See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/230691225 Differential migration of shorebirds in the East Asian-Australasian Flyway Article in The Emu: official organ of the Australasian Ornithologists' Union · January 2007 DOI: 10.1071/MU06006 CITATIONS READS 20 86 1 author: Silke Nebel The University of Western Ontario 38 PUBLICATIONS 659 CITATIONS SEE PROFILE All content following this page was uploaded by Silke Nebel on 14 March 2014. The user has requested enhancement of the downloaded file. All in-text references underlined in blue are linked to publications on ResearchGate, letting you access and read them immediately. CSIRO PUBLISHING Emu, 2007, 107, 14–18 www.publish.csiro.au/journals/emu Differential migration of shorebirds in the East Asian–Australasian Flyway Silke Nebel School of Biological, Earth & Environmental Sciences, University of New South Wales, Sydney, NSW 2052, Australia. Email: silke.nebel@unsw.edu.au Abstract. Differential migration involving intraspecific segregation of ages or sexes on the non-breeding grounds is common among migratory birds. Most of the existing data have, however, been collected in Europe and the Americas and very little is known about such migration patterns in the East Asian–Australasian Flyway, which links eastern Siberia with Australia and New Zealand. Spatial segregation of males and females during migration and at non-breeding grounds has clear implications for conservation management, as the loss of habitat predominantly used by members of one sex will disproportionately reduce effective population size. Here, I review the published data on differential migration in shorebirds in the East Asian–Australasian Flyway and discuss these data in the context of alternative hypotheses for differential migration and their relevance to existing conservation programs. Introduction Many migratory birds travel large distances between breeding and non-breeding grounds during their annual migrations. Intriguingly, choice of non-breeding site often varies with age or sex within species. Individuals of a certain age or sex may consistently migrate further from the breeding grounds than others, resulting in sex- or age-specific segregation on the non-breeding grounds. This so-called differential migration is common among migratory birds (see review in Cristol et al. 1999). Spatial segregation of males and females has clear implications for conservation management. Loss of habitat predominantly used by members of one sex will disproportionately reduce effective population size, which is the average number of individuals contributing genes to succeeding generations (Hartl 1988). It can be calculated using the following population genetic model: Ne = 4Nm × Nf / (Nm + Nf). If, for example, the population consists of 5000 males (Nm) and 5000 females (Nf), the loss of 2000 females will reduce the effective population size (Ne) from 10 000 to 7500, even though the total number of individuals in the population is 8000. In order to direct conservation efforts, we need to identify sites of conservation concern that are preferentially used by members of one sex, as the loss of such sites would contribute heavily to population declines. Although differential migration is common among migratory birds (Cristol et al. 1999), most of the data available to date have been assembled in the American and African–European flyways. Little is known about the occurrence of differential migration in the East Asian–Australasian Flyway, which links the breeding grounds in eastern Siberia with the non-breeding grounds in Australia and New Zealand. Here, I review the published information on differential migration in the East Asian–Australasian Flyway, using shorebirds as the focal group. This Flyway is used by more than five million migratory shorebirds annually (Shorebird Working Group of Wetlands International – Asia Pacific 2001); population estimates for the © Royal Australasian Ornithologists Union 2007 28 migratory shorebird species using the East Asian–Australian Flyway are provided in Table 1 (which see for scientific names of species mentioned in text). The main focus of this review is the distribution of sexes and age-classes of shorebirds on their non-breeding grounds in Australia. Differential migration of males and females To determine