128 research paper Wader Study 122(1): 128–134. doi: 10.18194/ws.00015
Morphological variation in spring migrant and wintering American GoldenPlovers Pluvialis dominica Joop Jukema1, John B. Dunning, Jr.2*, Piet Vlas3, Lauren Brierley4 & Paul Brooks5 1
Haerdawei 62, 8854 AC Oosterbierum, e Netherlands 2Department of Forestry and Natural Resources, Purdue University, 195 Marsteler Street, West Lafayette, Indiana 47907, USA 3 3 De Skatting 24, 8621 BW Heeg, e Netherlands 4 Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907, USA (Current address: College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27607, USA) 5 Natural Resources and Environmental Sciences Program, Purdue University, West Lafayette, Indiana 47907, USA (Current address: 7088 W. Oakmeadow Drive, La Porte IN, USA 46350) *Corresponding author:
[email protected] Jukema, J., J.B. Dunning, Jr., P. Vlas, L. Brierley & P. Brooks. 2015. Morphological variation in spring migrant and wintering American Golden-Plovers Pluvialis dominica. Wader Study 122(2): 128–134.
Seasonal variation in morphology may reflect the selective pressures experienced by migrating birds, especially for long-distance migrants such as shorebirds. We quantified variation in linear measurements, plumage scores, and body mass among sex and age classes in the American GoldenPlover Pluvialis dominica at different points in the nonbreeding period, from November to May. Most previously reported samples of morphological variation in this species were taken on the breeding grounds. Using birds caught in Uruguay and Indiana (USA), we show that there is little sexual dimorphism in size in samples from the wintering and migration seasons. First-year birds had shorter wings than adults on the wintering grounds, as previously found in the Pacific Golden-Plover P. fulva and Eurasian Golden-Plover P. apricaria. Molting American Golden-Plovers at a spring stopover site were not heavier than non-molting birds. We compared patterns of seasonal morphological variation in the American GoldenPlover to that of the Pacific and Eurasian Golden-Plovers and relate the differences and similarities of this variation to the migration strategies of these closely related species.
INTRODUCTION Populations of migratory shorebirds can show variation in their physical features throughout the year, including variation within different age-classes and sexes (Piersma & Jukema 1993, Scherer et al. 2014). This variation in size and plumage can affect pairing and mate choice (Edwards 1982, Höglund et al. 1990) or lead to differential success in migration and survival (Morrison et al. 2007). American Golden-Plovers Pluvialis dominica have one of the longest migrations of any bird in the Western Hemisphere, breeding in the Arctic coastal regions of Canada and Alaska, USA, and wintering in grasslands of the Southern Cone of South America. Quantification of seasonal morphological variation might be important in understanding the strategies of migrating individuals.
Keywords American Golden-Plover Pluvialis dominica migration molt morphology winter
However, the largest samples of morphological measurements for this species have been taken from breeding adults in the Arctic and subarctic (Irving 1960, Johnson & Connors 2010) with relatively few data from either wintering or migrating birds. The American GoldenPlover is closely related to two other species, the Eurasian Golden-Plover P. apricaria and the Pacific Golden-Plover P. fulva (Johnson & Connors 2010, Withrow & Winker 2014). It is thought that these three species arose from a single ancestor (Byrkjedal & Thompson 1998) but the three golden-plovers have different breeding and wintering areas, migration routes, migration distances, and phenology. All three golden-plovers therefore differ in the natural selection they have experienced and we suggest that the three species should have different levels of variation in
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their morphology that reflect these differences. Some patterns of morphological variation could be tested with data from nonbreeding birds. Johnson & Johnson (2004) found little variation in size between age classes of American and Pacific Golden-Plovers in fall, but showed in the Pacific Golden-Plover that by late winter or spring, first-year, juvenal birds had shorter wings than adults. They attributed this difference to wear of retained juvenal flight feathers on first-year birds during the winter. Data from the breeding grounds summarized by Johnson & Connors (2010) suggest that the two North American golden-plover species are not sexually dimorphic in size, similar to the Eurasian Golden-Plover which is sexually monomorphic (Jukema & Piersma 1992). The summary by Johnson & Connors (2010) showed no difference in linear measurements between the sexes in either species (American or Pacific) or in mass in Pacific Golden-Plover. A small sample of spring migrant American GoldenPlovers from Minnesota suggested that females might be heavier than males (n = 5 for each sex, Johnson 1973). A somewhat larger sample (n = 24–36) from breeding birds showed no differences (Irving 1960). In addition, some European shorebirds (Bar-tailed Godwit, Limosa lapponica, Piersma & Jukema 1993; Ruff, Philomachus pugnax, Jukema et al. 2001b) have higher mean body masses while molting at stopover sites during spring migration. This pattern has not been examined in Pluvialis plovers. As part of a project on migration and flight-feather molt in this species (Jukema et al. 2013), we took standard linear measurements, recorded body mass, and noted plumage condition of wintering and migrating individuals. We captured golden-plovers during spring migration in west-central Indiana, USA, to determine whether the birds varied by sex in terms of linear measurements or mass. The birds we captured were in the process of molting to alternate plumage and we assessed whether stage of molt was related to body mass. We also present morphological data for samples of American GoldenPlovers captured in Uruguay, early in the wintering period and at the start of their northward migration. Together, these samples present an opportunity to better understand morphological variation throughout the annual cycle for a species notable for its long-distance migration.
