Adrian H. Farmer

Effects of the landscape on shorebird movements at spring migration stopovers

We monitored the inter-wetland movements of 115 radio-tagged Pectoral Sand-pipers (Calidris melanotos) at three migration stopovers in the Great Plains of North America during April and May from 1992 to 1995. While resident at a stopover, individuals were very localized in their movements. Over 40% of the birds made no inter-wetland movements, and over 90% of individuals moved less than 10 km from their original release site. Characteristics of wetlands where birds were released did not affect bird movements. However, the structure of the surrounding landscape explained up to 46% of variation in individual bird movements. As the distance between wetlands decreased, and the proportion of the landscape composed of wetlands increased, individual birds moved between wetlands more frequently and moved longer distances from their release site. These movement patterns indicate that a more connected landscape allows shorebirds to exploit more feeding sites with reduced searching costs; a result consistent with foraging theory. We estimate a degree of landscape connectivity at which a wetland complex functions as a single large wetland as measured by sandpiper feeding patterns. Our data provide support for the idea that complexes of small, closely spaced wetlands can be important migration stopovers and may have significant conservation value.

MODELS AND REALITY: TIME–ENERGY TRADE‐OFFS IN PECTORAL SANDPIPER (CALIDRIS MELANOTOS) MIGRATION

We used a combination of modeling and field studies to determine the spring migration strategy of Pectoral Sandpipers (Calidris melanotos). We developed a dynamic programming model to predict patterns that should be detected along the migratory route if Pectoral Sandpipers use a strategy of early arrival at the breeding grounds (time minimization) or arrival at the breeding grounds with excess energy reserves (energy maximization). The predictions were then compared to data collected at stopover sites in the mid-continent of North America and at the breeding grounds in Alaska over a 5-yr period (1992–1996). During spring migration to their Arctic breeding grounds, Pectoral Sandpipers stop periodically to feed. The length-of-stay at such stopovers, for both time minimizers and energy maximizers, was predicted to vary inversely with date and body fat, and to vary directly with invertebrate abundance. We observed that: (1) length-of-stay was negatively correlated with capture date in Missouri and Nebraska, but not in Texas; (2) length-of-stay was not correlated with body fat at any site; and (3) length-of-stay was positively related to invertebrate abundance at the Nebraska and Missouri sites. As the population moves northward in the spring, three regional patterns are diagnostic of migration strategy. Length-of-stay was predicted to be bimodal (energy maximizer) or constant (time minimizer) with respect to latitude, but neither pattern was observed. The migration window, or period of time during which spring migrants occur, was predicted to decrease with increasing latitude for time minimizers, a pattern that was seen for both males and females. Body fat was predicted to increase with latitude for energy maximizers, a pattern that was seen for females but not males. The evidence suggests that males and females differ in their spring migration strategies. Both sexes attempt to arrive in the Arctic as early as possible after ice breakup in the spring. Additionally, females gain significantly higher fat loads than males (up to 60% body fat for females) during migration, and these energy reserves may later enhance female reproductive success. However, females gained large fat loads only during 1993 and 1995, which had above normal spring precipitation along the migration route. We believe that the correlation between female body fat and precipitation reflects an abundance of high-quality stopover habitat during wet springs. This view is supported by model sensitivity analyses showing that the spacing and quality of stopover habitat can strongly influence observed migration patterns. Our results suggest the need to focus additional research on the landscape-level features of the flyway through which shorebirds migrate.