the extent of differential migration of male and female shorebirds in the East Asian–Australian Flyway, I conducted a literature search in the ISI Web of Science (1900–2006, http://scientific.thomson.com/products/wos/, accessed 5 January 2007; Thomson Corporation, Philadelphia, PA) using ‘sexratio’ and the genus or species name of the Australasian shorebird species (Actitis, Arenaria interpres, Calidris, Charadrius, Gallinago, Heteroscelus, Limicola, Limnodromus, Limosa, Numenius, Pluvialis, Tringa, Xenus) as search terms. I also searched the online issues of Emu (Volumes 75–106, 1975–2006) for papers containing ‘sex-ratio’. I also looked through all volumes of the Stilt (Volumes 1–49, 1981–2006), the journal of the Australasian Wader Study Group (Birds Australia, Melbourne, Victoria), for sex-ratio data. The selected papers were then reviewed for relevant non-breeding sex-ratio data. Sex-ratio data from the non-breeding season in the East Asian–Australian Flyway are available for four species of shorebirds (Curlew Sandpiper, Eastern Curlew, Bar-tailed Godwit and Sanderling), and these are summarised in Table 2. Most data are for regions within Australia, although data are also available from Thailand and India for the Curlew Sandpiper. Non-breeding season is defined as November–February. Male Curlew Sandpipers migrated further south than females (Barter 1986, 1987), whereas female Eastern Curlews and Bar-tailed Godwits migrated further south than males (Barter 1989, 1990). Note, however, that sex ratio data for Eastern Curlews are available from a single site only and need to be treated with caution. In Sanderlings, the proportion of females in north-west and south10.1071/MU06006 0158-4197/07/010014 Differential migration of shorebirds Emu eastern Australia was lower than that of males, but it did not differ between the two sites, and among-year variation was very high (Gosbell and Minton 2001). The data available do therefore not support a sex-bias in distribution in Sanderlings. Differential migration of different age-classes To determine the distribution of age-classes among migratory shorebirds in the East Asian–Australian Flyway, the methodology for determining differences between sexes (as described above) was used but with ‘age-ratio’ instead of ‘sex-ratio’ as the search term. I also searched Arctic Birds (http://www.arcticbirds.ru/) and International Wader Studies (International Wader Study Group, Thetford, UK) for relevant age-ratio data. Information on the distribution of age-classes (either ‘adult’ or ‘1st-year birds’) exists for eight species of shorebirds, but for only two regions within Australia (the north-west and south-east), as shown in Table 3. Comparisons of age-ratios in north-western and southeastern Australia were made within years only, because the proportion of 1st-year birds in a given year depends on breeding success in that year, which can, however, vary greatly between years (Underhill et al. 1989; Minton et al. 2005a). Age-ratio data collected during a single year therefore need to be interpreted with caution. In Red Knots and Sanderlings, data from several years consistently showed that 1st-year birds migrated further south than adults (Minton et al. 2001, 2002, 2003, 2004, 2005b). In Greenshanks, data collected in a single year did not show an Table 1. Estimates of populations (December 2005) of migratory shorebirds using the East Asian–Australasian Flyway (Bamford et al. 2006) Common name Specific name Estimate population (birds) Asian Dowitcher Bar-tailed Godwit Black-tailed Godwit Broad-billed Sandpiper Common Greenshank Common Sandpiper Curlew Sandpiper Double-banded Plover Eastern Curlew Great Knot Greater Sand Plover Grey Plover Grey-tailed Tattler Latham’s Snipe Lesser Sand Plover Little Curlew Marsh Sandpiper Oriental Plover Oriental Pratincole Pacific Golden Plover Red Knot Red-necked Stint Ruddy Turnstone Sanderling Sharp-tailed Sandpiper Terek Sandpiper Whimbrel Wood Sandpiper Limnodromus semipalmatus 23 000 Limosa lapponica 325 000 Limosa limosa 160 000 Limicola falcinellus 25 000 Tringa nebularia 55 000 Actitis hypoleucos 30 000 Calidris ferruginea 180 000 Charadrius bicinctus 50 000 Numenius madagascariensis 38 000 Calidris tenuirostris 380 000 Charadrius leschenaultii 100 000 Pluvialis squatarola 125 000 Heteroscelus