Flight Range Estimates Research Articles

Arriving depleted after crossing of the Mediterranean: obligatory stopover patterns underline the importance of Mediterranean islands for migrating birds

AbstractHundreds of millions of birds reach the Mediterranean islands or Mediterranean coast of Europe every spring after having crossed the Sahara Desert and the Mediterranean Sea. Using data from three small insular stopover sites, we calculated body mass without fuel for 18 trans-Saharan passerine migrants. We subsequently used arrival fuel loads coupled with potential flight range estimates to assess the percentage of birds that are forced to perform an obligatory stopover after crossing the Mediterranean Sea due to fuel depletion. Average arrival fuel loads were among the lowest ever recorded in the Mediterranean region and minimum body mass values recorded for several species were lower than any other individual value reported. The percentage of birds that needed to replenish their energy stores before resuming their northward migration journey varied from 0% to 50% depending on the species and locality studied. Based on conservative estimates at least 180 million birds of our study species are expected to migrate through Greece, 14% of which would not be able to resume their migration without refueling. The significance of small islands and coastal sites in the Mediterranean as obligatory refuelling sites is discussed and their conservation value for migratory birds is highlighted under the perspective of climate change.

Pollinators Complicate Conservation of an Endemic Plant: Physaria obcordata (Cruciferae) in the Piceance Basin, Colorado

To the unnamed road grader operator, resident of Rangeley CO, who on May 13, 1992, idled his machine atop Calamity Ridge and jumped down to admonish us, as we collected insects from P. obcordata flowers, to use care in dealing with those plants with the small yellow flowers because “they grow nowhere else in the world but Rio Blanco Co.”: We dub thee Sir Dudley, Knight of the Twinpod, and thank you belatedly for your challenge, for your concern. Physaria obcordata is a rare endemic of western Colorado. Plants are self-incompatible; they cannot reproduce sexually unless pollinators move pollen between flowers of different genets. We found no evidence that: seed production was limited by inadequate pollination or that pollinations between near neighbor plants (inbreeding depression) or between distant plants (outbreeding depression) resulted in reduced fruit or seed set. Examples are given of the potentially devastating effect of grazing on fruit and seed production. Preliminary evidence suggests that P. obcordata can hybridize with its common congener, P. acutifolia. Only crosses with P. obcordata as pollen recipient and P. acutifolia as pollen donor seemed fertile. Primary flower-visitors of P. obcordata are ground-nesting native bees in the families Andrenidae and Halictidae. Most species visiting the flowers of P. obcordata are generalists. Only two bee species were likely mustard specialists. The only non-bee visitor of any import was a dipteran, Gonia (Tachinidae). Predictions of the distances flown by common bee visitors were made using the equations of Greenleaf et al. (2007). No bee is likely to travel more than 1 km from its nesting site to visit P. obcordata flowers and most are likely to fly distances that are significantly less. The implications of flight range estimates for gene flow between Physaria populations and for pollinator protection are discussed. Several recommendations are made for conservation of P. obcordata and its attendant bees.

Seasonal differences in energy requirements of Garden Warblers Sylvia borin migrating across the Sahara desert

The Sahara desert acts as an ecological barrier for billions of passerine birds on their way to and from their African wintering areas. The Garden Warbler Sylvia borin is one of the most common migrants involved. We used body mass of this species from Greece in autumn and spring to simulate the desert crossing and to assess how body mass relates to fuel requirement. The flight range estimates were adjusted to the seasonal extent of the desert, 2200 km in autumn and about 2800 km in spring. In autumn, with an average fuel load of about 100% of body mass without fuel, birds were not able to cross the desert in still air, but northerly winds prevail during September and with the average wind assistance only one in 14 was predicted to fail to make the crossing. Body mass data from spring, after the desert crossing, was used to estimate departure body mass from south of the desert. The average wind assistance in spring is close to zero and departure body mass of the average bird arriving at Antikythira, a small Greek island, under such conditions was estimated to be 34.6 g, which corresponded to a fuel load of 116%. Calculations based on 1% body mass loss per hour of flight showed slightly larger body mass loss than that calculated from flight range estimates. The results suggest that passerine birds about to cross the eastern part of the Sahara desert need to attain a larger fuel load in spring than in autumn.

