The Long-term Decline of the Rusty Blackbird
By Gregory Gough
The rusty blackbird's population has declined almost 90 percent since the mid-1800s. The decline has accelerated in recent decades.
While most other North American blackbirds (such as the red-winged, Brewer's, grackles, and cowbirds) have experienced population explosions, enough to warrant blackbird control programs, the rusty blackbird's decline has been largely unnoticed.
Rusty blackbirds nest in the boreal forests of Canada and the far northern United States. They migrate south to overwinter in flooded forests in the southeastern United States.
Systematic surveys since the 1950s (Christmas Bird Count, Breeding Bird Survey, Quebec Checklist Program), all show precipitous declines in rusty blackbird populations that are most severe in the 1970s.
Historical records in books and checklists indicate the decline stretches back into the 1800s. In the chart to the right, the rusty blackbird is listed as common or abundant on 56 percent of checklists before 1920. Between 1920 and 1950 only 19 percent of checklists list it as such and only 7 percent after 1950.
Conversely, it is not listed at all as uncommon before 1920 in any source. It is listed as uncommon on 43 percent of checklists after 1950.
The reasons for the decline are unknown but may include:
- Destruction of winter habitat
- Destruction of breeding habitat
- Acid rain
This article summarizes the information in this publication:
Greenberg, Russell S. and Droege, S. 1999. On the decline of the Rusty Blackbird (Euphagus carolensis) and the use of ornithological literature to document long-term population trends. Conservation Biology, 13: 553-559.
Unlike most North American blackbirds, Rusty Blackbirds (Euphagus carolensis) have shown steep population declines. Declines of approximately 90% are indicated for three recent decades from the Breeding Bird Survey, Christmas Bird Counts, and Quebec Checklist Program. Analyses of abundance classifications in bird distribution books and annotated checklists reveal an overlooked but long-term decline dating back to at least the early part of this century. Rusty Blackbirds were described as very common to abundant in 56% of the pre-1920 published accounts, 19% of the 1921-1950 accounts, and only 7% of the post-1950 accounts. Rusty Blackbirds were described as uncommon in none of the pre-1950 accounts,18% of the 1951-1980 accounts, and 43% of the post-1980 accounts. A similar pattern was found for analyses based on local checklists. Destruction of wooded wetlands on wintering grounds, acid precipitation and the conversion of boreal forest wetlands could have contributed to these declines. Systematic analysis of regional guides and checklists provides a valuable tool for examining large-scale and long-term poplation changes in birds.
Related research:
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