Longer resource waves support larger breeding populations of wading birds (Ardeidae, Ciconiidae, and Threskiornithidae)

Background/Question/Methods

Resource waves arise when phenological variation in resource availability is spatially autocorrelated. For consumers that can track resources, increased phenological variation resulting from resource waves can lengthen the window of peak resource availability, resulting in greater energy intake. Although several movement studies have demonstrated that consumers track waves, few studies have demonstrated a relationship between wave characteristics and demographic responses. Building on a model of wading bird prey densities at Lake Okeechobee, Florida, we mapped the daily probability that each foraging patch was available (PAI) over 14 breeding seasons (2006-2019). We estimated the number of available-patch-days, phenological variation in resource availability (S.D. peak PAI date), and Moran’s I of spatial autocorrelation in peak PAI to test their effect on breeding density and nest survival for three wading bird species (Ardeidae, Ciconiidae, and Threskiornithidae). We plotted semivariograms from the PAI maps and used Matérn models to quantify the strength (partial sill) and length (range) of annual resource waves and tested whether wave characteristics could be linked to the number of available patch days in the wetland, since this was a good predictor of nest abundance and survival.

Results/Conclusions

There was a positive relationship between annual available-patch-days and wading bird nest abundance for all three bird species, and between biweekly available-patch-days and daily nest survival rates for small herons. Neither variation, nor spatial autocorrelation in resource phenology were related to breeding densities. Great egret nest survival increased with phenological variation whereas small heron nest survival decreased. Annual survival rates likely decreased for small herons when phenological variation was high because they nested later in the dry season in those years, and daily survival rates generally decrease with day of year. We also showed that the range, which is analogous to the distance that the resource wave travels across the landscape, was strongly correlated with the number of available-patch-days/year and, as a corollary, nest abundance. The finding that wave range is indicative of available patch days, which is a strong predictor of annual nest abundance, suggests that resource phenology is an important parameter that should be integrated into estimates of resource availability and underscores the heuristic utility of the resource wave concept for management of landscapes for mobile consumers.