Spring passerine migrants stopping over in the Sahara are not fall-outs

The strategy of migrants crossing the Sahara desert has been the subject of debate, but recent evidence from radar studies has confirmed that most passerines use an intermittent migration strategy. The latter has also been suggested from previous studies in oases during autumn migration. It was found that migrants with relatively high fuel loads rest in the desert during daytime and continue migration during the following night, whereas lean migrants stopover in oases for several days to refuel. However, data from the Sahara are scarce for spring migration. We captured passerine migrants near Bîr Amrâne (22°47′N, 8°43′W) in the plain desert of Mauritania for 3 weeks during spring migration in 2004. We estimated flight ranges of 85 passerines stopping over in the desert to test whether they carried sufficient fuel loads to accomplish migration across the Sahara successfully. High fat loads of the majority of birds indicated that they were neither “fall-outs” nor too weak to accomplish migration successfully. The flight range estimates, based on mean flight speeds derived from radar measurements (59 km/h), revealed that 85% of all birds were able to reach the northern fringe of the desert with an intermittent migration strategy. Furthermore, birds stopping over in an oasis (Ouadâne, 370 km to the southwest of Bîr Amrâne) did not carry consistently lower fuel loads compared to the migrants captured in the desert.

Estimating fat and protein fuel from fat and muscle scores in passerines

Fat is the prime energy source for birds during prolonged exercise, but protein is also catabolized. Estimates of the amount of catabolizable fat and protein (termed fat and protein fuel) are therefore important for studying energetics of birds. As fat and protein fuel can only be measured by sacrificing individuals or by use of technically complex methods, scoring systems were invented to estimate fat and protein fuel of birds in the field. The visible subcutaneous fat deposits and the thickness of the flight muscles are each scored on an ordinal scale but these scales do not correspond linearly to fat and protein fuel within species, which is needed for analyses such as flight range estimates. We developed an anova‐type model to estimate fat and protein fuel from fat scores (FS) and muscle scores (MS) along with total mass and a size measurement. Using data from 11 337 individuals of eight passerine species (Common Nightingale Luscinia megarhynchos, Eurasian Reed Warbler Acrocephalus scirpaceus, Melodious Warbler Hippolais polyglotta, Willow Warbler Phylloscopus trochilus, Orphean Warbler Sylvia hortensis, Garden Warbler Sylvia borin, Common Whitethroat Sylvia communis, Subalpine Warbler Sylvia cantillans) mist‐netted in Mauritania, West Africa, we tested for independence of FS and MS and for variation in the relationship between scores and associated mass in response to physiological state. FS, MS and third primary length (size) explained variation in body mass of all eight species analysed (R2: 0.56–0.77). The parameter estimates of the model showed that fat and protein fuel increased monotonically with increasing fat and muscle scores. In two species we found small differences in the estimates between physiological states (seasons). We evaluated our model by comparing the predicted body mass of birds with both FS and MS equal to 0 with the mean body mass of individuals mist‐netted with both scores equal to zero. The values were very close. The amount of fat extracted from dead Garden and Willow Warblers was within the range of predicted fat fuel derived from the model. We conclude that our model is a useful non‐invasive method to estimate simultaneously mean fat and protein fuel of small passerines and we provide recommendations on its use.

Fast fuelling but light flight in Broad‐billed Sandpipers Limicola falcinellus: stopover ecology at a final take‐off site in spring (Sivash, Ukraine)

We studied phenology, staging time and refuelling in Broad‐billed Sandpipers Limicola falcinellus stopping over during spring migration in the Sivash (Black Sea, Ukraine) in May 1991–94. In the study area, peak staging numbers of 2000–2500 individuals occurred in the third week of May. In May 1993, 460 birds were marked with a yellow dye and 126 of these were colour‐ringed. Before 28 May no departure of birds dyed yellow could be detected; by 3 June all birds had departed. Colour‐ringed adults in mid May 1993 staged for a minimum of 8.2 days. After the observed departure of large flocks (24 May and later) the staging time of colour‐ringed birds decreased significantly with body mass at the time of capture. Of birds mist‐netted in 1991–94, 99.3% were in full summer plumage and 89% were adults. In second‐year birds, fuel deposition rate (measured between individuals) was 0.44 g/day. In adults caught from early May to 24 May, overall fuel deposition rate was 1.04 g/day (3.4% of lean body mass). Mean adult body mass in early May was 34.8 g, increasing to 45.5 g after 24 May. Estimated body mass at departure was 51 g. Departure body mass and flight range estimates suggest that although birds refuelled quickly, fuel loads are only just sufficient for an unbroken flight to Scandinavia and the Kola Peninsula. We suggest that Broad‐billed Sandpipers use the Sivash as a crucial final take‐off stopover site, and that they follow a ‘jumping’ migration strategy, performed under narrow time constraints.

