Introduction to the Maya Project Introduction to The Selva Maya Principal Agents of Tropical Deforestation The Maya Project Components - Activities and Results


Description and Results

Black Hawk-Eagle (Spizaetus tyrannus)

The following account of our research activities follows the outline just given. For each topic, we describe the associated conservation challenges, give some background knowledge on the topic, describe our research activities, and give a brief synopsis of our results.

1. Basic Biology of Poorly-known Tropical Forest Birds of Prey

Many tropical forest raptors are biologically poorly known. We know little in detail concerning the kinds of habitat these raptors need, nor the amount of space they require. We also know little about their diets, breeding biology, and population ecology.

Hector Madrid places a bal-chatri trap
 in a tree-top, in hopes of capturing 
an Ornate Hawk-Eagle.

Knowledge concerning these topics is basic to the development of conservation efforts geared toward these species, should such efforts prove necessary.

Because our organization is dedicated mainly toward the conservation of birds or prey, 90% of our project effort was devoted to answering these basic questions concerning tropical forest raptors.

Including migrants and uncommon species, 50 species of raptors (42 diurnal raptors and 8 owls) occur in the forests of Pet�n, Guatemala. However, in our main study site, Tikal National Park, about 21 species of raptors (not including the vultures) are common enough to study. Of these 21 species, we studied 20.

For each species, we studied the breeding biology and behavior, diet, habitat use, and spatial needs. Our basic strategy was to locate several breeding pairs of each species, and conduct extensive observations at the nest--usually from a platform in a nearby tree. In defining habitat needs, we also made use of various kinds of census or population sampling methods, described below.

Ready for release, this White Hawk 
(Leucopternis albicollis) has been
 fitted with a back-pack radio transmitter;
 the antenna wire is visible.
(Walter Mart�nez in photo)

In many cases we also captured and placed radio transmitters on several individuals over time. This allowed us to map out the birds' movements, and to estimate the amount of space used, i.e., home range size. This information, along with the distance between neighboring nests, allowed us to estimate the amount of habitat normally needed by a nesting pair (see below). Using radio telemetry also facilitated our observing the birds' behavior and habitat use.

From 1989 to 1996, we put in thousands of person-hours studying these 20 raptor species. As a result, we have greatly increased the sum total of scientific knowledge about many of these raptors. Much of this information has been published in scientific journals (see the Maya Project Bibliography given here), and much awaits publication for the first time in a book we are in the process of compiling.

The Ornate Hawk-Eagle (Spizaetus ornatus) was one of our main study species.  We studied nine territories during 45 territory-years. Slightly larger than a Red-tailed Hawk (Buteo jamaicensis), females in this species weigh about 40% more than males. Endowed with formidable talons, these handsome raptors hunt by stealth within and below the canopy, stalking prey and then attacking from a short distance. Among 325 identified prey items, 56% were birds and 44% mammals, though bird contributed 70% of the diet by biomass, and mammals 30%. Birds taken were largely toucan-sized but included some very large birds--guans, curassows, and Ocellated Turkeys (Meleagris ocellata). Most of the mammals taken were tree squirrels, but several opossums, young coatimundis (Nasua narica), and a few agoutis (Dasyprocta punctata)--large forest-floor rodents--were also taken (Whitacre et al. unpubl.).

Atop Tikal's Temple 4, Julio Madrid 
searches for a signal from a 
radio-tagged Ornate Hawk-Eagle .

These birds built large stick-nests in tall, emergent trees amidst mature forest. They laid a one-egg clutch, with most eggs hatching late in the dry season and young fledging during the first two-thirds of the rainy season. Incubation was mostly 44-46 days in duration and chicks fledged 66-92 days after hatching. These hawk-eagles displayed a prolonged period of juvenile dependence, as verified in many cases by placing radios on juveniles prior to fledging. Four young birds achieved independence on average at 15.2 months of age and 12.4 months after fledging.

Adults advertised their territories by soaring prominently above the canopy on warm, sunny mornings. Neighboring nests averaged 3 km apart, for an average density of one territorial pair per 10-12 km2 within local areas of suitable habitat. Two adult males used home ranges estimated at 10-14 km2. Early in the nesting period adult females ranged over quite small areas--no more than 3 km2--whereas during the period of fledgling dependency, they ranged over 11-19 km2, depending on the method of home range calculation used (Whitacre et al. unpubl.).

