1. W. R. Marion and R. A. Ryder "The Condor." 77, "Perch-site preferences of four diurnal raptors in northeastern Colorado." 350 (1975)
2. I. Newton, "Population Ecology of Raptors." (1979)
3. J. H. Rappole, "The ecology of migrant birds: a Neotropical perspective." (1995)
4. J. Fowler, L. Cohen, P. Jarvis, "Practical Statistics for Field Biology." (1998)
5. S. A. Akhtar, "Raptor." 1994, "Harriers in the Velavadar National Park, District Bhavnagar, Gujarat, India: Winter 1992–93." 51 (1994)