brevipes 40 000 Gallinago hardwickii 36 000 Charadrius mongolus 130 000 Numenius minutus 180 000 Tringa stagnatilis 100 000–1 000 000 Charadrius veredus 70 000 Glareola maldivarum 2 880 000 Pluvialis fulva 100 000 Calidris canutus 220 000 Calidris ruficollis 315 000 Arenaria interpres 31 000 Calidris alba 22 000 Calidris acuminata 160 000 Xenus cinereus 50 000 Numenius phaeopus 55 000 Tringa glareola 100 000 15 age-bias in distribution (Minton et al. 2003). In Ruddy Turnstones, Curlew Sandpipers, Bar-tailed Godwits, Red-necked Stints and Great Knots, age-ratio data from several years were inconsistent and did not reveal trends (Gosbell and Minton 2001; Minton et al. 2001, 2002, 2003, 2004, 2005b). Differential migration hypotheses The few data currently available suggest that in shorebirds using the East Asian–Australasian Flyway, differential migration according to sex and age occurs. Five hypotheses have been proposed to explain differential migration of males and females. The ‘dominance hypothesis’ (Gauthreaux 1978) predicts that dominant individuals monopolise areas closer to the breeding grounds to lower migration costs. According to the ‘body size hypothesis’ (Ketterson and Nolan 1976), larger individuals are better suited to survive the colder and less predictable climates at higher latitudes, as they can endure longer periods of fasting. The ‘arrival time hypothesis’ (Ketterson and Nolan 1976; Myers 1981) states that individuals that benefit most from early arrival should spend the non-breeding season closest to the breeding grounds. However, recent work has shown that longer migration distance can be offset by commencing northward migration earlier (Nebel et al. 2002). According to the ‘predation risk hypothesis’, latitudinal distribution of males and females varies with weight-dependent vulnerability to predation (Nebel and Ydenberg 2005). Escape performance in birds is generally reduced by extra body mass, as it leads to a decrease in take-off speed and manoeuvrability (e.g. Kullberg et al. 2000). The heavier sex is predicted to seek safety by migrating to areas closer to the Equator, where generally less fat is carried (e.g. Davidson 1984). Finally, the ‘resource partitioning hypothesis’ predicts that sexual and latitudinal differences in foraging niche lead to spatial segregation of males and females in species with sexual dimorphism in bill size (Nebel 2005). The relative availability of deeply buried prey increases with proximity to the Equator (Mathot 2005), possibly in response to increasing ambient temperatures (Elner and Seaman 2003; Nebel and Thompson 2005). The advantage derived from having a long bill is thought to result in distributional clines according to length of bill such that the sex with longer bills spends the non-breeding season closer to the Equator (Nebel 2005, 2006). In Curlew Sandpipers, males migrate further south than females (see above, and Table 4) and arrive on the breeding grounds earlier (Higgins and Davis 1996). Further, females are larger and heavier than males, and with longer bills (Higgins and Davis 1996), and are thus assumed to be dominant (Fretwell 1969; Gauthreaux 1978). Thus the dominance, predation risk and resource partitioning hypotheses are all consistent with their observed pattern of migration (Table 4), whereas the arrival time and body size hypotheses are not. In Eastern Curlews and Bar-tailed Godwits, females are again larger and heavier, and with longer bills, than males, but there are no data on arrival time available (Higgins and Davis 1996; McCaffery and Gill 2001). Females of both species migrate further south than males, which is consistent with the body size hypothesis. The dominance, predation risk and resource partitioning hypotheses are not supported. Note that in Bar-tailed Godwits, two separate populations occur in Australia: Limosa lapponica menzbieri in 16 S. Nebel Emu Table 2. Non-breeding distribution according to sex in migratory shorebirds within the East–Asian Australasian Flyway Catches were made with cannon- or mist-nests. NW Australia refers to the region around Broome in northern Western Australia; SE Australia refers to Victoria and New South Wales, south-eastern Australia Species Curlew Sandpiper Eastern Curlew Bar-tailed Godwit Sanderling Sex migrating further south Study area (latitude) Proportion of females (sample size (birds)) Age-classes involved Study period Source Males Thailand (13°32′N) Tamil Nadu, India (11°00′N) NW Australia (18°00′S) Victoria, SE Australia (37°50′S) Tasmania, Australia (42°50′S) SE Australia NW Australia SE Australia NW Australia SE Australia 44% (93) 46% (305) Adults 1978–85 Barter (1986); Barter (1987) Barter (1986); Barter (1987) Females Females No difference 40% (460) Barter (1986); Barter (1987) 34% (1520) Barter (1986); Barter (1987) 23% (331) Barter (1986); Barter (1987) 64% (142) 32% (450) 49% (667) 33% (371) 36% (1464), range 25–59% Adults Adults 1979–89 1979–88 Adults/1styear birds 1991–2000 Barter (1990)A Barter (1989) Barter (1989) Gosbell and Minton (2001)B Gosbell and Minton (2001)B ASample BData period January–December but most data collected October–November. collected October–early March. the north-west and L. l. baueri in the south-east (Higgins and Davis 1996) and the sex-ratio between the two populations differs (see Table 2). The difference in sex-ratio between the two regions may be a result of inter-populational rather than intraspecific variation in migration strategy, but the data available do not allow us to distinguish between these alternatives. In Sanderlings, females are larger than males (Table 4) but no difference in timing of arrival on the breeding grounds or in migration distance has been reported (Myers 1981). Interestingly, the difference in length of bill between male and female Sanderlings is very small (Table 4), and the resource-partition- ing hypothesis predicts a latitudinal sex-ratio bias only in species with pronounced sexual dimorphism in bill-length. The distribution of Sanderlings is therefore consistent with the resource-partitioning hypothesis. Implications for conservation More sex-ratio data from throughout the non-breeding range are needed to establish how common differential migration is in the East Asian–Australasian Flyway and to evaluate the validity of the competing hypotheses. Detection of geographical biases in sex-ratio also plays an important part in successful wildlife con- Table 3. Non-breeding distribution according to age in migratory shorebirds in Australia Age-classes are either adults or 1st-year birds. A given year refers to the beginning of the non-breeding season, e.g. 2002 stands for the non-breeding season of 2002–03. All catches were made by cannon-net. Annual totals of <10 birds caught are not included in the table. NW Australia refers to the region around Broome in northern Western Australia; SE Australia refers to Victoria and New South Wales, south-eastern Australia Species Age-class migrating further south Annual mean proportion of 1st-year birds in sample (range; sample size (number of birds)) NW Australia SE Australia Study period Source Minton et al. (2001), (2002), (2003), (2004), (2005b) Minton et al. (2002), (2003); Gosbell and Minton (2001) Minton et al. (2003) Minton et al. (2002), (2003), 2004), (2005b) Minton et al. (2001), (2002), (2003), (2004), (2005b) Minton et al. (2001), (2002), (2003), (2004) Minton et al. (2001), (2002), (2003), (2004), (2005b) Minton et al. (2001), (2002), (2003) Red Knot 1st-year birds 15% (2–18%; 653) 61% (3–92%; 958) 1999–2004 Sanderling 1st-year birds 6% (0–16%; 608) 21% (3–43%; 1870) Greenshank Ruddy Turnstone No difference No consistent trend 4% (–; 23) 13% (0–20%; 129) 5% (–; 41) 13% (7–21%; 687) 1996, 1998, 2001, 2002 2002 1999, 2001–04 Curlew Sandpiper No consistent trend 16% (9–24%; 907) 18% (7–27%; 2929) 1999–2004 Bar-tailed Godwit No consistent trend 10% (5–15%; 1850) 13% (1–38%; 693) 1999–2004 Red-necked Stint No consistent trend 23% (10–44%; 2318) 20% (10–34%; 31945) 1999–2004 Great Knot No consistent trend 9% (4–18%; 2049) (4–8%; 128) 1999–2001 Differential migration of shorebirds Emu 17 Table 4. Hypotheses on differential migration of males and females (B = body size hypothesis; D = dominance hypothesis; A = arrival time hypothesis; P = predation risk hypothesis; R = resource partitioning hypothesis) Species Curlew Sandpiper Eastern Curlew Bar-tailed Godwit Sanderling AOn Sex migrating further south Larger (= dominant) sexA Sex that arrives first Male Female Female Female Female Female MaleB ?B ?B,C No difference Female No differenceD Mean length of bill (mm)B,E Male Female 37.0 158.0 