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Nature Of Migration In The Species
Very variable; birds nesting in the interior and on Atlantic Coast (P. a. auritus) are highly migratory; all age groups migrate. Immature birds arrive later in spring but are not known to occupy different ranges. In other areas, mostly resident within breeding range, and very little is known of migratory habits. In Florida and the Caribbean, P. a. floridanus reported to be resident, but withdraws from the Carolinas in winter (Bent 1922). Its breeding range overlaps with migration and wintering areas of P. a. auritus, from which it is indistinguishable in the field. In Bahamas, P. a. heuretus is probably resident (Watson et al. 1991). The more northerly populations of P. a. cincinatus (in Alaska) may migrate south to s. British Columbia, especially the outer coast (Campbell et al. 1990, Johnsgard 1993) and Washington, but specific data are lacking. In Mexico and California, coastal and southern areas receive influx of birds in fall, probably P. a. albociliatus from inland or farther north. Birds banded in British Columbia encountered south to s. California.
A change in wintering areas (and migration routes) suggested by recoveries of cormorants banded in S. Dakota; from Louisiana in 1930 to Texas and Mexico in 1960s (Drewlen and Fredrickson 1969).
Timing And Routes Of Migration
Some birds leave Gulf of Mexico as early as Feb. They follow Atlantic coastline or river systems, as well as going overland. Earliest individuals reach Oklahoma and Virginia by 4–5 Mar (Sutton 1967, Kain 1987); Massachusetts, s. Great Lakes, Minnesota, S. Dakota, and s. Idaho by late Marearly Apr; upper Great Lakes by early to mid-Apr; coastal Maine, Alberta, and Saskatchewan by mid- to late Apr. First adults arrive at colonies in e. Lake Ontario when ice breakup is well advanced but large areas of pack ice usually still present (DVW). Similarly, first arrivals at North Channel (Lake Huron) may be confined to river or creek mouths that form the only open water (S. Elliott pers. comm.). Peak spring migrations occur 3–4 wk later, in Missouri in third week of Apr (Robbins and Easterla 1992), in Massachusetts in late Apr. Records of migrants in large numbers en route are rarely published, but a very large passage was reported on 24 Apr 1926, when “100,000–1,000,000” cormorants passed La Crosse, WI, flying up the Mississippi River (Robbins 1991). From banding recoveries, Dolbeer (1991) concluded that first-year birds return north about a month later than older birds.
On Pacific Coast, migration is poorly known because some individuals are resident year-round. Migrants leave Oregon in Mar (Gilligan et al. 1994). In British Columbia, migration period is mid-Mar–late May, with heaviest movements late Apr–early May (Campbell et al. 1990). In interior Oregon, migrants arrive Feb–Mar. In California, arrive at Farallon Is. early Mar–mid-Apr (Ainley and Boekelheide 1990).
Autumn migration is essentially the reverse of spring migration, but movements are well under way before open water becomes limiting. Earliest autumn migrants appear to reach wintering grounds faster than they reached breeding grounds in spring, although overall timing is more protracted. Several juveniles banded in Michigan have been recovered on Mississippi River from s. Minnesota to s. Missouri by late Jul (J. Ludwig unpubl.). More usually, migration is evident on Canadian prairies by mid- to late Aug, by then first migrants are on upper Mississippi River (Kirsch 1997) and in Ohio, Massachusetts, Virginia, and even Texas (Oberholser 1974). By Sep, migration is well under way throughout range. From Lake Champlain (Vermont/New York), all individuals depart within a few days in late Sep (D. Capen pers. comm.). Several areas report peak numbers of transients in Oct (Massachusetts and Rhode Island: first half; Minnesota: third week). Very few descriptions available of what must be large movements both down the Mississippi River/Flyway and out of the Great Lakes: Two reports from 1940s describe 9,500 and 15,000 birds, respectively, passing Havana, IL, on single days in early Oct (Mills et al. 1966). Near Rochester, NY, hundreds are seen each year flying westward along southern shore of Lake Ontario (B. Ewald unpubl.). Most banded migrants are recov-ered in Oct and Nov (Dolbeer 1991).
Patterns of coastal migration in New England, described by Nisbet and Baird (1959), continue with much greater numbers in 1990s (JJH). Flocks are generally observed along coast, except where they fly overland to bypass Cape Ann, MA, and again for Cape Cod. Small flocks appear in mid-Aug, but most migration observed 25 Sep–17 Oct. Individuals arrive in wintering areas Sep–Nov. Some fly across open ocean (now winters regularly in Bermuda), but rarely seen during cruises in search of pelagic seabirds.
On Pacific Coast, migrants are evident in British Columbia from late Aug to early Nov; influx into inner coastal waters peaks in early Sep (Campbell et al. 1990). In coastal Oregon, migrants arrive Jul–Aug; east of Cascade Mtns., the last migrants depart Oct–Nov (Gilligan et al. 1994). Depart from Farallon Is., CA, after last young fledge in late Sep (Ainley and Boekelheide 1990).
Most inland records of migratory flights are of small flocks, <50–100. Along coasts, near-shore flocks may be large (in the thousands). Over water, usually flies low, in wavering lines or irregular V-formations; over land, flies as high as 1,000 m. An autumn departure from Presqu’Ile Bay, Lake Ontario, was described as follows: “At mid-morning a flock of approximately a thousand cormorants began to take flight from a large protected bay and spiraled upward to a height of several hundred feet. They circled and then flew off to the south, as if to fly across the lake. Several minutes later, however, they returned, resumed their high altitude circling (much the way passerines do during migration before crossing a large body of water) then departed a second time to the south and did not return” (R. D. McRae pers. comm.).
Little information available on daily duration; flights start soon after dawn and continue all day (Nisbet and Baird 1959). Some autumn departures observed as late as 10:00 for overland flights from Boston, MA, toward Narragansett Bay, RI (JJH). Spring departures observed at first light from overnight roosts on Lake Ontario (DVW). No published observations of cormorants migrating by night as well as by day, but flocks have been observed flying in late evening and few have been seen to stop and settle (Nisbet and Baird 1959).
Control And Physiology
Largest numbers seen in cool, clear weather after passage of cold fronts (Nisbet and Baird 1959). No other data.