Ecology and Breeding Habits of the Savanna Hawk in the Llanos of Venezuela

-The ecology of the Savanna Hawk (Buteogallus meridionalis) was studied in the palm-savannas of Venezuela in 1978-1980. Pairs produced only one egg per clutch, no more than one young per calendar year, and had eggs or young in nests from February to December, coinciding largely with the wet season. In comparison to North American Red-tailed Hawks (Buteo jumaicensis), Savanna Hawks had significantly fewer young per pair per year, lower nest success (47.1% vs. 82.5%), mainly due to lower hatching success (64.1% vs. 84.4%) and a lower proportion of pairs laying eggs (73.3% vs. 88.0%). Survival of young Savanna Hawks from hatching to near fledging was 70.4%, and the adult survival rate, 7 1.1%; these values are approximately equal to those for Red-tailed Hawks. The brood size was artificially doubled from one to two at 10 Savanna Hawk nests, but the adults could find only enough food to feed and raise one chick in most cases. Food appeared to be both the ultimate and proximate factor controlling the timing of breeding. The Savanna Hawk has evolved interrelated habits that counter its low fecundity: (1) the capability of renesting after the initial eggs or young have perished; (2) a post-fledging dependency period of four to seven months to help insure survival of the single young; and (3) permanent or longlived pair-bonds. The ability to renest is enhanced by hunting versatility. Longlived pair bonds may improve intra-pair activities thereby enhancing the reproduct&e success of experienced breeders. Hawks and eagles in the tropics, in contrast to northern latitudes, may have evolved different reproductive habits in response to greater diversities of prey and predators, both avian and mammalian, and different climates. Little is known, however, about breeding habits and food relationships in neotropical falconiforms. Tropical birds generally lay smaller clutches than their temperate counterparts (Lack 1947, 1948, 1954). Of the several hypotheses that have been proposed to explain this contrast, I investigated two dealing with the evolution of tropical clutch sizes: Lack’s (1947) hypothesis of “maximum reproduction” and Skutch’s (1967) of “readjusted reproduction. ” The former theory, which is more widely accepted, states that birds produce as many eggs and thus offspring as they can feed. The latter maintains that birds produce fewer young than they are capable of feeding because the adult survival rate is high and the need to recruit breeders is low. Additionally, Skutch believed that a smaller clutch size may, in part, be a result of heavy predation pressures in the tropics, such that smaller broods would require fewer feeding visits and therefore less risk of nest detection by predators. The question of what selects for a smaller clutch size in a predatory bird has not been addressed. Here, I present the results of a 16-month study of the Savanna Hawk (Buteogallus meridionalis, following Mayr and Cottrell 1979) in the llanos of Venezuela which examined: (1) a test of the hypotheses of Lack and Skutch, (2) what breeding habits exist to help offset a low reproductive potential inherent with a clutch size of one, and (3) how the timing and duration of breeding were related to rainfall and food resources. STUDY AREA AND METHODS The Savanna Hawk ranges from Panama to central Argentina (Blake 1977). I studied a population for two breeding seasons and part ofa third (15 January through 15 October 1978, 25 April through 3 October 1979, and 26 April through 3 May 1980) on a cattle ranch in Guarice State, Venezuela. Colleagues continued checking nests after I left, from October to December. The habitat was primarily palm savanna, although a few nests were found in tree savanna. The region (llanos) has one dry season (generally December through April) and one wet season (May through November), which have a pronounced effect on the land. In February for instance, the palm savannas were dry and dusty while in August they were sometimes covered with over a meter of standing water. I chose a study area of 9.4 km2 containing 23 pairs of hawks for which I determined the number of eggs and young produced each year. Additional data, including clutch size and nesting success, were gathered on nests located