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Audubon's Crested Caracara in Florida


by
James N. Layne
Archbold Biological Station
Audubon's crested caracara (Caracara plancus audubonii) is a species characteristic of the grassland ecosystems of central Florida and is one of the state's most distinctive birds. The Florida population is threatened and widely separated from the main species' range, which extends from extreme southwestern Louisiana, southern Texas, and southern Arizona to the tip of South America, including Tierra del Fuego and the Falkland Islands. Another isolated population occurs on Cuba and the Isle of Pines.
Audubon's crested caracara (Caracara plancus audubonii) in Florida. Courtesy J.N. Layne
The number of Florida caracaras is believed to have undergone a substantial decline from the early historic level in the 1950's and 1960's (Layne in press), with the total state population estimated at 250 in the early 1950's (Sprunt 1954) and fewer than 100 birds in the late 1960's (Heinzman 1970). Based on the appar-ent continuing decrease in its numbers, Florida's population of Audubon's crested caracara was federally listed as threatened in 1987 (Federal Register 1987). As part of a general study of the life history, ecology, and behavior of the caracara in Florida, I monitored its distribution and population status from 1972 to 1991.
Information was obtained from road and off-road searches in all parts of the known range; systematic roadside and aerial surveys in a 5,116-km2 (1,975-mi2) area within the core portion of the range; published records; museum specimens; and sighting reports from over 500 cooperators. Logistical limitations prevented surveying the entire potential Florida range thoroughly enough in any given year to obtain a reasonably accurate picture of the distribution and total population. Thus, estimates of the statewide distribution and numbers were based on records combined over 5-year periods: 1972-76, 1977-81, 1982-86, and 1987-91. Searches were most intensive from 1972 to 1981 and in the final period 1987-91. Because areas along public roads were surveyed more intensively than those remote from highways, there was a lower probability of detecting caracaras whose territories did not overlap roads than those whose territories included roads. This bias appeared to be at least partially compensated for by a tendency of caracaras to concentrate along highways because of the attraction of roadkills as a food source.

Status and Trends
Fig. 1. Breeding range of Audubon's crested caracara in Florida based on records from 1987 to 1991; range boundaries shown by Howell (1932), and main species' range in western United States.
The breeding range of Audubon's crested caracara in Florida (Fig. 1), based on records from the most recent 5-year period of the study (1987-91), did not differ significantly from that during 1973-76 (Layne 1978). Caracaras were documented in 20 counties in central peninsular Florida, with most locations in the same 5-county area as in the earlier years. Counties with 10% or more of the 183 estimated locations during 1987-91 included (number of locations in parentheses) Glades (41), Highlands (34), Okeechobee (23), and Osceola (18). The data indicate no obvious change has occurred in the overall range or core area of the distribution of the caracara in Florida from that shown by Howell (1932). As there had been relatively little alteration of the natural habitats of the state up to that time, Howell's range map is assumed to reflect the early historical distribution.
The estimated number of adult caracaras during 5-year intervals from 1972 to 1991 ranged from 196 to 312 (Fig. 2). The variation between periods reflects differences in sampling effort rather than changes in actual numbers. Thus, the adult population over the 20-year period appears to have been stable with a minimum of about 300 individuals in 150 territories. Further evidence that the population remained generally stable between 1972 and 1991 is the similarity in adult-immature age ratios during this interval (Fig. 2). Although immatures could not be censused accurately because they tend to wander individually or in aggregrations after the break up of family groups, they are believed to have numbered between 100 and 200 in any one year, giving a total statewide population of 400-500.

Fig. 2. Estimated numbers of adult Audubon's crested caracaras in Florida over 5-year intervals from 1972 to 1991, based on the assumption that localities where adults were recorded represent territories occupied by an adult pair. Percentage of locations that had immature birds versus those that had adults are given above bars.
The estimate of the minimum adult population includes single adults observed in an area only once during a 5-year interval as representing a pair on an established territory. Assuming that such individuals were actually unmated transients reduces the estimated adult population to about 230 individuals. Regardless of which estimate of the adult population during 1972-91 is accepted, it is highly unlikely that the Florida population was reduced to fewer than 100 birds between 1967 and 1970 (Heinzman 1970).
Although the range of Audubon's crested caracara in Florida appears to have remained unchanged for the past 60 years and numbers have been stable over at least the past 20 years, the future status of the population is still of concern. Most birds occur on private ranchlands subject to habitat degradation or loss from intensification of agricultural practices or other development. The most immediate threat is large-scale conversion of native range and improved pasture habitats to citrus groves.
A decline in the Florida caracara population within the next 10 years appears likely if citrus conversion and other habitat losses continue at the present rate. Because caracaras are relatively long-lived and strongly attached to their territories, residents may persist in a territory despite unfavorable changes, but may not be replaced by new individuals when they finally leave or die. The result may be a significant time lag before the effects of deleterious habitat changes are reflected in an actual population decline. The magnitude of the time lag in detection of any trend in the Florida distribution and population of Audubon's crested caracara also will depend upon the effectiveness of future monitoring efforts.
For further information:
James N. Layne
Archbold Biological Station
PO Box 2057
Lake Placid, FL 33862
References
Federal Register. 1987. Endangered and threatened wildlife and plants: threatened status for the Florida population of Audubon's crested caracara. Federal Register 52(128):25229-25232.

Heinzman, G. 1970. The caracara survey, a four year report. Florida Naturalist 43:149.

Howell, A.H. 1932. Florida bird life. Florida Department of Game and Freshwater Fish, Tallahassee. 479 pp.

 Layne, J.N. 1978. Audubon's caracara (Caracara cheriway auduboni). Pages 34-36 in H.W. Kale II, ed. Birds, rare and endangered biota of Florida. Vol. 2. University Press of Florida, Gainesville.

Layne, J.N. Audubon's crested caracara (Polyborus plancus audubonii). In J.A. Rodgers, H.W. Kale II, and H. Smith, eds. Birds, rare and endangered biota of Florida. 2nd ed. University Press of Florida, Gainesville. In press.

Sprunt, A., Jr. 1954. Florida bird life. Coward-McCann, New York. 527 pp.


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