Sympatric Forest-Falcons of the Genus Micrastur

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

The forest-falcons of the genus Micrastur are found only in tropical and subtropical forests of the Western Hemisphere. This group of seven species breeds from Tamaulipas, Mexico south to northern Argentina, Paraguay and Brazil's southernmost state of Rio Grande do Sul.¹

Although Micrastur forest-falcons are secretive and often difficult to observe in dense vegetation, they are some of the most abundant raptors in Neotropical forests.

For example, Thiollay (2007) conducted surveys of all raptor species in many different forests of French Guiana and found that, because of their high densities (compared to other raptors), Micrastur forest-falcons were the numerically dominant raptors in all forest types. He also found that the smaller-sized Micrastur species (M. rufficollis and M. gilvicollis) were more abundant than the larger-sized ones (M. semitorquatus and M. mirandollei).

Although forest-falcons are members of the bird family Falconidae, they do not resemble normal falcons. Instead, they resemble Accipiter hawks of the bird family Accipitridae. Both forest-falcons and Accipiter hawks have long tails and short wings, adaptations for maneuvering among the dense vegetation of forests where they live.  In contrast, hawks and falcons living in open country usually have long wings and short tails.

Micrastur forest-falcons feed primarily upon birds, mammals and reptiles.  Like Accipiter hawks, they often hunt prey by sitting quietly on tree branches and waiting for their victims to appear.  When the latter arrive, the forest-falcons quickly ambush them, attempting to catch them with a brief, flying pursuit.  However, forest-falcons also use other techniques to hunt prey, such as chasing prey on foot (Thorstrom 2000), following army ant swarms (Willis et al. 1983, see below), and acoustical luring of birds (Smith 1969, Atkinson 1997).   

In addition to their long tails and short wings, Micrastur forest-falcons have other adaptations for living in tropical forests, where dense vegetation, and low levels of light may make it difficult for them to see prey.  Their ears have larger openings than other falcons, resulting in extraordinary hearing that enables them to better detect prey by sound. The facial ruff may also aid hearing, as it does in owls (Bierregaard 1994).

Micrastur forest-falcons nest in the holes of trees and cliffs, and usually have a clutch size of 2 or 3 eggs (Thorstrom et al. 1990, 2000ab, 2001; Baker et. al 2000; Gerhardt 2004).

Only two species have been adequately studied: the Barred Forest-Falcon (Micrastur ruficollis) and the Collared Forest Falcon (Micrastur semitorquatus). Collared Forest-Falcons are 3 to 4 times larger than Barred Forest-Falcons.  In some tropical forests, both forest-falcons are found.  When 2 or more species live in the same area, ecologists say they are sympatric. In this article, we compare the habits of sympatric Barred and Collared Forest-Falcons, and discuss the many important ways that they differ from each other.  We also briefly mention interactions with some of the other Forest-Falcons.

Habitat Preferences

In Guatemala, the Collared Forest-Falcon occurs in a wide variety of tropical forests, including mature forests, forest edges, and secondary woodlands and thickets (Thorstrom 2000).  In contrast, the Barred Forest-Falcon is generally restricted to mature tropical forests (Thorstrom 2000).  

In South America, however, the Barred Forest-Falcon lives in other kinds of forests.  For example, in Amazonia it occurs most often in second-growth forests, gallery forests, tidal swamp forests, semideciduous forests and forest edges, while the closely-related Lined Forest-Falcon (Micrastur gilvicollis) is found in mature tropical forests (Bierregaard 1994).

In Acre, Brazil, the Barred Forest Falcon is reported to prefer "disturbed forest types, both natural secondary and man made, including bamboo and more open seasonally drier forest on rocky outcrops," while the newly discovered Cryptic Forest-Falcon (Micrastur mintoni) is "strongly associated with undisturbed terra firme forest with dense understory" (Whittaker 2002).

Food Habits

In mature, semideciduous tropical forest at Tikal National Park, Guatemala, Thorstrom (2000) studied the food habits of sympatric Barred and Collared Forest-Falcons.  He found some similarities, but also significant differences.

The main prey of the Collared Forest-Falcon was mammals, particularly squirrels (Sciurus deppei and S. yucatanensis), followed by birds and then reptiles (see Figure 1).  The main prey of the Barred Forest-Falcon was reptiles, especially lizards, followed by birds and then insects, mammals and amphibians. 

