• Weatherhead, Patrick J.


Habitat loss and large-scale climate phenomena are widely implicated as causing decline in animal populations. I examined how both factors contributed to a precipitous decline in an Ontario red-winged blackbird (Agelaius phoeniceus) population using 16 years of data collected between 1974 and 1995. The decline was manifested as an almost 50% reduction in mean harem size, which reduced the opportunity for sexual selection threefold. Regional hay production, which should affect recruitment into the study population, also declined substantially. Correlation between blackbirds and hay may be coincidental, however, because annual changes in harem size were not associated with annual changes in hay production. This study coincided with an unprecedented positive phase of the North Atlantic Oscillation (NAO). Changes in harem size were correlated with winter NAO index values, suggesting that winter mortality contributed to the population decline. Positive correlation between harem size change and male return rates also supported the winter mortality hypothesis. Continued declines will cause this blackbird population to change from socially polygynous to socially monogamous. Study of red-winged blackbird winter ecology is needed to identify the proximate causes of mortality, whereas breeding studies can explore the consequences of relaxed sexual selection.


I used data from a series of studies conducted in 16 breeding seasons spanning the period from 1974 to 1995 at the Queen's University Biological Station in eastern Ontario (45 °37′ N, 76 °13′ W). All studies were conducted using the same group of marshes. Every marsh was not used in every year, but multiple marshes were used each year and every marsh was used in multiple years. Each year territories were mapped and beginning in 1985, territorial males were individually banded. Because returning males are highly site faithful, annual return rates of banded males were used to estimate male survival (Weatherhead & Clark 1994). Detailed methods are available from the following: 1974–1975 (Weatherhead & Robertson 1977), 1981 (Weatherhead 1983), 1982–83 (Weatherhead 1985), 1985–1995 (Weatherhead & Sommerer 2001).

I estimated harem size as the maximum number of simultaneously active nests on a territory. I estimated the opportunity for sexual selection (Wade & Arnold 1980) as the standardized variance (I=variance/mean2) in harem size for all territories sampled each year. I did not have measures of reproductive success in all years, but harem size explains a substantial proportion of the variance in male fledging success (Weatherhead & Boag 1997) and was the best measure of sexual selection available for most years.

I could not estimate the number of breeding males by summing territories across all marshes because the same suite of marshes was not used each year. Therefore, for each marsh I calculated the number of males present each year as the percentage of the mean number of males on that marsh over all the years it was sampled. I combined those percentages for all marshes sampled each year as ‘mean percentage of territorial males’. I excluded one marsh from these estimates because water levels fluctuated in response to the state of a beaver dam, altering availability of nesting habitat.

I obtained estimated land planted to hay in Ontario from Statistics Canada. The hypothesis I considered with regard to hay was that land area in year x−1 should affect the size of my blackbird population in year x. Note that I consider this hypothesis only for female blackbirds. Females start to breed at one year of age, whereas males usually delay breeding (Searcy & Yasukawa 1995). Thus, changes in regional production of females in year x−1 should be reflected by changes in harem size in my study population in year x.

I obtained climate and weather data from the National Oceanic and Atmospheric Administration. I used monthly mean NAO index (NAOI) and ENSO index (southern oscillation index (SOI)) values for 6 months (October–March) and 12 months (April–March) preceding the breeding season. To associate climate effects with weather on the birds' wintering grounds in southeastern US (Dolbeer 1978), I used mean temperature and precipitation records for December through February for the Southeast Region for each winter preceding a year for which I had breeding data.

In addition to regressing blackbird numbers against independent variables to identify associations, I examined how differences in blackbird numbers from year to year varied with annual changes in independent variables. The latter approach helped differentiate between coincidental and causal associations between variables, where both exhibited chronological trends.