Risks taken by female red‐winged blackbirds (Agelaius phoeniceus) in defence of their young were recorded for individuals with differing past investment but identical expected benefits. Investment was manipulated by switching clutches of equal size between nests differing in initiation date, thereby achieving incubation periods as brief as 6 days and as long as 17 days in addition to control nests with normal 11–12 day incubation. While this manipulation had no apparent effect on the females' ability to rear their offspring, during the late nestling stage (6–11 day old nestlings) there was a significant correlation between defence response and length of incubation for 1 of 2 measures used. If females use their cumulative time of investment to assess offspring age, the strong correlation between these two parameters under natural conditions would mean that the females would normally behave in an economically sound fashion. Some evidence is presented which supports this interpretation while contradictory evidence is also discussed. The general conclusion drawn from this study is that the validity of the assumption that animals invest in offspring in an economically optimal fashion remains to be adequately empirically demonstrated.
The study was conducted during the 1980 breeding season near the Queen’s University Biological Station in eastern Ontario employing the same population of red-winged blackbirds used by ROBERTSON and BIERMANN (1979). Breeding areas were searched every 2-3 days to locate nests as they were built and the clutches initiated. Whenever 2 nests with equal sized clutches and a difference of 4-8 days (normal incubation is 11-12 days) in clutch initiation date became available the clutches were exchanged. A “dummy” clutch of eggs was placed in these nests during the brief interval between removal of the eggs and replacement with the other clutch to avoid debcrtion by a female finding her nest empty. Experimental nests with shortened or lengthened periods of incubation will hereafter be referred to as short and long incubation iiests respectively. A control group of nests was made up of those nests for which a marched nest could not be found using the criteria detailed above. From the time the eggs hatched in a nest through to fledging the nest defence behaviour of the female was recorded. Following the approach of WEATHERHEAD (1979), the human observer was used as the nest threat. Nests were approached to within 1 m and then 2 mill were allowed for the female to respond. To reduce the likelihood of recording the response of a female other than the nest owner, a response was recorded only when the female gave a “check” call, termed here an alarm call. When a female began responding the number of alarm calls given in a 2-min period was recorded. During the same interval, the closest approach, furthest retreat and modal distance of the female from the observer were estimated with the mean of these values recorded as approach distance. Both increasing numbers of alarm calls and decreasing approach distance were assumed to represent greater risk-taking by females as both behaviours increase the exposure of the female to the animal to which she responds or to other predators that may be nearby (see ANDERSON et al. 1980). A minimum of 3 days was allowed between trials at the same nest.