Behavioral ecology of coastal peregrines (Falco peregrinus pealei)

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A long-term study, 1968-75, of Peregrine Falcons at Langara Island, British Columbia, produced much information on the behavior and ecology of this population. An ethogram summarizes descriptions and functions of 43 behavior patterns in courtship, 32 in territorial advertisement and defense, and 15 in self, nest and food defense. Males are more active in courtship, territorial advertisement and defense. Mainly same-sex intruders are chased, but males also evict females. Nine hypotheses of sexual size dimorphism are considered. I conclude that aerial combat with dangerous weapons selects for smaller males, better combatants; the proportion of aerial to ground fighting sets the lower limit to the size of males. The annual schedule of courtship, incubation, nestling, fledgling, and dispersal phases is described. Seasonal changes in courtship are not proximate causes of egglaying. Photoperiod is an early timer for laying. Ambient temperature is a "final" timer, initiating rapid follicle growth ea. two weeks before Egg 1 is laid. Early egglaying gives juveniles more experience before autumn-winter hardships. Productivity over eight years averaged 1.76 fledglings per territorial pair, and 2.32 per successful pair. Average breeding spans were: males, 6.0 years; females, 3.5 years (survival rates: 0.85, 0.75). A first year survival rate of ea. 0.45-0.55, and a floating population at least 50% of the size of the breeding population were estimated. The Langara falcons declined from ea. 21-23 pairs in the early 1950s to 5-6 pairs in 1968-?5. This decline paralleled a seabird decline, apparently throughout the Queen Charlotte Islands. Falcons amalgamate territories by means of pseudopolyandry; an orderly population decline results, toward a new "equilibrium" with the prey base. Peregrines occupy Type A, B-A, and B territories, from 0.3-0.5 km to ca. 15 km in diameter. Individuals establish and adjust territory size in relation to available food. They harvest on a conservative sustained-yield basis. The result is the "natural conservation" of V. C. Wynne-Edwards, but the cause is individual selection. Peregrines demonstrate Bergmann's Rule. Larger birds live in cooler climates and have higher mortality rates and larger clutch sizes. Clutch size offsets natural mortality plus provides a floating population of optimum size. Peregrines evolved a strategy which does not produce the most fledglings at independence (e.g. D. Lack), but which balances the survival of parents and the number and quality of fledglings. Smaller broods will produce young with better survival rates and competitive abilities. When balanced with quantity, and combined with philopatry, this strategy tends to increase genetic fitness.
Bibliography: p. 448-485.
Nelson, R. W. (1977). Behavioral ecology of coastal peregrines (Falco peregrinus pealei) (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from doi:10.11575/PRISM/24399