L. l. menzbieri 86.1 L. l. baueri 80.4 24.1 41.0 179.5 108.2 105.5 25.6 Consistent hypotheses D, P, R B B R the basis of bill, wing and body-mass measurements. BFrom Higgins and Davis (1996); CMcCaffery and Gill (2001); DMyers (1981). are of adults except for Sanderling, which are of ages combined. EMeasurements servation management, as the loss of habitat predominantly used by members of one sex would lead to a disproportionate reduction in the effective population size (Hartl 1988). Differential migration can therefore have significant consequences for the long-term survival of a species. This is particularly important for species with declining populations. In Australia, all migratory species and their habitats are considered to be of conservation concern of national importance under the Environment Protection and Biodiversity Conservation Act 1999 (http://www.deh.gov.au/epbc/, accessed 5 January 2007). Identification of habitat used predominantly by one sex is therefore important to meet Australia’s national and international obligations. Conservation of shorebird habitat is particularly urgent in the East Asian–Australasian Flyway. More than half of the human population lives in the Asia–Pacific region, and by the late 1980s, over 85% of the important wetlands of the region were under threat (Scott 1989; Asia-Pacific Migratory Fig. 1. The 45 sites of international importance for migratory shorebirds in the East Asian–Australasian Flyway, October 2006. (Reproduced with permission © Wetlands International – Oceania, Canberra.) Waterbird Conservation Committee 2001). In 1996, the East Asian–Australasian Shorebird Site Network was established, which is a key component of the Action Plan for the Conservation of Migratory Shorebirds (Shorebird Working Group of Wetlands International – Asia Pacific 2001). About 400 shorebird sites of international importance have been identified within this Flyway. The Action Plan for the Conservation of Migratory Shorebirds facilitates conservation action across this ecological network as it helps to provide training to site managers, improve information exchange between managers and conservationists, and collect biological data to facilitate conservation work across the Flyway (http://www.deh.gov.au/biodiversity/migratory/waterbirds/ infosrn1.html, accessed 5 January 2007). By October 2006, there were 45 sites participating in the Shorebird Site Network (Fig. 1). Evaluating data on sex-ratios will help in designating the most important sites for the Network. Shorebirds can help us to detect global ecosystem changes. As long-distance migrants, they use arctic, temperate and tropical areas during their annual cycle, and can therefore act as indicators of environmental changes occurring throughout their flyway. Permanent monitoring programs at spring and autumn stop-over sites allow the collection of information indicative of such environmental changes (Piersma and Lindström 2004). For example, philopatry to stop-over sites, body condition and moult stages of migratory birds during southward migration reflect the quality of the staging area in terms of food availability and predation risk. Body condition during northward migration, weather conditions on the breeding grounds, and predator densities all affect breeding success (Minton et al. 2005a). Detecting small changes in any of these key variables allows the design of research projects that best identify the underlying causes (Piersma and Lindström 2004). Better knowledge of the sites used by migratory shorebirds is certainly crucial for directing conservation effort, particularly if males and females are spatially segregated. However, such information will only be of use if society recognises the importance of protecting the vulnerable systems of migrating birds and their habitats. Directing public attention towards the ‘fragile phenomenon of shorebird migration’ (Piersma and Lindström 2004) and the factors that threaten them with extinction are important steps towards obtaining long-term solutions at regional, national and global levels to ensure shorebird survival into the next century (Piersma and Lindström 2004). 18 S. Nebel Emu Acknowledgements D. Rogers is thanked for an insightful discussion. D. Rogers, G. Thompson, D. 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