Red knot (Calidris canutus) COSEWIC assessment and status report: chapter 8
Biology
- Life Cycle and Reproduction
- Predation
- Physiology
- Dispersal/Migration
- Interspecific Interactions
- Adaptability
The information in this section is drawn mostly from material in the species account in Birds of North America (Harrington (2001) and in the US Red Knot status assessment Status of the Red Knot (Calidris canutus rufa) in the Western Hemisphere (Niles et al. 2005).
Breeding
Red Knots have a monogamous mating system. Egg laying occurs after a period of physiological reorganization, with eggs being formed from local food resources rather than from body stores brought from migration areas (Morrison and Hobson 2004). Pair bonds form soon after arrival and remain intact until shortly after the eggs hatch. Knots generally start breeding at age two. Mating occurs on the breeding grounds. Clutches consists of four eggs, occasionally three. Only one clutch is normally laid per year. Nests are simple scrapes usually placed in small patches of vegetation, and may contain lichens and other pieces of vegetation as lining. Incubation takes 22 days and is shared by males and females. Territories are large (and hence nest densities are low), with nests widely separated (typically 0.75-1 km apart). Although territories are defended from other conspecifics, off-duty birds tend to feed away from their territory in communal feeding areas. Knots often return to the same general breeding area from year to year, though little is known concerning the details of site or mate fidelity. The female departs a few days after hatch, leaving the male to care for the brood; fledging takes approximately 18 days. Hatching occurs in the first half of July. The young leave the nest within 24 hours, and as soon as they are mobile (within a day or two) the brood wanders over large distances (several kilometres or more) across the tundra. Following fledging, the males depart, being followed by the juveniles one to three weeks later. Nesting success may vary considerably from year to year, depending on weather conditions and predator cycles.
Migration and Wintering
During the non-breeding seasons, knots gather in large flocks on migration and wintering areas, feeding in coastal intertidal areas and roosting on nearby beaches, marshes or fields, where open undisturbed habitat is available. Not all juveniles reach the most southerly parts of the wintering range (Baker et al. 2004, 2005b). Second-year immatures are found on the major wintering areas, and move north to breed when two years old.
Longevity and Survival
The oldest Red Knot recorded (C. c. islandica) was originally banded on The Wash, SE England, in August 1968 as an adult and recaptured there in September 1992 (Wash Wader Ringing Group 2005). Since it could not have hatched later than July 1967, it was at least 25 years old when recaptured. One islandica knot bred near Alert, on the north coast of Ellesmere Island, over a period of at least 11 years between 1992 and 2002 (R.I.G. Morrison unpublished data). The oldest recorded rufa knot was originally banded as a juvenile at Punta Rasa, Argentina, in October 1987 and recaptured on the wintering grounds at Bahía Lomas, Tierra del Fuego, in February 2003, making it 16 years old (Niles et al. unpublished data). Although these records demonstrate that the potential lifespan of a Red Knot is considerable, most live much shorter lives. Annual adult survival in stable populations has been estimated at around 80% and the survival of juveniles is about half that (Boyd and Piersma 2001). Very few knots therefore live for more than about 7 - 8 years.
On the breeding grounds in the Canadian Arctic, major predators of nests/eggs include Arctic foxes (Vulpes lagopus) and Long-tailed Jaegers (Stercorarius longicaudus), and sometimes Arctic wolves (Canis lupus arctos); these species, as well as other species of jaegers (Parasitic Jaeger Stercorarius parasiticus, Pomarine Jaeger Stercorarius pomarinus), gulls (Herring Gull Larus argentatus and Glaucous Gull Larus hyperboreus), falcons (Gyrfalcon Falco rusticolus and Peregrine Falcon Falco peregrinus), and owls (Snowy Owl Bubo scandiaca), may take chicks and sometimes adult birds. Predation pressure may vary considerably in different years depending on the abundance of lemmings: in years of high lemming abundance, most of the above predators focus on lemmings as their major prey, whereas in years of low lemming abundance, predators are forced to turn to alternative prey, including shorebirds.
On migration and wintering areas, the most common predators of Red Knots are large falcons, such as the Peregrine Falcon, harriers, accipiters, smaller falcons such as Merlin (Falco columbarius), owls (Short-eared Owl Asio flammeus), and large gulls (Great Black-backed Gull Larus marinus). Butler et al. (2003) and Ydenberg et al. (2004) have pointed out that the increase of raptor populations over the past several decades has affected the behaviour of small shorebirds, and it is possible that knots are also being affected. For instance, it appears that falcons have influenced shorebird distribution between and within the upper arms of the Bay of Fundy (NSDNR 2004). In addition to increasing the risk of direct predation, the presence of aerial predators has the potential to affect energy budgets of the birds through disturbance and forced additional flight. No information is available on whether raptor predation is a significant pressure on the wintering areas.