The Collared Forest-Falcon preyed upon both a larger number of species, and a wider size-range of species, than the Barred Forest-Falcon.  For example, the prey of Collared Forest-Falcons ranged in size from frogs (20 grams) to Ocellated Turkeys (Agriocharis ocellata) at 3 kilograms.  Many other large birds were captured, including Crested Guans (Penelope purpurascens), Plain Chachalacas (Ortalis vetula) (See Figure 2), Great Curassows (Crax rubra), Tinamous (Crypturellus spp.), Keel-Billed Toucans (Ramphastos sulfuratus), Collared Aracaris (Pteroglossus torquatus), and Brown Jays (Psilorhinus morio).  In contrast, the prey of Barred Forest-Falcons ranged in size from insects (1.5 grams) to doves (160 grams). 

Some individual forest-falcons deviated from the average diets just given.  For example, Thorstrom (2000) observed an adult Collared Forest-Falcon helping a mated pair of Collared Forest-Falcons feed their two offspring, 4 to 11 weeks after the latter had fledged (left the nest).  This third adult forest-falcon showed a preference for catching toucans.  Of 36 prey items it delivered to the young, 27 were Keel-Billed Toucans (Ramphastos sulfuratus) and 2 were Collared Aracaris (Pteroglossus torquatus).  On some days it delivered 2 toucans to the young.

Thorstrom (2000) reported that the Barred Forest-Falcon usually hunted its prey by surprise flying attacks from concealed perches.  Although the Collared Forest-Falcon also used this method of hunting, Thorstrom observed it using other techniques such as running on the ground around trees after prey, and chasing prey on foot along large tree branches.  He hypothesized that the long legs of the Collared Forest-Falcon, and its long arched tail, gave it greater maneuverability while chasing prey on foot.

The smaller Barred Forest-Falcon fed more frequently on insects than did the larger Collared Forest-Falcon.  Eight percent of prey delivered to female Barred forest-Falcons, nestlings and fledglings at Tikal National Park were insects, while no insects were recorded in the diet of the Collared Forest-Falcon (Thorstrom 2000). 

In the same park, Thorstrom et al. (2000a) observed 8 young Barred Forest-Falcons associating with swarms of army ants (Eciton spp.), 3 to 4 weeks after fledging.   These young forest-falcons usually perched 1 to 2 meters above the ant swarm, and fed on cockroaches, crickets and beetles that fled the advancing ant army.  Thorstrom and his colleagues also observed these young falcons chasing small birds that were often attracted to the army ant swarms.   

Other observers confirm that the Barred Forest-Falcon is a frequent associate of army ants.  Willis et al. (1983) found Barred Forest-Falcons most numerous as ant followers in "mesic woodlands of central and southern Brazil," while in the wet forests of the upper Amazon they did so only rarely.  At Reserva Ducke, Amazonas, Brazil, these researchers reported seeing one or more Barred Forest-Falcons associating with army ants on 78 different occasions.

At Manu National Park in the Amazon region of Peru, Robinson (1994) twice observed Barred Forest Falcons associating with army ant swarms, and saw one of these raptors catch a large katydid fleeing the ants.   Collared Forest-Falcons are seen less frequently at army ant swarms than Barred Forest-Falcons and, when they are, seem more interested in catching the birds following the ants than the insects fleeing them (Willis et al. 1983).

Nest Site Preferences

In Tikal National Park, Thorstrom (2001) found that the Collared Forest-Falcon nested in larger trees than the Barred Forest-Falcon.  He explained this difference by the fact that the Collared Forest-Falcon is larger in size than the Barred Forest-Falcon, and therefore needs the larger nest cavities found in bigger trees.

Home Ranges

In Tikal National Park, home ranges for the larger-sized Collared Forest-Falcon were much greater in area than those of the smaller-sized Barred Forest-Falcon (Thorström 2007).  In one case, the home range of a nesting male Collared Forest-Falcon encompassed the home ranges of six nesting male Barred Forest-Falcons.

Perch Heights

In the forests of French Guiana, larger forest-falcons (M. semitorquatus and M. mirandollei) are usually seen perched 15-35 meters above the ground, while smaller forest-falcons (M. rufficollis and M. gilvicollis) are usually seen perched 3-16 meters high (Thiollay 2007).

Enemies

In Guatemala, Thorstrom et al. (2000a) reported predation on recently fledged Barred Forest-Falcon young, 2 or 3 days after they left the nest.  One was killed by a Gray Fox (Urocyon cinereoargenteus), the other by the snake Boa constrictor.

Footnotes

1.  The seven species of the genus Micrastur are:  Collared Forest-Falcon (Micrastur semitorquatus), Barred Forest-Falcon (Micrastur ruficollis), Lined Forest-Falcon (Micrastur gilvicollis), Plumbeous Forest-Falcon (Micrastur plumbeus), Slaty-Backed Forest-Falcon (Micrastur mirandollei), Buckley's Forest-Falcon (Micrastur buckleyi) and the Cryptic Forest Falcon (Micrastur mintoni - see Whittaker 2002).