Fundamental limits to the accuracy of deuterium isotopes for identifying the spatial origin of migratory animals

Deuterium isotope analyses have revolutionized the study of migratory connectivity because global gradients of deuterium in precipitation (δDP) are expressed on a continental scale. Several authors have constructed continental scale base maps of δDP to provide a spatial reference for studying the movement patterns of migratory species and, although they are very useful, these maps present a static, 40-year average view of the landscape that ignores much underlying inter-annual variation. To more fully understand the consequences of this underlying variation, we analyzed the GNIP deuterium data, the source for all current δDP maps, to estimate the minimum separation in δDP (and latitude) necessary to conclude with a given level of confidence that distinct δDP values represent different geographic sites. Extending analyses of δDP successfully to deuterium in tissues of living organisms, e.g., feathers in migratory birds (δDF), is dependent on the existence of geographic separation of δDP, where every geographic location has a distribution of values associated with temporal variability in δDP. Analyses were conducted for three distinct geographic regions: North America, eastern North America (east of longitude 100°W), and Argentina. At the 80% confidence level, the minimum separation values were 12, 7, and 14° of latitude (equivalent to 53, 31, and 32‰) for North America, eastern North America, and Argentina, respectively. Hence, in eastern North America, for example, one may not be able to accurately assign individual samples to sites separated by less than about 7° of latitude as the distributions of δDP were not distinct at latitudes <7° apart. Moreover, two samples that differ by less than 31‰ cannot be confidently said to originate from different latitudes. These estimates of minimum separation for δDP do not include other known sources of variation in feather deuterium (δDF) and hence are a first order approximation that may be useful, in the absence of more specific information for the system of interest, for planning and interpreting the results of new stable isotope studies.

Optimal migration schedules depend on the landscape and the physical environment: a dynamic modeling view

We developed a dynamic state variable model of individual migrating shorebirds for use in testing hypotheses about spring migration strategies of the Pectoral Sandpiper Calidris melanotos. We conducted model sensitivity analyses to determine how predicted migration schedules might vary with respect to the landscape and the physical environment. In landscapes with closely spaced, high-quality stopovers, female Pectoral Sandpipers can vary widely in their migration schedules and still arrive on the breeding grounds early enough and with sufficient energy reserves to achieve maximum reproductive success. Such a population might appear quite variable, and show no stopover patterns, even if all individuals were making optimal decisions. Latitudinal gradients in temperature and photoperiod differentially affect a birds energy budget as it moves northward in the spring. Stopovers at more northerly locations are associated with higher metabolic rates, lower food abundance in early spring, and longer days for feeding. The optimal migration schedule in these conditions can be quite different from that in a homogeneous environment, and patterns observed in the field can be misinterpreted if the environmental gradients are not considered. The landscape and the physical environment shape migration schedules and influence ones ability to interpret patterns observed at stopovers. Modeling these factors may lead to new insights about migration adaptations in heterogeneous environments.

ESTIMATING SHOREBIRD NUMBERS AT MIGRATION STOPOVER SITES

Abstract ABSTRACT We describe a method for estimating the totalnumber of shorebirds that use a migration stopoversite during spring and fall migration. We combinedweekly shorebird counts with parameter estimatesfor detection probability, sampled proportion, andlength of stay on the Squaw Creek National WildlifeRefuge. Double sampling was used to determinedetection probability and estimated values variedamong wetland units from a low of 0.07 to a high of0.82. The sampled proportion of most wetland unitswas 100% but was lower in some of the largerunits. Length of stay (measured for Pectoral[Calidrismelanotos] and LeastSandpipers [C.minutilla] combined)averaged 10.0 days in spring and3.7 days in fall. Spring shorebird numberswere approximately five times greater than fallnumbers on the Refuge. Annual shorebird numbersvaried among years from an estimated low in 2003 of15 734 to a high in 2002 of 69 570.Peak daily counts during study years averaged only12% of estimated spring totals and 4%of fall totals. An estimate of shorebird numbersbased on summing weekly counts, not corrected fordetection probability or sampled proportion, wouldhave been only 21% (spring) to31% (fall) of the total number ofbirds. These results reveal that peak counts andnonadjusted counts can significantly underestimatethe number of shorebirds that use migrationstopover sites in the midcontinent of NorthAmerica.

