The Pomacea Project - Snail Kite Information

The Florida apple snail (Pomacea paludosa) is the nearly-exclusive food of the endangered snail kite (Rostrhamus sociabilis plumbeus) in Florida. The kite’s upper mandible is described by Sykes et al. (1995) as slender, deeply hooked, and sharp-tipped, and it is used to cut the snail’s columellar muscle (the only muscle holding the snail to its shell) to remove the edible snail parts from the shell. Snails are captured at the surface down to about 16 cm when snails are engaged in aerial respiration, moving, or resting on vegetation (Sykes et al. 1995).

Snail kites forage in marshes, lake littoral zones, ponds, ephemeral wetlands, and along the shallow banks of rivers and some manmade areas (Sykes et al. 1995). Snail kites are visual hunters, either by aerial course-hunting or perch hunting. Perch hunting involves scanning the water for surfacing snails, from perches 0.15 to 4.6 meters above the water (Sykes et al. 1995). When course-hunting, kites fly 1.5-3 meters above the water surface (Bennetts et al. 1994). When the kite detects a snail, it swoops down to the water surface and extends its feet to take the snail with its talons.

After capturing a snail, kites transport snails in their talons or bill to a perch to feed (Sykes et al. 1995). The kites’ toes are long and slender with curved, long, slender claws described as acute (see above picture - photo by Rob Bennetts). It has been reported that air and water temperatures affect both the number of foraging efforts and capture rates (Cary 1985). Lower capture rates during colder temperatures are likely due to snail inactivity (Hanning 1979). Stevens et al. (2002) quantfied snail inactivity at lower temperatures.

The kite is described as being nonmigratory, but seminomadic in response to factors such as water depth, hydrooperiod, food availability, vegetation growth, nutrient loads, and other habitat changes (Sykes et al. 1995). As noted previously, suitable foraging habitat is characterized by emergent vegetation scattered amongst open-water communities (Bennetts et al. 1994). The emergent vegetation is important for apple snails for egg laying and climbing to the water’s surface to breathe air. Low to moderate stem density areas are important for kite foraging because dense vegetation (e.g., high density cattail and sawgrass) is an impediment to effective foraging (Bennetts et al. 2006).

In addition to areas with too-dense vegetation, snail kites do not forage in areas experiencing a dry down (i.e., no standing water) because snails would be temporarily unavailable (Sykes 1979). However, it should be noted that the movements of kites may also reflect the seminomadic nature of the species—not necessarily foraging conditions (Darby et al. 2006). In other situations when it is thought that kites leave a given wetland due to poor foraging conditions, there may still be snails present—they simply may be present under conditions not amenable to kite foraging. Additionally, as discussed under the “Research” section below, assessments of habitat quality for snail kites and other snail predators should include abundance estimates of snails; Darby et al. (2006) found that little or no use of an area by snail kites does not necessarily reflect a low snail density.

Links to Websites for More Detailed Information on the Snail Kite

Recent Snail Kite – Apple Snail Research

This section discusses findings of recent research related to apple snails and snail kites conducted by Philip Darby, Robert Bennetts, and Laksiri Karunaratne.

Habitat Structure and Apple Snail Density: Karunaratne et al. (2006) compared snail densities in two broad habitat types, wet prairie (with emergent macrophytes) and slough (without emergent macrophytes), and explored the effects of structural attributes within wet prairie habitats. They found that: 1) snail densities were greater in prairies than sloughs; 2) within wet prairie habitats, snail densities were higher in Panicum hemitomon -dominated areas than in Eleocharis cellulosa -dominated areas; and 3) lower snail densities were found in dense E. cellulosa compared to less dense E. cellulosa (see Figures 1 and 2 below). These results are significant for water management because continuous inundation of wetlands may convert wet prairie (containing higher snail densities) to lily pad dominated slough habitat (containing lower snail densities).

Snail Densities and Foraging Kites: For the first time since Florida snail kites were listed as endangered, concurrent reports of snail densities and foraging kites have been made. Darby et al. (2006) reported snail densities and presence/absence data for foraging kites in wet prairies. There was a positive correlation between snail density and the number of foraging kites (Figure 3). Overall, snail density estimates ranged from 0 to 1.8 snails per square meter. The study suggests that snail densities < 0.14 snails per square meter are unlikely to support foraging kites. Darby (pers. comm.) considered this a preliminary indication of what constitutes an adequate prey base for snail kites; nesting kites may require greater snail densities to support foraging by the adults for their nestlings.

Kite Selection of Foraging Habitat: Bennetts et al. (2006) examined foraging habitat selected by snail kites, and quantified the role of snail abundance and vegetation structure in that selection. The researchers found that vegetation structure (dense vs. sparse) was more influential in foraging habitat selection than site, the dominant emergent vegetation species, or abundance of snails. Snail kites captured fewer snails from habitat patches with relatively dense vegetation and high snail densities compared to more open habitat with similar or lower snail densities. It is thought that dense vegetation may limit the kites’ ability to see and/or capture prey (Bourne 1985; Beissinger 1983; Sykes 1987; Bennetts et al. 2006).

Kites Foraging on Non-native Snails: A recent study has also found that the large, non-native channeled apple snail (Pomacea insularum) may present difficulties for foraging snail kites. Darby et al. (2007) found that kites foraging on P. insularum on Lake Tohopekaliga (Osceola County) dropped a greater proportion of the non-native snails compared to the native Florida apple snail (P. paludosa). Prior to reaching a perch to eat, kites dropped 44% of the non-native snail, compared to 0% of the native snails observed in the study (and the 1% reported by another study). Whether or not a non-native was dropped varied considerably between individual kites. The variation in drop rates may reflect kite age/experience differences. The much larger and heavier P. insularum (see picture, P.i. on left, P.p. on right) also took longer to consume, although it would also contain more calories per snail. As suggested in Darby et al. (2007), the size or dimensions of the kites’ claw, and/or the higher weight of P. insularum (37-45% of the weight of a kite), may be the reason for the relatively high drop rate and food handling difficulty.