| Contact details
(phone/fax//e-mail//address): |
| 907-543-1014/907-543-4413(f) // brian_mccaffery@fws.gov // U.S. Fish and Wildlife Service, Yukon Delta National Wildlife Refuge, P.O. Box 346, Bethel, AK, 99559, USA
|
|
| Rodents abundance evaluation: |
|
|
|
Breeding conditions:
|
In 2002, we completed the fifth consecutive year of shorebird studies at our long-term study site on the central Yukon-Kuskokwim Delta.
Since at least 1984, arvicoline rodents, primarily voles (Microtus spp.), on the Yukon-Kuskokwim Delta have exhibited a 4-year population cycle. The last peak occurred in 2000; the next is expected in 2004. Unlike other shorebird species in other areas of the arctic, nest success of Western Sandpipers is not obviously correlated with rodent cycles at our study site. The years of highest and lowest nest success, 1998 (0.55) and 2002 (0.11), occurred at the same stage of the rodent cycle (i.e., 2 years after peak rodent numbers). Nest success during the rodent population high in 2000 was intermediate (0.37) and was not statistically different from the 5-year mean at our site. We hypothesize that this lack of correlation is due to 1) the lower amplitude of rodent population highs in our region compared to other regions in the arctic, and/or 2) the high annual abundance of alternate prey species for predators which may buffer them against dramatic fluctuations in rodent populations. In addition to mink, both Red and Arctic Foxes were observed on our study site in 2002. Between the first Western Sandpiper clutch initiation and the final fledging (~60 days), Red and Arctic Foxes were detected on 5 and 17 days, respectively. Among avian predators, both Long-tailed and Parasitic Jaegers were seen almost daily throughout the breeding season.
As in past years, Western Sandpipers (Calidris mauri) were the focus of our investigation. The first Western Sandpiper clutches were initiated on 17 May, which was 8 days earlier than in 2001, and 2 days earlier than the previous early date at our site (19 May in 1999). In 2002, 50% of first nests were initiated as of 23 May. By comparison, in 2001, it was 5 June before 50% of first nests had been initiated. In past years, nest density has been calculated as simply the number of nests found divided by the study area size (= 16 ha). By this measure, nest density in 2002 was the lowest recorded to date, 2.63 nests/ha versus a mean of 3.01 nests/ha (range 2.95-3.06) for 1999-2001. The number of nests found, however, is at least partially a function of nest predation (i.e., fewer nests found when predation rates are high). Nest predation in 2002, probably primarily by mink (Mustela vison), was very high; Mayfield nest success was only 0.11, compared to a mean of 0.34 (range 0.21-0.55) for 1998-2001. When rates of nest loss are considered, nest density in 2002 may have been as high as 4.56/ha.
High rates of nest loss led to high rates of re-nesting; 35% of pairs that lost nests re-nested. The mean interval between the loss of a first nest and the initiation of a pair's second nest was 6.4 days. There was no correlation between the age of the nest lost, and the interval required to initiate the second clutch. By eliminating second nests from consideration, we estimated the density of breeding pairs on the study area (i.e., number of pairs that initiated at least 1 clutch) to be 3.56 pairs/ha.
Among clutches which hatched, fledging success (defined as a clutch fledging one or more young) was only 39%, compared with 58% in 2001. Overall in 2002, only 4% of clutches initiated resulted in fledged young.
|
|