Tracing the pathways of Neotropical migratory shorebirds using stable isotopes: A pilot study

We evaluated the potential use of stable isotopes to establish linkages between the wintering grounds and the breeding grounds of the Pectoral Sandpiper (Calidris melanotos), the White-rumped Sandpiper (Calidris fuscicollis), the Bairds Sandpiper (Calidris bairdii), and other Neotropical migratory shorebird species (e.g., Tringa spp.). These species molt their flight feathers on the wintering grounds and hence their flight feathers carry chemical signatures that are characteristic of their winter habitat. The objective of our pilot study was to assess the feasibility of identifying the winter origin of individual birds by: (1) collecting shorebird flight feathers from several widely separated Argentine sites and analyzing these for a suite of stable isotopes; and (2) analyzing the deuterium and 18O isotope data that were available from precipitation measurement stations in Argentina. Isotopic ratios (δ13C, δ15N and δ34S) of flight feathers were significantly different among three widely separated sites in Argentina during January 2001. In terms of relative importance in separating the sites, δ34S was most important, followed by δ15N, and then δ13C. In the complete discriminant analysis, the classification function correctly predicted group membership in 85% of the cases (jackknifed classification matrix). In a stepwise analysis δ13C was dropped from the solution, and site membership was correctly predicted in 92% of cases (jackknifed matrix). Analysis of precipitation data showed that both δD and δ18O were significantly related to both latitude and longitude on a countrywide scale (p < 0.001). Other variables, month, altitude, explained little additional variation in these isotope ratios. Several issues were identified that will likely constrain the degree of accuracy one can expect in predicting the geographic origin of birds from Argentina. There was unexplained variation in isotope ratios within and among the different wing feathers from individual birds. Such variation may indicate that birds are not faithful to a local site during their winter stay in Argentina. There was significant interannual variation in the δD and δ18O of precipitation. Hence, specific locations may not have a constant signature for some isotopes. Moreover, the fractionation that occurs in wetlands due to evaporation significantly skews local δD and δ18O values, which may undermine the strong large-scale gradients seen in the precipitation data. We are continuing the research with universities in Argentina with a focus on expanding the breadth of feather collection and attempting to resolve the identified issues.

Trace Elements Have Limited Utility for Studying Migratory Connectivity in Shorebirds that Winter in Argentina

Abstract. Trace-element analysis has been suggested as a tool for the study of migratory connectivity because (1) trace-element abundance varies spatially in the environment, (2) trace elements are assimilated into animals tissues through the diet, and (3) current technology permits the analysis of multiple trace elements in a small tissue sample, allowing the simultaneous exploration of several elements. We explored the potential of trace elements (B, Na, Mg, Al, Si, P, S, K, Ca, Ti, Cr, Mn, Ni, Cu, Zn, As, Sr, Cs, Hg, Tl, Pb, Bi, Th, and U) to clarify the migratory connectivity of shorebirds that breed in North America and winter in southern South America. We collected 66 recently replaced secondary feathers from Red Knots (Calidris canutus) at three sites in Patagonia and 76 from White-rumped Sandpipers (C. fuscicollis) at nine sites across Argentina. There were significant differences in trace-element abundance in shorebird feathers grown at different nonbreeding sites, and annual variability within a site was small compared to variability among sites. Across Argentina, there was no large-scale gradient in trace elements. The lack of such a gradient restricts the application of this technique to questions concerning the origin of shorebirds to a small number of discrete sites. Furthermore, our results including three additional species, the Pectoral Sandpiper (C. melanotos), Wilsons Phalarope (Phalaropus tricolor), and Collared Plover (Charadrius collaris), suggest that trace-element profiles change as feathers age. Temporal instability of trace-element values could undermine their application to the study of migratory connectivity in shorebirds.

