Decline of the Rusty Blackbird
Most species of North American blackbirds (family Icteridae)
are highly adaptable and have expanded, at least historically,
in the face of human development. The success
of the group as a whole may explain why it has taken a particularly
long time to recognize both acute and long-term chronic
declines in at least two species: the tricolored blackbird
(Agelaius tricolor) in the west (Beedy and Hamilton
1999) and the rusty blackbird in the northern and western
portions of the North American continent.
The Tricolored Blackbird is at present the focus of a concerted
monitoring and conservation effort with participation from
scientists at the Point Reyes Bird Observatory, United States
Fish & Wildlife Service, United States Geological Survey,
and the California Department of Fish & Game, among
others. No such similar program exists for the rusty blackbird,
despite the fact that it was recently featured in a report
by the National Audubon Society as the
North American species showing the sharpest decline.
Conservation efforts are most effective when they are
initiated while the target species is still common. However,
it is impossible to develop a meaningful conservation strategy
for a species when the cause or causes of decline are undetermined.
Despite the fact that the combined breeding and wintering
ranges of the species cover much of North America north
of the U.S.-Mexican border, the causes of the species' decline
are still far from certain.
Clues to the cause of the precipitous drop in numbers may be gleaned from
the species' unique natural history. The rusty blackbird
is arguably the most ecologically specialized of the North
American blackbirds, both in its feeding habits and habitat
uses. Throughout the year this species feeds
to a considerable extent on animal prey and is one of the
few bird species restricted year-round to wooded wetlands.
The species breeds either in isolated or small clusters
of pairs in boreal wetlands from northern New England and
the Maritime Provinces of Canada, north and west to central
Alaska (Figure 1, from Avery 1995). In fact, the rusty blackbird
is the passerine species whose breeding is most closely
tied to boreal forest wetlands for breeding, where it nests near open water
and feeds primarily on the adults and aquatic larvae of
Rusty blackbirds winter primarily in wooded wetlands of
the southeastern United States. An analysis of Christmas
Bird Count (CBC) data suggests that the greatest winter
concentrations are found in the Mississippi River Valley
(Niven et al. 2004). The species seems to roost with many
other blackbird species, but often is found foraging in
single species flocks or together with common grackles (Quiscalus
quiscula) in or near wooded wetlands and only occasionally
in agricultural fields with other blackbirds.
Fig. 1. Rusty blackbird breeding and winter range (from Avery 1995)
An analysis of the literature on the distrubution of North
America birds from the late-18th to the late-19th century
shows a consistent long-term decline in the qualitative
assessment of this species' abundance.
More alarmingly is that both national indicators
of songbird abundance, the Breeding Bird Survey (BBS) and
the CBC, show sharp declines over the past three decades
BBS data indicate a population trend that averages a
decline of more than ten percent per year for the last 30
This precipitous decline equates to a loss of more than
95 percent of the population that existed when the Breeding
Bird Survey was initiated. Therefore, it appears that rusty
blackbirds have shown both long-term chronic and short-term
acute patterns of decline. Despite the severity of the declines
in this species, the bird research and conservation community
has been slow to recognize and investigate the plight of
The major features of its life history are known, but
none are known thoroughly enough to support the various
hypotheses that have been proposed to account for the species'
decline. In the following paragraphs we list what is known
about the decline followed by potential explanations.
Fig. 2. Population trends over the last 30 years (line with dots is CBC index trend, line with diamonds is BBS index trend, from Niven et al. 2004)
The Decline and Possible Explanations
What is known
- The species seems to be rare to absent from a number of areas in the boreal forest where it had been known to be a common breeder. This includes northern Alberta and Saskatchewan, Northwest Territories, and northern Ontario. The only area we know where researchers report reliably large breeding populations is Alaska and Northern Yukon Territories.
- Although not well sampled on BBS, the species has shown a sharp global decline
for the past 35 years.
The rate of decline has been quite variable,
with the highest rates in the central and eastern portion
of the boreal forest
- Rusty blackbirds were common on the CBC until about
1970 when the species slipped into a decline from which
it has not recovered. Although the decline occurred across
the entire winter range of the species, interestingly,
the onset of the decline varied between regions.
The magnitude of the declines has also been quite variable, with the greatest
drop in numbers found along the South Atlantic coastal
Possible Reasons for Decline
- Winter habitat loss due to conversion of wetlands to
agriculture. The long-term and more recent conversion
of wooded wetlands to agriculture has been well documented.
At least 80 percent of the bottomland hardwood habitat has been
converted since European colonization. Whether the rate
of habitat conversion in the past 30 years is sufficient
to account for the recent declines in rusty blackbirds
- Large losses during blackbird control programs in the 1960s and 1970s
- Joining large roosts of blackbirds in winter exposes
this species to a disease for which it has low immunity.
- Increased competition with other blackbird species.
Habitat loss may have caused the rusty blackbird to feed
in more open habitats where it is more exposed to competition
with common grackles (Quiscalus quiscula) and
red-winged blackbirds (Agelaius phoeniceus).
Particularly females are at risk of suffering from such
- Breeding habitat loss and degradation, including boreal
wetland drying and changes in water chemistry due, directly
or indirectly, to global warming. Birds associated with
boreal wetlands have shown consistent cross-species declines.
In additions to rusty blackbirds, lesser yellowlegs (Tringa
flavipes), solitary sandpipers (T. solitaria),
lesser scaup (Aythya affinis), and boreal-nesting
scoters (Melanitta fusca and M. perspicillata)
have experienced among the highest rates of decline of
North American species.
In addition to rusty blackbirds, severe long-term declines
have been documented across all or major portions of the
breeding ranges of co-occurring lesser scaup, white-winged
and surf scoters, horned grebe, lesser yellowlegs, and
solitary sandpiper. Global warming is suspected
to be causing major changes in the extent of boreal wetlands,
the chemistry of the waters, and the structure of invertebrate
communities. Peat production, logging, and reservoir formation
have contributed both to direct loss of boreal wetland
and profound changes in hydrology, particularly in the
eastern portion of the species range. The eastern portion
of the range is where, historically, the species may have
achieved the highest breeding densities.
- Acid rain is effecting boreal wetlands, particularly
for the eastern portion of the breeding population, affecting trophic resources for a bird restricted
to already low-Ph environments.
- Mercury accumulation in tissue, known to affect other
blackbirds including common grackles, may decrease reproductive
success. The rusty blackbird may have a higher risk of
accumulating mercury than other blackbird species because
of its preference for feeding on aquatic invertebrates
and small fish.
- Loss or change of habitats along the migratory route.
- The rusty blackbird may be less able to adapt to environmental changes due
to its ecological specialization. Even slight changes
in the environment may render a habitat unsuitable for
- As yet unknown causes of mortality or reproductive failure that will not
be known until research is undertaken.