Breeding Ecology of the Crane Hawk in Tikal National Park, Guatemala
Type of Culminating Activity
Master of Science in Raptor Biology
Marc J. Bechard
The Crane Hawk (Geranospiza caerulescens) is a lanky hawk of the forests and swampy woodlands of tropical America. Deep wing beats reminiscent of a large heron, interspersed with short sails on slightly arched wings, lethargically carry the Crane Hawk above the forest's humid penumbra. Its dark form is interrupted by a pair of bright white tail bands and a diagnostic white crescent across the base of the primary flight feathers. Rarely soaring or flying high above the canopy, except during courtship or territorial displays, these astute predators rely less on speed of flight to secure prey than on a methodical reconnaissance of their large, diverse territories. Using their long legs and unusually flexible intertarsal joints, these hawks forage largely by reaching within nooks and crannies for hidden pry. The brilliant reddish-orange glow of their long legs haunts the daylight dreams of roosting nocturnal animals throughout all strata of the moist lowland forest. In reality, nary a small animal is safe from this active and agile hunter. As either a silent forest skulker or an adroit cavity snatcher, the Crane Hawk uses an array of foraging styles to capture a wide variety of prey types. The evolution of several anatomical features such as long legs, a highly flexible intertarsal joint, scutellate tarsi, and miniature outer toes facilitate this generalized hunting repertoire and contribute to a rather catholic diet. Although the Crane Hawk is a dietary generalist, it seems to be found most commonly near water in forested habitat, and less so in remnant or gallery forests. Inconspicuous flight behavior and dark plumage make the Crane Hawk an elusive research subject.
I studied the breeding ecology of Crane Hawks in Tikal National Park, Petén, Guatemala, from 1993-1995. Breeding biology, adult and nestling behavior, and home ranges of Crane Hawks in Tikal are presented in Chapter 1. I located and recorded data on the breeding biology of 14 breeding pairs. I documented productivity at 12 nesting attempts and studied behavior during the incubation, nestling, and post-fledging periods. Nesting was relatively synchronous among pairs, with egg laying and hatching coinciding with the dry season, and young fledging at the onset of the rainy season. Crane Hawks had relatively high reoccupancy of nest sites, often using alternative nest sites in the same general area in consecutive years. Clutches ranged from 1-2 eggs, and mean clutch size was 1.7 ± 0.5 eggs. Eighty percent of eggs hatched, and 50 % of those resulted in fledged young. At 10 nests, 42.9 % of nestlings fledged. Mean productivity was 0.7 young fledged per nesting attempt (n = 12), and 1.6 fledglings per successful attempt. Overall reproductive success was 41.7 %. Females conducted the bulk of incubation and nest attendance, while males provided the majority of prey. During incubation, one adult or the other was on the nest 89.7 % of the time. The duration of the incubation period was at least 39 days, and four nestlings fledged at an average age of 36.8 ± 4.3 days. The estimated duration of the post-fledging dependency period for one juvenile Crane Hawk was 17 weeks. Crane Hawks held regularly-spaced breeding sites 2.6 ± 0.5 km apart, and the two methods we used to estimate breeding densities yielded similar results (526-714 ha) and were consistent with estimates of home range size (513-1045 ha).
My findings on breeding diet and hunting behavior are presented in Chapter 2. I identified prey items during nest observations, while observing radio-tagged birds, and during opportunistic sightings. Rodents, lizards, frogs, bats, birds, snakes, and a juvenile skunk comprised 47.5 %, 19.9 %, 16.0 %,6.6 %, 6.1 %, 2.8 %, and 0.6 %, respectively, of 181 identified prey items. Rodents (n = 86) accounted for 77.3 % of observed biomass captured. Terrestrial, cursorial, and arboreal rodents of at least eight species occurred frequently in Crane Hawk diets. Over half (52.2 %) of all prey items weighed <20 g, and 39.7 % weighed >50 g. I described hunting behavior and collected data on vegetation structure and floristics at hunting sites in order to classify hunting habitat. My field assistants and I observed hunting attempts in all strata of the forest and in all forest types except low-canopied seasonal swamp forest. Hunting behavior included still-hunting from a perch and probing with head and feet in holes, bromeliads, epiphytes, palm leaf axils, crotches of branches, behind bark in both live and dead trees, and in puddles. Crane Hawks foraged in upright postures as well as hanging at varying degrees of inclination including upside down.
Crane Hawk nesting habitat and nest site characteristics are presented in Chapter 3. I measured structural and floristic variables at 14 occupied and recently occupied nest sites. I described nest sites at three spatial scales: nest location, nest tree, and nest area (forest type surrounding nest tree). Crane Hawk nests were located high within vine tangles in large, emergent trees isolated from the surrounding forest canopy. Nest trees most often were located in seasonally inundated swales between karst hill-tops. These low areas were dominated by a forest community of large canopy-emergent trees (Swietenia, Aspidosperma) interspersed between canopy palms (Sabal mauritiiformis), and with a relatively closed understory characterized by a palm species (Cryosophila stauracantha) and the virtual absence of Piper c.f. psilorachis.
Like many similarly-sized sympatric tropical species, Crane Hawks laid relatively small clutches, exhibited relatively low productivity, and experienced a relatively long adult- dependency period. The Crane Hawk's unique anatomical features enable it to utilize a wide variety of forest habitats and to capture a broad diversity of prey types, including many diurnally reclusive species. In addition, the outcome of this foraging behavior may allow for lower dietary overlap with similarly-sized sympatric raptors. Raptor surveys and related research in the human-altered agricultural mosaic outside the park's boundaries suggest that the destruction and subsequent loss of mature forest types and associated nesting habitat may have serious implications for not only Crane Hawks but other forest- dwelling raptors.
Sutter, Jason A., "Breeding Ecology of the Crane Hawk in Tikal National Park, Guatemala" (2000). Boise State University Theses and Dissertations. 446.