 39 mutica), should continue to be controlled, as it was rarely used for nesting. More desirable 40 vegetation, such as Pickleweed, should be made available throughout wetlands to encourage 41 larger spacing between nesting pairs, which may help to reduce egg depredation pressure. 42 Increasing mammalian predator control later in the nesting season may also increase hatching 43 success of later nesters. Alternatively, mammal-exclusion fencing may provide year-round 44 protection from mammalian predators, increasing both egg and chick survival. More data is 45 needed to form conclusions regarding home range and survival of Hawaiian Stilt chicks. 46 Improved detection methods and radio-tagging attachment styles will be used in the 2020 nesting 47 season, which will reduce uncertainties and improve statistical power of analyses. 1 48 INTRODUCTION 49 This report fulfills the annual reporting requirements for the permit agreement listed below. The 50 reporting period is January 1, 2019– December 31, 2019. 51 52 Permit 53 Number: TE-25955C-2 54 Effective date: March 27, 2019 55 Expiration Date: March 26, 2024 56 57 Nesting Ecology of the Hawaiian Stilt (Himantopus mexicanus knudseni) on O‘ahu 58 59 The Hawaiian Stilt (Himantopus mexicanus knudseni) is an endangered subspecies of the 60 Black-necked Stilt (Himantopus mexicanus) that inhabits wetlands throughout the Hawaiian 61 Islands (U.S. Fish and Wildlife Service 2011). The Hawaiian Stilt nests from February to 62 September across the Hawaiian Islands. Nest depredation has been identified as a major threat to 63 Hawaiian Stilt nesting success (U.S. Fish and Wildlife Service 2011). Hawaiian Stilts have a 64 variety of potential mammalian, avian, and aquatic predators, including introduced rats (Rattus 65 spp.), feral cats (Felis catus), Small Indian Mongooses (Herpestes auropunctatus), Cattle Egrets 66 (Bubulcus ibis), Barn Owls (Tyto alba), catfish (Order: Siluriformes), and American Bullfrogs 67 (Lithobates catesbeianus), as well as native Black-crowned Night-Herons (‘Auku‘u; Nycticorax 68 nycticorax hoactli) and Hawaiian Short-eared Owls (Pueo; Asio flammeus sandwichensis) (U.S. 69 Fish and Wildlife Service 2011). Where and when a bird decides to nest can impact the 70 likelihood of nest depredation. Nesting in close proximity to water may decrease depredation 71 rates, as water can act as a barrier to mammalian predators (Picman 1988, Hoover 2006). 72 Alternatively, some mammalian predators may be attracted to water (Bonesi and Palazon 2007). 73 Vegetation height is also an important factor for nest survival, as taller vegetation may help 74 conceal nests from predators, particularly aerial species (Kristiansen 1998, Jedlikowski et al. 75 2015). Further, nest depredation is likely not constant across the breeding season (Thyen and Exo 76 2005, Wilson et al. 2007, Polak 2016) and may vary due to changes in nest abundance 77 (Tinbergen et al. 1967, Holt 1977, Nams 1997) or parental activity (Skutch 1949, Martin et 78 al. 2000). 79 Hawaiian Stilts have a prolonged nesting season compared to Black-necked Stilts, which 80 generally nest from April to August in temperate regions (Carmona et al. 2000, Conway et al. 81 2005, Ackerman et al. 2014). Increased nesting opportunities due to a prolonged breeding season 82 suggest within-season timing of nesting may not be as important for determining nesting success 83 of Hawaiian Stilts, as is demonstrated in other waterbird species in temperate regions (Thyen and 84 Exo 2005, Cuervo 2010, Ackerman et al. 2014). However, precipitation on most islands in 85 Hawai‘i varies temporally, with the “wet season” occurring October through April, and the “dry 86 season” May through September (Price 1983). The Hawaiian Stilt nesting season begins during 87 the “wet season” and coincides with a seasonal decline in rainfall across the islands, which may 88 cause temporal changes in available nesting habitat, leading to differences in nest depredation 89 risk. 90 Habitat characteristics may also impact chick habitat use and survival, as nesting near 91 vegetation and water is important for both foraging and protection from predators. Sordahl 92 (1982) found that Black-necked Stilts that nest in close proximity to vegetation had greater 93 survival, as chicks used vegetation to hide from predators. Use of nearby water bodies was also 2 94 important for decreasing chick depredation, as open water does not hold scent and has few 95 obstacles, providing a safe escape route from mammalian predators (Sordahl 1982). In order to 96 effectively guide management decisions, more research is needed that examines how habitat use 97 impacts Hawaiian Stilt chick survival. 98 Ground nesting birds tend to benefit most from predator removal (Lavers, Wilcox, and 99 Donlan 2010). However, targeted removal of a single predator type, such as cats, may lead to 100 predator release of rats or other mesopredators, which may increase in number and increase 101 depredation on eggs or chicks (Rayner et al. 2007). Thus, predator control approaches that target 102 multiple species or exclude entire taxonomic groups of invasive predators, such as mammal- 103 exclusion fencing, may be warranted in some cases (Smith et al. 2010; Young et al. 2013). 104 Fencing that excludes mammalian predators, often referred to as predator-proof fencing or pest- 105 proof fencing, prevents the depredation of eggs and chicks by excluding invasive mammalian 106 predators (Burns, Innes, and Day 2012). Multiple methods for invasive predator removal are 107 currently being used in protected Hawaiian Stilt nesting habitat, including traps, fences, and 108 poison baiting. For endangered species, in which depredation significantly contributes to 109 extinction risk, fencing may be the most effective option for species recovery. In contrast, in 110 species nearing population sizes that may warrant delisting, the question remains whether 111 mammal-exclusion fencing is a cost-efficient option for recovery. Resource managers use 112 changes in population through time to measure the effectiveness of management actions (Reed et 113 al. 2007); however, there may be a lag in population growth immediately following conservation 114 actions. Measures of reproductive success and mortality provide more immediate measures of 115 potential changes in population demographics. Studies of target species following the 116 implementation of a management action allows resource managers to evaluate its cost 117 effectiveness in mitigating extinction risk.