Foraging White Ibis target inter-habitat prey movements in the Florida Everglades

In the 1800s John James Audubon wrote of the Florida Everglades, “We observed great flocks of wading birds flying overhead toward their evening roosts …. They appeared in such numbers to actually block out the light from the sun for some time.” Though diminished over the past century, wading birds still nest in the tens of thousands during the dry spring months and become an important top predator in the ecosystem. Nesting populations are highly dependent on the availability of aquatic prey, which can become accessible to the birds in high densities as water levels recede across the vast ridge and slough wetland landscape.

The White Ibis is an iconic bird of the Everglades and the most abundant of the nesting wading bird species, acting as an important ecological indicator for hydrologic management and restoration. They feed extensively on crayfish during the nesting season using their long, sensitive, decurved bills to feel for prey in the water column and substrate. Previous studies have demonstrated that a flux of large numbers of crayfish move down from slightly higher elevation, and thickly vegetated, ridge habitats into adjacent lower elevation, open-water sloughs when ridges begin to dry (0 – 10 cm of water) and sloughs are relatively deep (20 – 30 cm of water). It is at these depths that crayfish reach their peak densities in the sloughs. However, literature on White Ibis foraging and prey availability purport that White Ibis forage mainly in slough depths of 9-19 cm, a depth limitation that would preclude them from feeding on the highest densities of crayfish.

The apparent dissonance in optimal foraging conditions for Florida’s most abundant wading bird remained a mystery.  Accurate information on habitat availability in relation to water depths and seasonal timing of dry downs is important for Everglades management plans and is essential for creating foraging opportunities to support nesting populations.

We explored the puzzling potential discrepancy in a series of large, man-made wetlands at the Loxahatchee Impoundment Landscape Assessment (LILA), in Boynton Beach, Florida. These wetlands were specifically constructed as a landscape-scale model of the Everglades ridge and slough system. We used time-lapse imagery from game cameras to observe daily White Ibis foraging activity over two months of the nesting season. We also quantified crayfish biomass throughout the study season to track densities and timing of crayfish movement between ridge and slough habitats as water levels declined.

The results of our study were clear: Crayfish biomass in sloughs peaked in density when ridges were almost dry (2 – 10 cm of water), just as the previous crayfish studies suggested – but White Ibis foraging depths did not agree with previous literature. Foraging activity corresponded with the high-density movements of crayfish, with most (≥50%) foraging occurring over 2-3 days of the two-month study; when the ridges were almost dry and water in sloughs averaged >29 cm deep.

Our results might be explained by the focal prey type of the respective studies. In the previous studies, free-swimming fish were used as the prey type. Fish can move through the water column in all directions with densities increasing or decreasing as water levels change, but crayfish mainly stay on the bottom of the water column regardless of the water depth. Because White Ibis are tactile foragers and crayfish are bottom-dwelling crawlers, they may be able to perceive and capture crayfish almost as easily in 30 cm of water as in 15 cm. Fish also concentrate in sloughs at different times and depths than crayfish, so it’s unlikely that White Ibis were foraging on fish in our study.

We also noticed an interesting spatial pattern in the foraging: White Ibis selected slough edges during the 2-3 days of peak foraging and crayfish migration, suggesting that White Ibis fed on crayfish as they migrated between habitats. This supports the idea that White Ibis foraging activity was triggered by an inter-habitat prey flux and was not limited by a restricted range of water depths. Observations of foraging White Ibis in other wetlands suggest these foraging patterns could be general; White Ibis can and will forage on vegetation edges and even in relatively deep water when crayfish are abundant and vulnerable.

We encourage other researchers to closely observe predator and prey characteristics as well as environmental drivers when considering where and how predators naturally access their prey. Although much of the prey flux work in ecology relates to movement over ecosystem boundaries, we suggest additional studies to address prey availability in the context of inter-habitat prey fluxes, an important, ecological process that has been disrupted by human activity in most, if not all, ecosystems. As human populations continue to grow and more habitats are simplified or eliminated, important inter-habitat prey fluxes that support biodiversity and food webs may be lost before they are found.

 

This research was recently published in the Journal of Field Ornithology:

Binkley, E. E., N. J. Dorn, and M. I. Cook. 2019. Feeding on the edge: foraging White Ibis target inter‐habitat prey fluxes. Journal of Field Ornithology. https://doi.org/10.1111/jofo.12302

 

Guest post by:

Erin Binkley
Department of Biological Sciences
Florida Atlantic University