Long-billed Curlew (Numenius americanus): a technical Conservation Assessment Prepared for the usda forest Service, Rocky Mountain Region, Species Conservation Project December 12, 2006 James A. Sedgwick
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- Table 1
- Table 1 ). Figure 9 a and Figure 9b
- Parameter Numeric value Interpretation
24 25 often forage outside of territories, and broods leave territories after hatching. Curlews quickly disperse after they arrive on the breeding grounds to establish territories in suitable habitat. Returning breeders establish territories with less agonistic interaction than first-time breeders do. After pairing, primarily the males defend territory boundaries only against conspecifics. One study (Nebraska) reported that some breeders defended a feeding territory in addition to a nest territory (Bicak 1977). Territory size at Humboldt Bay, California averaged 3.0 ha ± 2.1 SD (range = 1.3–7.5, n = 8) in summer, with overlap averaging 28.5 percent ± 29.7 SD (range = 1.3–88.1, n = 12; Mathis 2000). Away from breeding areas, territoriality has been reported for intertidal habitat; territoriality was not observed in wet pastures (Dugger and Dugger 2002). Not all individuals establish territories during migration or on winter areas. Territories at Humboldt Bay, California, which are occupied during winter and summer, averaged 2.4 ha ± 1.6 SD (range = 1.3–4.2, n = 3), similar in size to summer territories (Mathis 2000). Dispersal Juvenile curlews may move extensively after hatching. One brood that was relocated 6 days after banding was 6.5 km from the nest site (Sadler and Maher 1976). Curlew chicks fledge at 38 to 45 days after hatching (King 1978, Allen 1980, Redmond and Jenni 1986) and depart the breeding grounds relatively early (most from mid-June to mid-August) and in small flocks, with no real evidence of fall staging (Campbell et al. 1990). During late summer and migration, flocks of 10 to 50 birds are common (range = 100–500) (Allen 1980, Renaud 1980, Pampush 1981, Campbell et al. 1990, Roy 1996). Small migratory flocks in Utah during fall contain one or two adults and two to four juveniles, suggesting that family groups sometimes migrate together (Paton and Dalton 1994). There is little information on natal philopatry, but no birds marked as chicks were ever seen in subsequent years on breeding areas as yearlings (Redmond and Jenni 1986). Compared to females, male curlews are more likely to return and breed (first time) near the natal nest site (Redmond and Jenni 1982). Return rates of breeding adults (based on resighting data) over three years in Idaho were 89 percent ± 0.10 SD, 64 percent ± 0.10 SD, and 0.84 percent ± 0.16 SD.
There is no evidence of source-sink dynamics in this species. Because there has been only one long- term study of a marked population (Redmond and Jenni 1986), and few recoveries of banded individuals, there is no information on the possible linkage of populations or metapopulation dynamics.
Curlew deaths have been reported from predators, disease, and contaminants. There are no mortalities reported due to exposure, but the impact of climate on prey abundance and availability may influence population growth. Decreased food availability—either on breeding areas or along migration routes to the south—is presumed to be responsible for interannual variation in clutch size, and may limit population growth (Redmond and Jenni 1986). With the exception of habitat loss, predation on eggs and chicks is probably the single greatest factor limiting population growth. Gopher snakes (Pituophis spp.) and a variety of mammalian and avian predators are known to depredate nests. In Idaho, 28.6 percent of the nests were destroyed by canids (coyote [Canis latrans], red fox [Vulpes vulpes], feral dog [C. familiaris]), badgers (Taxidea taxis), or other undetermined mammals; 6.7 percent were destroyed by birds (primarily black-billed magpie [Pica hudsonia]); 4.2 percent of nests were abandoned due to disturbance by livestock (n = 119; Redmond and Jenni 1986). In Oregon, nest predators (including badger, coyote, and various corvids) destroyed 10 to 16 percent of 101 nests (Pampush and Anthony 1993). Other potential nest predators include feral cat (Felis catus), striped skunk (Mephitis mephitis), raccoon (Procyon lotor), and long-tailed weasel (Mustela frenata). Livestock destroy curlew nests by trampling (Dugger and Dugger 2002). Predation of adult long-billed curlews has not been confirmed, but prairie falcons (Falco mexicanus) have been observed making unsuccessful attempts on curlews, and raptors took two radio-marked chicks in Idaho one week after they fledged (Redmond and Jenni 1986). During one year, almost all chick deaths were attributed to raptors (a long-tailed weasel ate one chick) (Jenni et al. 1981).
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The studies of Dugger and Dugger (2002) provided the basis for formulating a life cycle graph for long-billed curlew that comprised two stages (censused at the fledgling stage and “adults”). The scanty data on survival suggested highest survival of yearlings (20 of 30 males returning) and lower survival of older birds (5 of 12 returning). We further assumed considerably lower survival in the first year, a value for which we solved by assuming λ (population growth rate) was 1.003. This “missing element” method (McDonald and Caswell 1993) is justified by the fact that, over the long term, λ must be near 1 or the species will go extinct or grow unreasonably large. In addition, we assumed that first-year reproduction was lower than that of “adult” birds (Table 1). From the resulting life cycle graphs (Figure 8), we produced a matrix population analysis with a post-breeding census for a birth-pulse population with a one-year census interval (McDonald and Caswell 1993, Caswell 2001). The models had two kinds of input terms: P i describing survival rates and m i describing number of female fledglings per female (Table 1).
for the matrix corresponding to the life cycle graph of Figure 8. The model assumes female demographic dominance so that, for example, fertilities are given as female offspring per female; thus, the fledgling number used was half the total annual production of fledglings, assuming a 1:1 sex ratio. Note also that the fertility terms (F ij ) in the top row of the matrix include both a term for fledgling production (m i ) and a term for the survival of the mother (P i ) from the census (just after the breeding season) to the next birth pulse almost a year later. The population growth rate, λ, was 1.003, based on the estimated vital rates used for the matrix. Although this suggests a stationary population, the value was used as an assumption for deriving a vital rate, and it should not be interpreted as an indication of the general well-being of the population. Other parts of the analysis provide a better guide for assessment. Sensitivity analysis A useful indication of the state of the population comes from the sensitivity and elasticity analyses.
i and m i ) that make up the vital rates in the projection matrix for long-billed curlew.
1 1.4 Number of female fledglings produced by a first-year female m a 1.9 Number of female fledglings produced by an “adult” female P 21 0.28 First-year survival rate P 32 0.67 Second-year survival rate P a 0.42 Survival rate of “older adults” P 21 m 1 1 2 3 P 21 P 32 P a P a m a P 32ma Figure 8. Life cycle graph for long-billed curlew. The numbered circles (“nodes”) represent the three stages (first- year birds, second-year birds and “older adults”). The arrows (“arcs”) connecting the nodes represent the vital rates – transitions between age-classes such as survival (P ji ) or fertility (F ij ) (the arcs pointing back toward the first node). Yüklə 1,74 Mb. Dostları ilə paylaş:
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