Foraging Wading Bird (Ciconiiformes) Attraction to Prescribed Burns in an Oligotrophic Wetland

Venne and Frederick: Burns Attract Wading Birds Page 78 Fire Ecology Volume 9, Issue 1, 2013 doi: 10.4996/fireecology.0901078 ReseaRch aRticle FORAGING WADING BIRD (CICONIIFORMES) ATTRACTION TO PRESCRIBED BURNS IN AN OLIGOTROPHIC WETLAND Louise S. Venne* and Peter C. Frederick Department of Wildlife Ecology and Conservation, University of Florida, 110 Newins-Ziegler Hall, Gainesville, Florida 32611-0430, USA *Corresponding author: Tel.: 001-678-622-5559; e-mail: ABSTRACT Many wetland communities are fire prone or fire dependent, especially those dominated by forbs and grasses. Despite our considerable knowledge about fire effects on wildlife in uplands, there is a relative paucity of information about effects of fire in wetland systems. Long-legged wading birds (herons, egrets, ibises, storks, spoonbills; order Ciconiiformes) may benefit from fire through the exposure of prey after vegetation removal, or through a trophic response of prey to increased availability of nutrients and increased light. We conducted aerial surveys of foraging wading birds in prescribed burns and adjacent unburned areas in the central Everglades, Florida, USA, to determine if wading birds select for burned habitats. We measured aquatic prey density in burned and unburned sawgrass (Cladium mariscus [L.] Pohl ssp. jamaicense [Crantz] Kük ), and densities of prey injured or killed in the fires. We also observed foraging great egrets (Ardea alba L.) in and adjacent to prescribed burns to determine whether foraging success (i.e., capture efficiency and capture rate) differed between burned and unburned areas. Great egrets and white ibises (Eudocimus albus L.) selected for burns and areas of deeper water adjacent to burned areas, and avoided dense, tall, unburned vegetation. Measured densities of prey killed by the fire were very low. Live aquatic prey densities did not differ between burned and unburned sawgrass. Great egrets had higher capture rates in sloughs adjacent to burns than in burned areas, but were more efficient at capturing prey in burned areas than in adjacent sloughs. Prescribed fires created short-term shallow water habitats (burned areas) with limited submerged and emergent vegetation, making prey in burns more vulnerable despite lower densities (availability) compared to adjacent sloughs. This research suggests that prescribed fire in grass-dominated wetlands may attract predators like wading birds primarily because removal of vegetation makes prey easier to capture. Keywords: Ardea alba, capture efficiency, capture rate, Eudocimus albus, Everglades, great egret, habitat selection, prey accessibility, prey density, white ibis Citation: Venne, L.S., and P.C. Frederick. 2013. Foraging wading bird (Ciconiiformes) attraction to prescribed burns in an oligotrophic wetland. Fire Ecology 9(1): 78-95. doi: 10.4996/ fireecolgy.0901078 Fire Ecology Volume 9, Issue 1, 2013 doi: 10.4996/fireecology.0901078 INTRODUCTION Disturbance of upland areas via machinery, fire, and other physical disturbances often attract predatory birds to forage or scavenge for displaced, injured, or recently killed prey (e.g., Komarek 1969, Smallwood et al. 1982, Tewes 1984, Toland 1987). The removal of vegetation can increase the availability of prey (Vickery et al. 2001) even when abundance or density of prey does not increase (Vickery et al. 2001, Munro et al. 2009). Thus, intake efficiency of predatory birds may increase because of improved availability of prey, but not due to increased abundance or diversity of prey (Devereux et al. 2006). Disturbances like fire can also affect the abundance and diversity of invertebrates in uplands and in some peatlands in the longer term (Warren et al. 1987, Hochkirch and Adorf 2007), although directionality of responses is not consistent among species. While changes in prey availability due to disturbance are known from some upland habitats (e.g., Boyd and Bidwell 2001, Benson et al. 2007), it is unclear whether disturbance from fire results in similar effects on aquatic prey animals. The absence of appropriate habitat and vegetative structure appears to strongly affect species specific responses to fire (Gabrey et al. 1999, Baldwin et al. 2007). Many wetland bird species are not found in recently burned habitat for a year or more, while other species are observed in burns where they were previously not found (Venne 2012). For example, savannah sparrows (Passerculus sandwichensis) tend to be found in areas with sparse vegetation within one year post-burn, while sedge wrens (Cistothorus platensis) prefer dense vegetation that has not burned in the previous two years (Baldwin et al. 2007). However, studies of post-burn responses of wildlife typically have not determined the mechanisms driving avoidance or preference of burned areas. Fire may positively affect foraging conditions for many wetland dependent birds Venne and Frederick: Burns Attract Wading Birds Page 79 through nutrient release and resetting vegetation composition, thereby exposing roots and seeds (Gabrey et al. 1999), increasing nutritive content in vegetation (Smith et al. 1984), and increasing abundance of food resources such as invertebrates (de Szalay and Resh 1997, Hochkirch and Adorf 2007). However, changes in resource availability and quality depend in part on the time of year that the burn is conducted (Brennan et al. 2005, McWilliams et al. 2007). Also, fire may change patterns of resource availability in wetlands both in the short and long term, which could have management and conservation implications. Foraging success of long-legged wading birds (Ciconiiformes) depends largely on prey availability (Bancroft et al. 2002, Gawlik 2002). Water depth is a primary determinant of prey availability since wading birds are limited to foraging in water no deeper than their leg length (Powell 1987, Gawlik 2002). Emergent vegetation density also plays an integral role in prey availability in two important ways. Dense vegetation can impede access to prey, but may also increase prey density by improving cover to hide from predators. Thus, sparse vegetation may be preferred by wading birds compared to dense or no emergent vegetation (Lantz et al. 2011), and edges may be preferred over open water (Stolen 2006). The Everglades is a large oligotrophic wetland in southern Florida, USA, where primary production is strongly phosphorus limited (Noe et al. 2001). Wading birds are an iconic, abundant group of species and also serve as one of the indicators of restoration of the Everglades (Frederick et al. 2009). Sawgrass (Cladium mariscus [L.] Pohl ssp. jamaicense [Crantz] Kük ) is the dominant vegetation, forming elevated, elongate “islands” (i.e., ridges) surrounded by deeper, open water sloughs (Gunderson 1994). Sawgrass is a fire-adapted plant, growing quickly and recovering within two years post burn (Wade et al. 1980). Furthermore, as sawgrass grows, the leaves spread away from the culm and senesce, which hel (...truncated)