CONTINUED RANGE EXPANSION BY BRONZED COWBIRDS IN THE SOUTHWESTERN UNITED STATES

Abstract The brood-parasitic bronzed cowbird (Molothrus aeneus,) continues to expand its range in New Mexico and Texas. The current breeding range is 280 km outside of the historic breeding range. In Texas, spread rate increased from 6.22 km per year prior to 1956 to 13.75 km per year after 1956. Spread rate in New Mexico was constant at 12.72 km per year. Flight-range estimates based on fat indices of spring migrant cowbirds trapped at their northern range boundary in Texas suggest the potential for migration 341 km further north. We discuss factors that might limit or promote further range expansion.

BLACKPOLL WARBLER (DENDROICA STRIATA) FAT DEPOSITION IN SOUTHERN NOVA SCOTIA DURING AUTUMN MIGRATION

There is considerable evidence that the Blackpoll Warbler (Dendroica striata) undertakes a non-stop, transoceanic, fall migration from New England and Atlantic Canada to South America. However, this unique migration strategy is not yet clearly understood. Based on flight range estimates, Blackpoll Warblers must require large amounts of fat for this flight. I captured Blackpoll Warblers during fall migration on Bon Portage Island, a small island off southern Nova Scotia, to determine if Blackpoll Warblers either arrive with, or remain on the island and deposit, the large fat reserves necessary for their migration. Most Blackpoll Warblers did not arrive at Bon Portage with sufficient fat reserves to complete transoceanic migration from Nova Scotia to South America. Furthermore, although young Blackpoll Warblers did show moderate rates of diurnal fat deposition while on Bon Portage, most did not remain long enough to accumulate the necessary fat reserves for transoceanic migration. I conclude then, that Bon Portage Island is not one of the final sites for Blackpoll fat deposition, and I discuss several possible explanations for these results.

The Northward Migration of Arctic Waders in New Zealand: Departure Behaviour, Timing and Possible Migration Routes of Red Knots and Bar-tailed Godwits from Farewell Spit, North-West Nelson

SummaryThe departure behaviour of northward migrating Bar-tailed Godwits and Red Knots was studied on Farewell Spit, New Zealand, from 8–27 March 1994. Most flocks left in the evening and on rising tides. An exception occurred when waders responded to weather conditions favourable for migration, having had their departure delayed by strong winds for the previous five days. On average, flocks left with a slight tailwind, although some left with headwinds of up to 12 km h−1. Most departures occurred after the passage of a low-pressure system or at the approach of a high-pressure system, which produced favourable southerly winds. Despite observed departure directions being intermediate between those expected for flights to either northern Australia or northern New Zealand, most flocks probably encountered easterly winds from high-pressure systems and flew to Australia with some wind assistance. Varied assumptions of flight speed and the level of protein deposition show that flight range estimates can differ so widely that predicting migration routes is difficult.

Flight Costs, Flight Range and the Stopover Ecology of Migrating Birds

1. Flight range equations are a central component in the analysis of avian migration strategies. These equations relate the distance that can be covered to the fuel load that the birds carry. Models of stopover decisions deal with the question of how birds should react to variations in fuel deposition rates. Time-minimization models generally predict an increasing relationship between departure fuel load and fuel deposition rate. 2. We show that quantitative details of predictions derived from optimality models depend critically on the flight range equation that is used. We use two classes of flight range equations: one class is based on theoretical assumptions of aerodynamics; the other is based on empirical measurements of metabolism during flight. 3. Most empirically derived equations can be written as Y(x) = c[1-(1 + x) -ζ ], where 0 < ζ < 1, and c is a constant that includes morphological traits and lean body mass. 4. Patterns of site use and departure loads in environments with discrete stopover sites depend in significant ways on flight costs. 5. Flight range estimates that are based on empirically derived, multivariate equations are sensitive to errors in the estimates of exponents of the equations. Varying some exponents within their confidence limits can alter flight ranges by an order of magnitude.

Spring Stopover of Intercontinental Migratory Thrushes along the Northern Coast of the Gulf of Mexico

The narrow woodlands and wooded barrier islands along the northern coast of the Gulf of Mexico provide the first available landfall for Neotropical landbird migrants after their spring trans-Gulf flight. We studied the timing, volume, fat stores, length of stay, and fat deposition of thrushes (n = 1,903) during stopover at a coastal Louisiana woodland in 1987, 1988, and 1990. Peak migration occurred around mid-April in Wood Thrushes (Hylocichla mus- telina) and was one to two weeks later for Catharus thrushes. More than one-third of the thrushes carried no observable subcutaneous fat stores upon initial capture, and only 5% of the birds had estimated fat stores that exceeded 20% of lean body mass. Body mass and con- dition index (mass/wing length) were negatively related to time of capture, suggesting that birds captured later in the day arrived with lower fat stores. Wing size varied negatively with capture date, suggesting differential migration between sexes and among age classes and/or populations. Recapture probability was related to the amount of fat stores upon initial cap- ture, and length of stay was predictable from estimated fat stores, rate and amount of fat re- plenishment, and capture date. Flight range estimates suggest that most thrushes captured in spring along the northern coast of the Gulf of Mexico are unable to reach breeding destina-

