Little is known of how territoriality of tropical forest raptors may be influenced by habitat reduction or fragmentation. We compared territory size and territorial defense behavior of Collared Forest-Falcons (Micrastur semitorquatus) in four 2500-ha survey quadrants with differing percent cover of tropical moist forest in a modified landscape in east-central Mexico. We delineated territories of Collared Forest-Falcons using playbacks of a simulated intruder, and calculated territory size and core area size using GIS. We also measured latency, duration, and type of territorial response of falcons to playbacks. We estimated a mean territory size of 205.5 ± 98.7 ha for seven Collared Forest-Falcons, although territories were significantly larger in low-forest-cover landscapes (mean 288.6 ± 76.9 ha), compared to those with high forest cover (mean 143.1 ± 58.3 ha). Collared Forest-Falcons showed stronger territorial behavior when defending territories with greater forest cover, responding for a longer time (5.9 ± 3.3 min), and including vocalizations with flights toward the speaker, compared to territories with low forest cover, where falcons mainly vocalized for 2 ± 1.1 min. Our results suggest that the Collared Forest-Falcon is sensitive to the extent of forest cover in the landscape, compensating for reduced forest cover by expanding the area defended, but investing less in defense of this area. This may have long-term implications for the acquisition of resources such as nest sites, and population dynamics of Collared Forest-Falcons in areas of reduced forest cover.