Population and Community Energetics

Physiologists and ecologists tend to view energetics in different contexts. The physiological perspective, which is the focus of the preceding chapters, emphasizes individual organisms, the mechanisms by which they obtain, process, and allocate energy, and how these mechanisms are constrained by phylogeny, age, sex, behavior, and the environment. The ecological perspective, on the other hand, centers on populations and communities and on the consequences rather than the mechanisms of individual energetics. Beginning from the premise that the amount of energy available to individuals in a given habitat is limited, ecologists have developed theories relating such phenomena as foraging behavior, clutch size, reproductive trade-offs, population dynamics, competition and niche partitioning, community diversity, food-web structure, or the trophic-dynamics of ecosystems to energetics (Lindeman 1942; Lack 1947; Williams 1966; Faaborg 1977; Brown 1981; Yodzis 1984, 1993; Stephens and Krebs 1986; Maurer 1990; Afton et al. 1991; Nilsson 1991; Roff 1992; Tiebout 1993; Wright et al. 1993; Higashi et al. 1993; Chap. 8). Much of this theory has been founded on operational assumptions of optimization or equilibrium. Thus, foraging patterns are assumed to be optimized by a maximum rate of energy gain (e.g., Schoener 1971) or community packing with species is assumed to be limited by resource (i.e., energy) availability and competitively limited niche overlap among species (e.g., MacArthur and Levins 1967). Factors that may constrain systems from attaining the optimal or equilibrial configuration predicted by such theories, such as environmental variation (e.g., May and MacArthur 1972; Wiens 1977a), have also been related to energy availability, at least in a general sense.

Population Variation in Isotopic Composition of Shorebird Feathers: Implications for Determining Molting Grounds

Abstract. Stable isotope analyses have revolutionized the study of migratory connectivity. However, as with all tools, their limitations must be understood in order to derive the maximum benefit of a particular application. The goal of this study was to evaluate the efficacy of stable isotopes of C, N, H, O and S for assigning known-origin feathers to the molting sites of migrant shorebird species wintering and breeding in Argentina. Specific objectives were to: 1) compare the efficacy of the technique for studying shorebird species with different migration patterns, life histories and habitat-use patterns; 2) evaluate the grouping of species with similar migration and habitat use patterns in a single analysis to potentially improve prediction accuracy; and 3) evaluate the potential gains in prediction accuracy that might be achieved from using multiple stable isotopes. The efficacy of stable isotope ratios to determine origin was found to vary with species. While one species (White-rumped Sandpiper, Calidris fuscicollis) had high levels of accuracy assigning samples to known origin (91% of samples correctly assigned), another (Collared Plover, Charadrius collaris) showed low levels of accuracy (52% of samples correctly assigned). Intra-individual variability may account for this difference in efficacy. The prediction model for three species with similar migration and habitat-use patterns performed poorly compared with the model for just one of the species (71% versus 91% of samples correctly assigned). Thus, combining multiple sympatric species may not improve model prediction accuracy. Increasing the number of stable isotopes in the analyses increased the accuracy of assigning shorebirds to their molting origin, but the best combination - involving a subset of all the isotopes analyzed - varied among species.

Marbled Godwit migration characterized with satellite telemetry

ABSTRACT Marbled Godwits (Limosa fedoa) breed in 3 disparate areas: The majority breed in the prairies of midcontinental North America, but there are also 2 small and widely separated tundra-breeding populations, 1 in eastern Canada and 1 on the Alaska Peninsula, USA. The major winter ranges include the Atlantic, Pacific, and Gulf coasts of the USA and Mexico. Bear River Migratory Bird Refuge at Great Salt Lake, Utah, USA, is a major stopover site, hosting large godwit populations in the spring and fall. Although the distributions of Marbled Godwit populations and their habitats across the landscape are generally known, the linkages between them are not. We tracked 23 Marbled Godwits equipped with satellite transmitters from sites in Utah, Mexico, Canada, and coastal Georgia during 2006–2010. Our goals were to characterize the migration strategy of Marbled Godwit populations and to determine migratory connectivity of major breeding, staging, and wintering areas. We found that: 1) godwits breeding in the western USA and Canada followed an overland route to winter sites in Mexico after departing their Utah stopover site; 2) godwits tagged in eastern Canada migrated across the continental USA and wintered at sites along the Gulf of California, Mexico; and 3) godwits wintering in coastal Georgia bred in North and South Dakota. We believe this to be the first demonstration of a continental “crisscross” migration pattern in a shorebird. We identified differences in migration elements such as distances traveled, timing of migration, duration, residency, and stopover strategy between the subpopulations, but not between males and females.