During the final stopover on northward migration, knots undergo a host of physiological changes (Piersma et al. 1999; Baker et al. 2004). For example, during the middle of the stopover period, their digestive organs tend to increase in size, at a time when they are rapidly laying down fat. Towards the end of their stay, the fat deposition continues, while at the same time the “exercise organs” (or “flight machinery” - heart, flight muscles, fat) that will power the flight increase in size; in contrast, the “digestive organs” and muscles that will be used less or not at all during the flight (the “baggage” - leg muscles, gizzard, gut, liver) actually decrease in size, so that by the time the bird departs, it is adapted for the long flight to the Arctic. Knots arrive on the breeding grounds with stores of fat and protein still remaining. These fat and protein stores are lost at the same time that gut, gizzard, heart (which decreased in size during flight), liver, gonads, etc. are regrown in preparation for the breeding attempt (Morrison et al. 2005). Failure to accumulate the needed stores and to undergo the series of physiological transformations before migration and before breeding appear to have severe survival consequences (Baker et al. 2004; Morrison 2005; see Threats section).
Rufa undertakes one of the longest migrations of all the knot subspecies, moving from breeding grounds in the central Canadian Arctic to wintering grounds at the southern tip of South America (Morrison 1984). The southern “wintering” areas in Tierra del Fuego and Patagonia are occupied from approximately October to February. Knots are found on migration at the Valdes Peninsula and at San Antonio Oeste on the Patagonian coast of Argentina during March and April, before moving on to Lagoa do Peixe in southern Brazil, where they occur from late March through April. By late April or early May the birds move northwards through Maranhão on the north-central coast of Brazil, and then fly to the eastern seaboard of the US, where they pass up the coast of the Carolinas, Virginia, and Maryland, and into Delaware Bay. Rufa refuel in Delaware Bay in May, departing from the last week of May to early June. Small numbers are found migrating through areas such as Point Pelee National Park and Presqu’ile Provincial Park on the Canadian Great Lakes from around mid-May to early June (McRae 1982; Weir 1989; Morrison and Harrington 1992; V. MacKay pers. comm.). They are observed passing northwards through the southern part of James Bay in the last days of May or early June, and arrive on the breeding grounds in early June (Morrison and Harrington 1992).
The return migration begins in the latter half of July, with adult birds passing through James Bay, the north shore of the St. Lawrence River, and the Maritime Provinces in late July or early August (see references in caption for Figure 7). Passage through the New England coast and areas farther south on the Atlantic coast of the US is somewhat later. Juveniles migrate southwards in August, passing through the east coast of Canada from mid-August to around mid-September. Passage through Suriname (Spaans 1978) is in the second half of August and first half of September for adults, a second peak in October probably referring to juveniles. The birds may move farther east along the north coast of South America, before flying across Amazonia, passing through the Atlantic coast of southern Brazil, Uruguay, and northern Argentina from September onwards. Passage down the coast of Argentina occurs during October, with arrivals on the southern wintering area in that month (Baker et al. 2005b).
Most of the rufa occurring in Tierra del Fuegoare adults; juveniles made up 6% of the population in 2004 (Baker et al. 2005b). Many juveniles are thought to remain in southern areas during their first northern summer, perhaps moving some way towards the breeding grounds, but not completing the full migration (e.g., Belton 1984). Birds in their second year (immatures) are found in Tierra del Fuego; percentages in flocks fell from 19% in 1995 to 10-13% in 2001-2004 (Baker et al. 2005b).
Knots concentrate to a remarkable degree at favoured sites during the winter and on migration. In recent years, 97-98% of rufa knots on the wintering areas occurred in Tierra del Fuego, with about 83% of the population at one site, Bahia Lomas (Morrison et al. 2004, unpublished data). On migration, Gonzalez et al. (1996) estimated that at least 20% of the population passed through San Antonio Oeste (based on a high total population estimate of 125 000 knots: the figure was likely closer to half this, indicating about 40% of the population used this site). In Delaware Bay, Atkinson (2005) recently estimated that about 50% of rufa pass through that area in spring.
Knots appear highly site faithful in all parts of their range. On the wintering grounds, Baker et al. (2005b) reported many recaptures of birds previously banded at Rio Grande at the same place, and many resightings and recaptures of the same (colour marked) individuals have been made at San Antonio Oeste and Delaware Bay (P. Gonzalez, R.I.G. Morrison, A.J. Baker, L.J. Niles unpubl. data). At Alert, many islandica return to the same breeding area from year to year (Morrison et al. 2005, unpublished data), and studies on Southampton Island in 2000-2004 indicate that the same occurs with rufa (Niles et al. 2005).
Rufa and the Florida/SE US and Maranhão, Brazil populations of C. c. roselaari are dependent on the eggs of the horseshoe crab as their main prey in Delaware Bay and other coastal areas of the US during spring migration.
Given the high site fidelity of knots on their wintering, migration, and breeding areas, it is difficult to predict whether they would adapt readily to new or different areas under changing environmental conditions, even if such areas were available. The fact that knots have declined rapidly in the face of decreasing food resources in Delaware Bay suggests that they have not successfully been able to find alternative foods or foraging areas to meet their requirements on northward migration. Knots show many specializations in life history and physiology (Piersma and Baker 2000; and see Physiology section), and their traits of low fecundity (clutch size ≤4 eggs, high nest failure, only one brood per year), delayed maturity, and high annual survival (70 - 90%) (Sandercock 2003) would appear to make them vulnerable to rapid environmental change. Knots have very low genetic variability and it is not known if this may imply reduced behavioural plasticity and a greater susceptibility to environmental perturbations (Baker et al. 1994; Piersma and Baker 2000).
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