corn Zea mays stubble from the previous growing season (Braile 1999, Stodala et al. 2014). Therefore we deployed the wilsternet in untilled soybean fields. Once deployed, we placed plover decoys in front of the net and lured passing plover flocks in by imitating their calls (Jukema et al. 2001a). Captured birds were measured as described below, banded, and released. We also captured plovers on the wintering grounds in the Laguna de Rocha region of SE Uruguay. We netted birds using the wilsternet early in the austral summer (18–28 November 2010) and later (8–16 February 2011) when the birds were beginning northward migration. We placed the wilsternet in grazed natural grasslands. In the early capture period, first-year birds could be distinguished as they were still in distinctive juvenal plumage (Johnson & Connors 2010). By February, plovers had finished prebasic body molt and distinguishing second-year from older (after-hatching-year) birds was more difficult. We classified February-caught individuals as either presumptive second-year individuals (those hatched the previous summer) or presumptive adults based on wing and tail molt conditions (Jukema et al. 2011, Jukema et al. 2013). We measured a series of morphological characters on all birds caught in Indiana and Uruguay. Some measurements were inadvertently skipped on some birds so that sample sizes varied to a small degree among measurement categories. Metrics included body mass, wing length (measured as maximum flattened and straightened wing, Pyle 2008), bill length (tip of bill to feathering on head), length of head and bill combined, and tarsus as described in Prater et al. (1977).
METHODS
On the birds caught in Indiana during spring, we quantified the degree to which individual plovers had attained alternate plumage by measuring the extent of breeding plumage with a 7-point scale on the basis of coloration of head, breast and belly feathers: 1 = basic plumage, 2 = trace of alternate plumage, 3 = ¼ alternate plumage, 4 = ½ alternate plumage, 5 = ¾ alternate plumage, 6 = trace of basic plumage, and 7 = full alternate plumage (Piersma & Jukema 1993). Females were scored separately from males, based on sexual plumage differences in alternate plumage (e.g. “females [in Alternate Plumage] less colorful, with varying numbers of whitish feathers on breast and face producing mottled appearance,” Johnson & Connor 2010).
We captured American Golden-Plovers in two study regions, one in spring stopover habitat and the other on the wintering grounds. In April–May 2009 we caught plovers in large agricultural fields located in Benton County, Indiana, using a ‘wilsternet’, a traditional Dutch clap-net which lays flat on the ground and catches birds by being raised into position by attendants pulling on cables (Jukema et al. 2001a). In previous studies of the species’ spring stopover period in Indiana, plovers were strongly associated with untilled fields that had been planted with soybean Glycine max the previous year (Erickson 1992, Braile 1999, Stodala et al. 2014). The plovers are less likely to be found in tilled fields or in untilled fields with
We also scored individual birds on the molt phase of new growing breast feathers (Piersma & Jukema 1993: Fig. 1). We used a 6-point score for molt phase (0 = no molt, 1 = pin feathers present, 2 = feathers ¼ grown, 3 = feathers ½ grown, 4 = feathers ¾ grown, and 5 = new feathers; Piersma & Jukema 1993). We assessed molting intensity as the density of molting feathers on the back, breast and belly, using a 4-point scale (0 = no molt, 1 = light molt, 2 = medium molt, and 3 = heavy molt). All birds were scored by JJ and PV who have used these scales in other studies (Piersma & Jukema 1993). Spring migrants in Indiana could be aged by molting characteristics of the secondaries (Jukema et al. 2013) and could be sexed by
Mass (g)
130 Wader Study 122(1) 2015
Low
High
Molt intensity
Low
High
Molt phase
Fig. 1. Mean masses of spring migrant American Golden-Plovers captured in Benton County, Indiana; comparing birds with low values (scores = 1–2) and high values (= 3) of molt intensity index and low values (=1–2) and high values (= 3–5) of molt phase index. Indices are defined in Methods.
