Skip to main content
Dryad logo

Nesting White Ibis prey use in Everglades National Park in 2017 and 2018

Citation

Cocoves, Tasso (2021), Nesting White Ibis prey use in Everglades National Park in 2017 and 2018, Dryad, Dataset, https://doi.org/10.5061/dryad.gqnk98skv

Abstract

As avian reproductive success is generally prey limited, identifying important prey types or sizes, and understanding mechanisms governing prey availability are important objectives for avian conservation ecology. Irruptive White Ibis (Eudocimus albus) nesting at coastal colonies in the southern Everglades numbered over 100,000 nests in the 1930s. A century of drainage and altered hydrologic patterns reduced aquatic prey availability and eliminated large nesting events; nesting activity in recent decades has been typically <5% of historical peaks. Hydrologic restoration is expected to increase ibis nesting activity, but which prey types will support high nesting effort is less clear. In 2017 and 2018 we collected food boluses from White Ibis chicks at coastal colonies in Everglades National Park. We also monitored regional nesting activity from 1999-2018. In 2017 the region had 1,075 nests, typical of the past several decades, but in 2018 there were 30,420 nests, representing the highest recorded nesting activity in 87 years. Prey composition varied between years; estuarine crabs dominated nestling boluses in 2017 while crayfish and fish were dominant prey in 2018. Crayfish, especially Procambarus alleni, were heavily exploited by ibis early in the 2018 breeding season while fish were used more at the end. Crayfish abundances in wetlands near the colonies were higher prior to 2018, and more crayfish-producing short-hydroperiod wetlands remained available for ibis foraging in 2018. Our results support previous studies indicating that crayfish are important prey for breeding ibises and suggest that unprecedented, extensive flooding of seasonal wetlands promoted crayfish production and initiated the irruptive breeding in 2018. Our observations indicate re-hydration of the southern Everglades could restore ibis nesting activity at coastal colonies, but further investigations of hydrologic variation, crayfish production, and ibis foraging and nesting activity will be helpful to understand these dynamics and the importance of short-hydroperiod wetlands.

 

Methods

Prey composition of White Ibis nesting in Everglades National Park in 2017 and 2018 

We quantified prey use of ibises nesting in the ecotone of Everglades National Park in 2017 and 2018 by examining the contents of regurgitated food boluses collected from nestlings. We collected boluses weekly for ~5 weeks each year during the peak provisioning period when chicks are between 12 and 28 days old. Most boluses (~90%) were collected from the ground or nesting substrates following voluntary regurgitation and the rest were collected via esophageal massage.Each bolus was picked through twice, and all useful prey parts were recorded, counted, and sorted into vials of 70% ethanol similar to methods used in previous diet studies (Boyle et al. 2014). We measured lengths of prey parts and back-calculated dry masses of fish and most invertebrates with regressions developed for previous assessments of ibis diets (Boyle et al. 2014).

White Ibis nesting activity

We conducted aerial surveys of nesting ibis at seven historical colonies in the ecotone of Everglades National Park from 1999 and 2018. We monitored active nesting colonies monthly from January through June using fixed-winged aircraft and helicopters flown at an altitude of 150-250 meters above ground level. During each flight, we took photos of the entire colony using a high-resolution DSLR camera and telephoto lens. We examined the photos to determine the stage of nesting and number of nests. We counted nests during the incubation stage, when the white incubating adult could be detected against the darker background of the nesting colony; we assumed each horizontally sitting (incubating) adult equated to one nest. We estimated total nest counts by rounding the number of nests up to the nearest 5, 50, or 100 nests for colonies with <100, 100-1000, and >1000 nests, respectively. We used the total nest count for each colony as a measure of colony-level nesting effort. We summed total nest counts for all colonies in the ecotone each year to examine temporal trends of total nesting effort in the coastal ecotone of Everglades National Park.

Crayfish Production

We quantified wet-season crayfish (Procambarus alleni and P. fallax, combined) catch rates at five spatially-fixed sampling sites in marl prairies and sloughs east of the colonies in each October preceding the 2017 and 2018 dry seasons (i.e. October 2016 and 2017). At each of the five sites we set six non-baited minnow traps (three 3.0 mm and three 6.4 mm wire mesh traps) for 22-24 hr. We considered the aggregate catch of six traps at each site a single effort. Crayfish were identified to species and measured (mm carapace length), and dry weights were calculated using length-weight regressions (Boyle et al. 2014). We compared mean crayfish catch rates (crayfish effort−1) and biomass (g dry mass effort−1) between 2016 and 2017 with paired one-tailed t-tests. Crayfish biomasses were natural log transformed to help meet assumptions of the analyses.

Usage Notes

Biomasses (in mg) for White Ibis boluses are summed for all prey types per bolus.

Aggregate crayfish biomasses and abundances in traps set in October 2016 and 2017 (DW2016 and DW2017) are provided in two separate files.