Age-Specific Mortality in the Eggs and Nestlings of Blackbirds

Abstract

STUDIES of population dynamics are recognized as an important component of modern biology, and such studies have been made on many forms, both vertebrate and invertebrate. At the present time one of the needed forms of information for these studies is life curves for the various animal types. In ornithology some data for this use have been gathered by banding, but these relate primarily to adult and subadult segments of the populations. Vertebrate population studies in general have been handicapped by a shortage of data on age-specific mortality, particularly that occurring at early ages (Deevey, 1947; Hickey, 1952). Most ornithological data of this sort have pertained to pre-cocial game species, where the early survival of young is extremely difficult to determine. Estimates can be made by observing brood-shrinkage over a period of time (Randall, 1940; Leedy and Hicks, 1945), but it is seldom possible to determine when specific individuals are lost. Studies on the breeding success of altricial birds can be particularly useful in gaining information on early mortality in certain natural populations. Age-specific mortality rates can be approximated by keeping careful records of the appearance and disappearance of eggs and young in the nest. For comparison with mammals, egg-loss can be considered equivalent to intrauterine mortality. Studies of avian breeding success have occupied many ornithologists, and excellent papers on this topic are abundant. However, these have concerned themselves mainly with what has come to be considered as standard information on breeding success: per cent of nests successful (fledging at least one young), per cent of eggs hatching, and per cent of young fledging. Aside from the papers of Paynter (1949), Petersen and Young (1950), and Young (1955), there does not appear to have been much effort to gather age-specific mortality data for the early stages of avian life cycles in altricial species. This paper presents such information for these early stages (egg and nestling) of the Red-winged Blackbird, Agelaius phoeni-ceus, and the Yellow-headed Blackbird, Xanthocephalus xanthocephalus. METHODS Studies were made during the spring and summer of 1959 and 1960 in a cat-tail (Typha) marsh in the lowlands of the Mississippi River near Stoddard, Wisconsin. As each nest was found it was marked by a stake to which a numbered 145 The Auk, 80: 145-155. April, 1963