Justification of Red List Category
This species has an extremely large range, and hence does not approach the thresholds for Vulnerable under the range size criterion (Extent of Occurrence <20,000 km2 combined with a declining or fluctuating range size, habitat extent/quality, or population size and a small number of locations or severe fragmentation). The population trend appears to be increasing, and hence the species does not approach the thresholds for Vulnerable under the population trend criterion (>30% decline over ten years or three generations). The population size is extremely large, and hence does not approach the thresholds for Vulnerable under the population size criterion (<10,000 mature individuals with a continuing decline estimated to be >10% in ten years or three generations, or with a specified population structure). For these reasons the species is evaluated as Least Concern.
The overall population trend is increasing, although some populations may be stable and others have unknown trends (Delany and Scott 2006). In North America, this species has undergone a large and statistically significant increase over the last 40 years (2200% increase over 40 years, equating to a 120% increase per decade; data from Breeding Bird Survey and/or Christmas Bird Count: Butcher and Niven 2007).
The Double-crested Cormorant is widely distributed across North America, from the Aleutian Islands and Alaska (U.S.A.) down to north-west Mexico on the Pacific coast, and from North Carolina (U.S.A.) down to Cuba on the Atlantic coast. Summer breeding grounds also include much of the U.S.A. and southern-central and eastern Canada.
This species utilizes a variety of habitats, including sheltered marine waters such as estuaries, bays and mangrove swamps, rocky coasts and coastal islands, and inland lakes, rivers, swamps, reservoirs and ponds. Its diet it almost exclusively fish with a few crustaceans, with the prey species changing depending on locality. Prey is caught by pursuit-diving, and individuals can fish co-operatively, sometimes with thousands of birds together at one time. It begins laying from April to July, nesting on a wide variety of substrates forming colonies sometimes over thousands of pairs strong (del Hoyo et al. 1992).
In the 1950s and 1970s, the species was significantly impacted by pesticide contamination of waterways in California and the Great Lakes region. Reproductive output was dramatically reduced, followed by rapid population declines (Hatch 1995). Subsequent regulation of pollutants, including the ban on DDE in the U.S.A., allowed populations to recover and increase beyond their previous abundance (Wires and Cuthbert 2006).
Text account compilers
Butchart, S., Fjagesund, T., Martin, R., Ekstrom, J., Calvert, R.
BirdLife International (2020) Species factsheet: Nannopterum auritus. Downloaded from http://www.birdlife.org on 26/02/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 26/02/2020.