This yearling Ornate Hawk-Eagle (Spizaetus ornatus
was followed for the first three years of her life.

In 45 territory-years, an egg was laid in 50-58% of territory-years, depending on the method used. Our best estimate was that 61% of eggs produced a fledgling and 53% of eggs produced an independent adult. Hence we estimate that on average each territory produced 0.305-0.35 fledglings per year and 0.265-0.306 independent adults. If the territory densities we discovered (ca 10 territories per 100 km2) hold true over large areas, then about 2.7-3.1 independent subadults were produced yearly per 100 km2 of appropriate habitat.

Though these hawk-eagles nest principally among large expanses of mature forest, we also studied a few nests in forest fragments or peninsulas amidst the partly-deforested farming landscape. Such instances, however, may represent cases of adults clinging to traditional nest sites in the face of progressive deforestation. It remains unknown precisely how much forest fragmentation these hawk-eagles may be able to tolerate while still maintaining a viable population.

The forest-falcons (genus Micrastur) are a strictly Neotropical group of some half-dozen species. Though members of the Falconidae, they are very different from "typical falcons" of the genus Falco. These raptors are secretive dwellers of the forest understory, rarely venturing into the open or above the canopy. These fascinating raptors are most active in the early morning and late evening hours. Rather than soaring above the canopy, they maintain territorial spacing by voicing loud calls that carry great distances through the still pre-dawn forest. They have a distinct facial ruff like that of an owl or harrier (Circus spp.), which probably assists them in detecting prey by ear. About the size of a merlin (Falco columbarius), these forest-falcons are relatively size-dimorphic, with females averaging nearly 42% heavier than males.

The Barred Forest-Falcon 
(Micrastur ruficollis
was another of our 
main study species.

Russell Thorstrom studied Barred Forest-Falcons at Tikal, gathering data on 20 territories during 98 territory-years (Thorstrom 2000, Thorstrom et al. 2000).

Of the more than 400 prey items identified at Tikal, reptiles (mainly small lizards) made up 61.5% of the diet, birds 22%, insects 8%, mammals 6% and frogs 2.5% (Thorstrom 2000). In terms of biomass, birds made up 55% of the diet, lizards 31%, mammals 10%, frogs 3%, and snakes and insects less than 1% each. These raptors hunted stealthily below the canopy, and sometimes frequented army ant swarms, preying on insects and lizards fleeing from the ants.

Barred Forest-Falcons mainly utilized mature, "upland" forest at Tikal, and 69 of 70 nests were judged to occur in this forest type. Forest-falcons, like most falconids, do not build a nest, and lay their eggs in cavities in trees. Clutches were of two or three eggs, averaging 2.6 eggs. Laying occurred mainly late in the dry season, with hatching taking place at the onset of the rainy season, a time of increasing prey abundance. Eggs hatched 33-35 days after laying, and nestlings fledged 35-44 days after hatching (Thorstrom et al. 2000). Radio-tagged fledglings dispersed from their parents' territories within four to seven weeks after fledging, presumably achieving independence at that time.

Nesting territories were occupied year after year. On average, occupied nests were 1.0 km apart, yielding an estimated density of one territorial pair per 1 km2 of habitat. For several radio-tagged breeding males, home ranges averaged 98-115 hectares in area, depending on the method used, with 2.8 to 5.5% overlap in adjacent home ranges. Russell's best estimate of exclusive space per pair was 114 hectares, or one pair per 1.14 km2 of habitat.

On average 1.1 young fledged per breeding attempt, and 0.8 young fledged per territorial pair-year. Annual survivorship of adults was relatively high, estimated at 92.3% for females and 95.3% for males; several adults were at least nine years old at the end of Russell's study.

Our studies showed these birds to be less common in the fragmented and half-deforested farming landscape than within Tikal's mature forest. Because of their modest home-range size, these raptors may be more able to persist in farming landscapes than some other forest raptors. However, forest remnants as large as the mean home range (about 1 sq km) are uncommon in many farming landscapes. Hence, we suspect that these small raptors display a modest ability to persist in the face of deforestation, but will probably decline or disappear in excessively deforested landscapes.

Literature Cited, Basic Biology of Poorly-known Tropical Forest Birds of Prey


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