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Thorstrom, Russell K. – Breeding ecology of two species of Forest-falcons (Micrastur) in northeastern Guatemala. 1993.

Photo of a barred forest-falcon in a tree
Barred forest-falcon

Alumni Research

I studied the breeding biology, food habits, and home ranges of the barred forest-falcon (Micrastur ruficollis) and the collared forest-falcon (M. semitorquatus) in Tikal National Park of northeastern Guatemala from 1988 to 1992. Barred and collared forest-falcons are residents and maintain territories throughout the year.

Barred forest-falcon breeding coincides with the end of the dry and beginning of the wet seasons. Nesting sites were occupied in April, May, and June and average reoccupancy rate was 86%. Thirty-eight nests were located in tree cavities. The onset of egg-laying was in mid April to early June. The incubation period was 37 days from laying of the first egg to hatch (n=6 nests) and the average age of fledging was 38.3 days (SD3.3, range=35-44 days). Fledging occurred in July and August. Of 34 fully documented nesting attempts 16 (47%) produced young. A total of 1.1 young fledged per breeding attempt. Most reproductive losses in 17 nests resulted from egg predation (n=11). Survivability of adult breeding females was less than that for males. Food habits were based on 600 prey deliveries during the breeding season. On a numerical basis, lizards made up 40.8% (n=245) and birds 14.6% (n=88) of the diet. Biomass estimates showed lizards 37.3% and birds 36.8% to be the most important prey items delivered during the breeding season. Home range estimates of 85% harmonic means averaged 114.628.8 ha for breeding males (n=11).

Photo of a Collared forest-falcon
Collared forest-falcon

Collared forest-falcons breeding occurs at the end of the rainy season through the dry to the beginning of the rainy season in June and July. Nesting sites were occupied in February and March. Eight nests were located, seven in Tikal and one outside the park. Laying occurred in late February to mid May. The incubation period was 47 days (n=1 nest) and the average age of young fledging was 50.04.2 days (ranging from 46 to 56 days). Fledging occurred in June. Overall reproductive success was 57% (4 nests fledged young out of 7 attempts). Productivity was 1.0 young fledged per breeding attempt. Nest failures were caused by predation on eggs. Food habits were based on 258 prey items delivered during the breeding season. On a numerical basis (n=222 prey items) mammals 35.1 % (n=78) and birds 26.8% (n=59) made up the major proportion of the diet. Biomass estimates showed mammals 45.6% and birds 49.4% to be the most important prey delivered during the breeding seasons. An extra adult took over parental duties of feeding fledglings in 1990 and delivered 36 prey items. Home range estimates for the 85% harmonic means averaged 877.6301.6 ha for males (n=3, two breeding and one nonfreezing) and 712.0578.7 ha for females (n=4, one breeding and three nonfreezing).


Thesis Abstract

Where Are Graduates Now? (Russell K. Thorstrom)


Thorstrom R., R. Watson, A. Baker, S. Ayers, and D.L. Anderson. 2002. Preliminary ground and aerial surveys for Orange-breasted Falcons in Central America. Journal of Raptor Research 36:39-44.

Thorstrom R., J. Hart, and R.T. Watson. 1997. New record, ranging behaviour, vocalization and food of the Madagascar Red Owl (Tyto soumagnei). Ibis 139:477-481.

Thorstrom, R., R.T. Watson, B. Damary, F. Toto, M. Baba, and V. Baba. 1995. Repeated sightings and first capture of a live Madagascar Serpent Eagle (Eutriorchis astur). Bulletin of the British Ornithologists’ Club 115:40-45.

Thorstrom, R. 1996. Fruit-eating behavior of a Barred Forest-falcon. Journal of Raptor Research 30:44.