breeding plumage (Johnson & Connors 2010). We compared linear measurements and body masses among categories using Student’s t-tests as the linear metrics and mass variables were normally distributed. The categorical variables such as the plumage scores were assessed using non-parametric chi-square tests. All tests were calculated in Statistica (StatSoft Inc., Tulsa OK 2014). Results are presented as mean ±SD, range, N.
RESULTS Indiana We captured 97 plovers, including 47 males and 50 females. Flocks were present in west-central Indiana from 4 April to 9 May 2009 (JBD, K. Gretencord, pers. obs.). We caught our first plover (a female by plumage) on 20 April. Most of the plovers in our sample were caught on 26, 27, and 28 April when 24, 26, and 41 birds were netted, respectively. On each of these three days, most of the birds were caught simultaneously (e.g. from the same flock) and in each of these flocks the sex ratio of the captured birds was close to 1:1 (26 April – 12F:12M; 27 April – 11F:15M; 28 April – 22F:19M). When plovers were first seen in Indiana during the spring, most birds in the first flocks appeared to be in basic plumage or to have just started pre-alternate molt (JBD, JJ, pers. obs.). For instance, when evaluated using the scale defined above for extent of breeding plumage, a flock of 225 birds seen by JJ on 24 April 2009 consisted of
about 10% birds with a score of 0–1, 70% with a score of 2–3, and only 20% with a score of 4–7. This set of scores, obtained by visually inspecting the flock, was in agreement with the plovers we captured on 26–28 April, most of which were halfway through their pre-alternate molt (mean plumage extent score 3.9 ±1.8, 0–7, 97). By the time the last flocks left in mid-May, virtually all birds appeared to be progressing towards alternate plumage. Most birds that we captured were judged to be in medium molt, defined as a molt phase score between 2 and 3 (mean molt score = 2.2 ±0.82, 1–5, 95; mean molt intensity = 2.2 ±0.70, 1–3, 96). Thirty-one birds were scored as being in heavy molt (molt intensity = 3). On the scale measuring extent of breeding plumage, most birds were scored as having obtained at least ½ alternate plumage (mean extent score = 3.9 ±1.8, 1–7, 96). Seven females were scored as being completely in basic plumage, while eight males and one female were scored as being in full alternate plumage. The males and females sampled in Indiana did not differ in any morphological measurement (Table 1). Males and females differed in extent of breeding plumage (Table 1, χ² = 27.16, d.f. = 7, p = 0.0003) but not in molt phase or intensity scores. The body mass of birds with high scores for molt intensity and molt phase did not differ from the mass of birds with low values for either metric. Birds with low levels of molt intensity (scores 1–2) averaged 148 g (±12.4, 126–175, 63) while birds with high levels (scores 3) also averaged 148 g (±13.3, 127–180, 31; Fig. 1). Birds with low levels of
Jukema et al. l Morphological variation in spring migrant and wintering American Golden-Plovers-- 131
molt phase (= 1–2) weighed 147 g (±12.1, 127–180, 67) while those with high molt scores (= 3–5) weighed 149 g (±14.0, 126–172, 28; Fig. 1). Uruguay We captured and measured 219 plovers in the November and February periods combined. First-year and adult birds captured in November did not differ in bill or body mass measurements (Table 2). First-year plovers had shorter wings (182 vs. 187 mm, t = 7.62, p
