Nesting Ecology of the Hawaiian Stilt (Himantopus mexicanus knudseni) on O‘ahu

Abstract

The Hawaiian Stilt (Himantopus mexicanus knudseni) is an endangered subspecies of the Black-necked stilt (Himantopus mexicanus) that inhabits wetlands throughout the Hawaiian Islands. Depredation of eggs and chicks by introduced predators is a major threat to Hawaiian Stilt populations. Where and when a bird decides to nest may impact the likelihood of egg or chick depredation. Nesting in close proximity to water may decrease depredation rates by mammals, as water can act as a barrier to mammalian predators, does not hold scent, and provides an obstacle-free escape route for chicks. Alternatively, some mammalian predators may be attracted to water, and a number of aquatic species have been identified as predators of Hawaiian Stilt chicks, including the American Bullfrog (Lithobates catesbeianus). Vegetation height is also an important factor for egg and chick survival, as taller vegetation may help conceal nests and chicks from predators, particularly aerial species. Additionally, depredation is often not constant across the breeding season due to changes in parental activity, nest and chick abundance, or habitat characteristics. The Hawaiian Stilt nests from February to September across the Hawaiian Islands. The nesting season coincides with a seasonal decline in precipitation, which may alter habitat characteristics and thus impact depredation rates. Further, management tools, such as mammal-exclusion fencing, are currently in use and may greatly increase egg and chick survival. The objectives of this project were to: 1) identify habitat characteristics important for nest-site selection and chick habitat use; 2) identify factors that impact hatching and fledging success. We found that stilts preferred to nest in shorter vegetation than what was available and preferred Pickleweed (Batis maritima) rather than other available plant species. However, nest-site characteristics, such as vegetation height and distance to water, did not have an impact on egg depredation risk. Early nests had a higher chance of hatching than late nests. The number of depredated nests peaked later in the nesting season, following a peak in nest initiation. Introduced mammals were the primary egg predators and included rats (Rattus spp.), feral cats (Felis catus), and Small Indian Mongooses (Herpestes auropunctatus). The number of eggs laid, as well as hatching success, was greater inside the mammal exclusion fence at Honouliuli Wetland, compared to a nearby site without a fence, Waiawa Wetland, where mammalian predators are only excluded via trapping. The average home range size for 12 tracked pre-fledglings was 0.94 ± 1.42 acres, and most chicks were observed using vegetated mudflats near open water. Of the 20 chicks that were tracked in this study, 7 fledged (35%), 6 had unknown fates (30%), 4 died due to unknown causes (20%), 2 were depredated by a feral cat (10%), and 1 died due to emaciation (5%). Our results suggest that management of predators, particularly mammals, is key to improving stilt hatching success, as preferred nest-site characteristics do not reduce the likelihood of egg depredation. Tall, invasive vegetation, such as California Grass (Brachiaria mutica), should continue to be controlled, as it was rarely used for nesting. More desirable vegetation, such as Pickleweed, should be made available throughout wetlands to encourage larger spacing between nesting pairs, which may help to reduce egg depredation pressure. Increasing mammalian predator control later in the nesting season may also increase hatching success of later nesters. Alternatively, mammal-exclusion fencing may provide year-round protection from mammalian predators, increasing both egg and chick survival. More data is needed to form conclusions regarding home range and survival of Hawaiian Stilt chicks. Improved detection methods and radio-tagging attachment styles will be used in the 2020 nesting season, which will reduce uncertainties and improve statistical power of analyses.