Summary
Male parental care generally is assumed to improve survival of nestlings and increase female reproductive success (fitness). Consequently, the need for male assistance is considered by some to be responsible for the evolution of monogamy when the investment of the male is not shareable among mates (Wittenberger and Tilson 1980). However, Gowaty (1983) found that reproductive success was not different between lone and paired female Eastern Bluebirds (Sialia sialis) and argued that monogamy is not necessarily maintained only by the need for male care. Ban and Tomes (1989) cite several examples of male-removal studies that indicate the presence of a male is of little value in many species, but they conclude that apparent care by the male does tend to benefit the young.
In this study we attempted to determine the impact of male parental care on female reproductive success in the monogamous Eastern Kingbird (Tyrannus tyrannus). Male Eastern Kingbirds help to feed, guard, and defend their young throughout the breeding cycle. Males also aid in the 3- to 4-week period of post-fledging care (Morehouse and Brewer 1968).
Methodology
This study was conducted at Eastern Kingbird nests on and around Lake Opinicon, Ontario, from May to August 1986. The study site is described in detail by Blancher and Robertson (1985). Five resident males were removed experimentally by shooting when the nestlings were one day old. Shooting (Canadian Wildlife Service Permit #EK290) was necessary because we did not know of any way to keep these aerial insectivores alive in captivity for the necessary time period. For this reason a very small sample size was used. Seven nests that corresponded in nesting time and habitat to the experimental nests were chosen at the time of the removals and were used as unmanipulated controls. We measured feeding rates per nestling by watching nests for 60 min intervals every other day during the nestling period. These observations were carried out at approximately the same time each day. Nests were watched from a distance of approximately 30 m to avoid disturbing the birds. Following each watch, nestlings were weighed with a Pesola spring balance, and each nestling's 9th primary was measured to the nearest mm. We also quantified nest defense at each nest four times during the nesting cycle (incubation days 1-6, 7-14; and nestling days 1-4, 5-8) by scoring the aggressiveness of the birds during a 5-min trial to a simulated predator (plastic crow) placed approximately 1 m from the nest. Responses were scored on a scale of 0 to 5 (0--no response; 1--silent observation; 2--vocalization and hovering; 3--few single dives; 4--continuous diving; 5--striking the crow). Repeat testings on nests were carried out with a minimum of four days between presentations in an effort to avoid habituation. Males and females were distinguished by their behavior (for example, favorite perch locations) and by noting which bird was at the nest. In nesting pairs, one of the birds is almost always guarding the nest, resulting presumably in fewer losses to predators. Smith (1966) noted this trade-off pattern, and found that the male was present 82% of the time when the female left the nest and 91% of the time when she returned. Predation was assumed if the entire brood disappeared at one time with no evidence of poor weather or dead nestlings.