The Collared-Forest Falcon has been observed and photographed in the lower Rio Grande Valley of Texas, USA (e.g. Bensten - Rio Grande Valley State Park, Hidalgo County), but the species is not known to breed there (Lasley et al. 1994; DeBenedictis 1994). 

References

Atkinson EC  (1997)  Singing for your supper: acoustical luring of avian prey by Northern Shrikes.  Condor 99: 203-206

Baker AJ, Aguirre-Barrera OA, Whitacre DF, White CM  (2000)  First record of a Barred Forest Falcon (Micrastur ruficollis) nesting in a cliff pothole.  Ornitología Neotropical 11: 81-82

Bierregaard RO  (1994)  Species accounts - Genus Micrastur.  In: Del Hojo J, Elliott A, Sargatal J (eds) Handbook of the Birds of the World, Volume 2.  Lynx Ediciones, Barcelona, pp 252-254.

Cobb J  (1990)  A nest of the Collared Forest-Falcon (Micrastur semitorquatus).  Aves Mexicanas 2(3), 90-1:8

DeBenedictis PA  (1996)  ABA Checklist Committee Report, 1995.  Birding 28: 399-405

Gerhardt RP  (2004)  Cavity nesting in raptors of Tikal National Park and vicinity, Peten, Guatemala.  Ornitologia Neotropical 15: 477-483

Lasley GW, Sexton C, Luckner GD  (1994)  Texas Region.  National Audubon Society Field Notes 48 (2) 224-228

Mader WJ  (1979)  First nest description for the genus Micrastur (forest-falcons).  Condor 81: 320

Mays NM  (1985)  Ants and foraging behavior of the Collared Forest-Falcon.  Wilson Bulletin 97: 231-232

Robinson, S.K.  (1994)  Habitat selection and foraging ecology of raptors in Amazonian Peru.  Biotropica 26: 443-458

Smith NG  (1969)  Provoked release of mobbing - a hunting technique of Micrastur falcons.  Ibis 111: 241-243

Thiollay J-M  (2007)  Raptor communities in French Guiana: Distribution, habitat selection, and conservation.  Journal of Raptor Research 41: 90-105

Thorstrom R  (2000)  The food habits of sympatric forest-falcons during the breeding season in northeastern Guatemala. Journal of Raptor Research 34: 196-202

Thorstrom R  (2001)  Nest-site characteristics and breeding density of two sympatric forest-falcons in Guatemala.  Ornitología Neotropical 12:337-344

Thorstrom R  (2007)  Home ranges of Barred (Micrastur ruficollis) and Collared (Micrastur semitorquatus) Forest Falcons during the breeding season in Tikal National Park, Guatemala.  Ornitologia Neotropical 18: 395-405

Thorstrom R, Turley CW, Ramirez FG, Gilroy BA  (1990)  Description of nests, eggs and young of the Barred Forest-Falcon (Micrastur ruficollis) and of the Collared Forest-Falcon (M. semitorquatus).  Condor 92: 237-239

Thorstrom R, Ramos JD, Morales CM  (2000a)  Breeding Biology of Barred Forest-Falcons in northeastern Guatemala.  Auk 117: 781-786

Thorstrom R, Ramos JD, Castillo JM  (2000b)  Breeding Biology and behavior of the Collared Forest-Falcon (Micrastur semitorquatus) in Guatemala.  Ornitología Neotropical 11: 1-12

Thorstrom R, Morales CM, Ramos JD  (2001)  Fidelity to territory, nest site and mate, survivorship, and reproduction of two sympatric forest-falcons.  Journal of Raptor Research 35: 98-106

Whittaker A  (2002)  A new species of forest-falcon (Falconidae: Micrastur) from Southeastern Amazonia and the Atlantic rainforests of Brazil.  Wilson Bulletin 114: 421-445

Willis EO, Wechsler D, Stiles F.G.  (1983)  Forest-falcons, hawks, and a pygmy-owl as ant followers.  Rev. Brasil. Biol. 43: 23-28

Information about this Review

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The photo at the top of the page shows a Barred Forest-Falcon (Micrastur ruficollis).  The photo was taken by Carlos Reis of Lisbon, Portugul.

The author is:  Dr. Paul D. Haemig (PhD in Animal Ecology)

The proper citation is:

Haemig PD  2012   Sympatric Forest-Falcons of the Genus Micrastur.  ECOLOGY.INFO 8

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