Drought and Birds: Snail Kite

In one of the first Federally-funded field studies of an Endangered species in the United States, Paul W. Sykes Jr. (1979) examined population status, nesting success, and movements of the Snail Kite (Rhostrhamus sociabilis plumbeus)—formerly known as the Everglade Kite—in the human-altered marshes of south Florida over a span of 11 years, 1968-1978. The bill of the Snail Kite is uniquely adapted for feeding almost exclusively on apple snails (Pomacea sp.), which makes them wetland obligates, and thus particularly susceptible to drought.

Drought-related impacts on Snail Kites are emphasized in this excerpt from Sykes’s Summary:

The severe drought of 1971 resulted in a significant decrease in the population for that year and 1972, with no nesting attempts being observed in the dry year. From 1974 through 1978 the population increased significantly (r = 0.92, P < 0.025), apparently the result of favorable water conditions and increased food supply. The loss of suitable habitat is the major problem facing the species in Florida. A high water level is essential, as it affects food supply and its availability, as well as nesting success.

More specifically, in the two drought-influenced years (1971-1972), nesting attempts and number of young fledged were reduced to 3 and 1.5/year, respectively. By contrast, these same variables averaged 15 and 16/year in three pre-drought years (1968-1970) and 33 and 26/year in four post-drought years (1973-1976). In other words, drought conditions reduced nesting attempts to 20 and 9 percent of pre- and post-drought levels, and number of young fledged to 9 and 6 percent of pre- and post-drought levels.

Regarding population movements, Sykes notes: "My field work, beginning in 1967, has shown that kites are nomadic in Florida. Since widespread water manipulation has affected their food supply, kites must be nomadic to survive." Sykes concludes that "The nomadic behavior exhibited by this kite in recent years, probably represents a normal response to changes in water levels and food availability."

In discussing mortality, Sykes adds that "some individuals probably starved in drier years."

Additional details are revealed in the Discussion:

Each year, following breeding, some birds disperse, but during the drought of 1971 they were scattered more widely than usual over the entire Florida peninsula. The reduced food supply resulting from dry conditions apparently raised the mortality rate. In 1971 there was no recruitment to the population and when the census was taken only 72 individuals could be found. Although dispersal might have affected the actual number of birds seen, it was obvious that the population had decreased. In 1972 nesting attempts were at least 60% below the 1968—1970 period, and only 65 individuals were recorded on the census. Dry conditions prevailed in 1974 and there was a corresponding decrease in the kite population (Figure 4).

Sykes’s article can be read in full by clicking on the highlighted title below.

Citation:

Sykes, Paul W., Jr. 1979. Status of the Everglade Kite in Florida—1968-1978. Wilson Bulletin 91: 495-652. [.PDF]