Residency Patterns of Migrating Sandpipers at a Midcontinental Stopover

Arctic-nesting shorebirds require several refueling stops during their long migrations between breeding grounds and Central and South American wintering areas. The protection of stopover habitats for transcontinental migrants depends on whether birds fly long distances between a few select sites or fly short distances and stop at several wetlands. Although the Great Plains historically provided a vast array of wetlands for use by migrants, wetland loss and conversion have reduced the availability of stopover sites in recent decades. In this study, we examined (1) residency periods, (2) fat dynamics, and (3) migration chronology of two shorebird species, the Semipalmated Sandpiper (Calidris pusilla) and White-rumped Sandpiper (C. fuscicollis) at Quivira National Wildlife Refuge (NWR), Kansas. Semipalmated Sandpipers had prolonged periods of species residency with overlapping arrivals and departures. Individual residency periods were highly variable and were unrelated to lipid reserves upon arrival. In contrast, White-rumped Sandpipers arrived and departed more synchronously. Birds that arrived in poor condition stayed longer than those with more body fat in 1991, but not in 1992. Wind direction did not influence patterns of departures of either species. We hypothesize that Semipalmated Sandpipers are ecologically eurytopic when migrating across the Great Plains in the spring. Highly variable patterns in arrival, residency, and lipid levels indicate that spring migration of this species is relaxed and opportunistic. White-rumped Sandpipers showed a pattern of reduced flexibility. Flight range estimates suggest that most birds require intermediate stopovers before reaching the breeding grounds. Interior wetlands appear to function as migration stopovers rather than staging areas for shorebirds.

Optimal fat loads and long‐distance flights by migrating Knots Calidris canutus, Sanderlings C. alba and Turnstones Arenaria interpres

Arctic waders often build up large fat loads and complete their migratory journeys by a few long‐distance flights between traditional staging sites. Optimal fat loads and choices of staging sites differ depending on whether the birds are adapted to minimize energy or time spent on migration. In the latter case, we predict that the birds will depart for the next staging site when the instantaneous speed of migration expected after arrival at the next site, exceeds the corresponding speed at the departure site. The instantaneous migration speed is a function of the rate of fat deposition and the current fat load. As a consequence of this, overloading (birds deposit larger fat loads than needed merely for covering the flight distance to the next destination) and by‐passing of possible, but low‐quality staging sites, are expected under specific conditions in time‐selected migration.Estimates of fat deposition rates and departure fat loads were obtained by captures of Knots Calidris canutus, Sanderlings C. alba and Turnstones Arenaria interpres in W. Iceland during spring migration. Further fat deposition data referring to spring migration of these species were compiled from the literature. Fat deposition rates at different sites, as measured by the daily gain in mass relative to lean body‐mass, range between 1.0 and 3.6%/day, and departure fuel loads (in % of lean body‐mass) between 27 and 73%.Comparison with flight range estimates suggests that overloading may be a regular phenomenon during spring migration of Knots, Sanderlings and Turnstones. Furthermore, fat deposition rates at different staging sites, and the general difference in migration patterns between spring and autumn, indicate that by‐passing of possible staging sites may well occur. Hence, it cannot be excluded that the waders' migratory habits primarily serve to maximize the overall speed of migration.

How valid are flight range estimates for waders?

Assessments of fat loads and flight ranges are important in studies of migration phenology. Departure condition, which is an essential input for flight range calculations, is difficult to measure accurately, especially for live birds. Problems arise from the estimation of fat load and the rate of fat storage, water loss, and departure time. Flight range models give range estimates for waders differing by up to 2.5 times for the same fat load. The validities of the models are tested against known migrations of waders. Ranges instill air from flight metabolism models (McNeil & Cadieux 1972, Greenewalt 1975, Summers & Waltner 1979, this study) predict observed range most accurately. However, range modifiers, particularly wind speed and direction, altitude, flight speed, and aerodynamic drag, will all increase predicted range, when applied to waders. With information on actual conditions of migration, Pennycuick's (1975) aerodynamic model may be the best range predictor. Since range modifiers are difficult to apply, flight range can be approximated by calculation of a still‐air range from the flight metabolism model derived in this paper. Before they are used for other groups of birds, models should be tested against known migrations of these groups.