Created By: Cynthia Galang
Vol. 18, No. 1 (Mar., 1964), pp. 105-110
 The evolutionary trend among birds has been towards producing relatively few young, but bestowing upon them protracted parental care. One evolutionary reason for this is that the complex physical skills of flight and food getting require a certain time for maturation and perfection. The climax of this trend is found in certain large, long-lived birds.
In a few of these, less than a dozen among 8,600 species, the nesting cycle is longer than a year. These species rear only one young per brood. It does not attain sexual maturity until several years old. Related species provide intermediates in the direction of these extremes. The need for recruitments in these slowly reproducing, long-lived birds is very low on an annual basis. Competition for food as the population of each species approaches the carrying capacity of the environment will place a premium upon physical endurance and food-getting skills, especially during the nesting season.  As a result those pairs producing the best endowed young, even though few in number, will contribute more individuals to the following generations. In some species such as eagles territorial behavior has evolved and by safeguarding the food supply during the nesting season helps to insure or to increase productivity.
In other species such as vultures and albatrosses this type of territory does not exist because food territories could not be maintained in such wide-ranging and (in the albatrosses) oceanic species, or because they are otherwise disadvantageous. But in both the territorial and non-territorial species, the reproductive rate, however low it may seem, is believed to represent the maximum attainable by these large specialized birds. Hence, contrary to opinions expressed by Wynne-Edwards, they are not considered incompatible with Lack's exegesis of the selective basis for reproductive rates in birds.
(The article wouldn't let me copy the entire text.)
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