Fuel Deposition Rate Research Articles

Stopover Strategies in Passerine Bird Migration: A Simulation Study

Long-distance bird migration consists of a series of stopovers (for refuelling) and flights, with flights taking little time compared to stopovers. Therefore, it has been hypothesized that birds minimize the total time taken for migration through efficient stopover behaviour. Current optimality models for stopover include (1) the fixed expectation rule and (2) the global update rule. These rules maximize the speed of migration by determining the optimal departure fuel load for a given fuel deposition rate. We were interested in simple behavioural rules approaching the stopover behaviour of real birds and how these rules compare to the time minimizing models above with respect to the total time taken for migration. The simple strategies were to stay at a site (1) until a fixed fuel load was reached or (2) for a constant number of days. We simulated migration of small nocturnal passerine birds across an environment of continuously distributed but variable fuel deposition rates, and investigated the influence of different stopover strategies on the duration of migration. Staying for a constant number of days at each stopover site, irrespective of the fuel deposition rate, resulted in only slightly longer than minimum values for migration duration. Additionally, the constant stopover duration, e.g. 10 days, may change by a day or two (per stopover) without having a large effect on total migration duration. There is therefore a possibility that real birds may be close to optimal migration speed without the need for very complex behaviour. When assessing the sensitivity of migration duration to factors other than stopover duration, we found that flight costs, search and settling time, mean fuel deposition rate and the accuracy in the choice of flight direction were the factors with the largest influence. Our results suggest that migrating birds can approximate optimal stopover duration relatively easy with a simple rule, and that other factors, e.g. those above, are more relevant for travel time.

Variation of fuelling rates among sites, days and individuals in migrating passerine birds

Summary The seasonal migration of birds is divided into alternating phases of stopover and flight. The fuel deposition rate at stopover sites is the crucial factor determining overall speed of migration and its success. Therefore, field data about the variation in fuel deposition rates at different levels (among sites, seasons, days, individuals) are essential to explain the observed behavioural reactions to environmental variability and migration strategies. Fuel deposition rates of four species of passerine migrant birds captured at 14 stopover sites from northern Europe to sub‐Saharan Africa during autumn and spring were analysed. Plasma concentrations of triglycerides and β‐hydroxy‐butyrate were used to estimate relative fuel deposition rates (named fattening index). The largest variation in fattening indices was between sites which was only weakly explained by geographical position relative to ecological barriers and did not differ between spring and autumn. Therefore it is expected that the selection of appropriate stopover sites is of paramount importance for the success of migration. Variation in fattening index between days within sites was comparatively small with no obvious seasonal trend over the restricted period of mid‐migration and with no correlation between subsequent days. However, only samples during good weather days were examined and the full range of this variation could not be estimated. Air temperature and duration of rainfall could explain part of this variation, probably because invertebrate availability is higher during warm and dry weather. The variation of fattening rates between birds on the same day and site was low, and could partially be explained by body mass. Heavy birds had higher fattening rates than light birds. From this analysis, it appears to be difficult for migrating birds to predict the fattening rate of the next stopover site, but relatively easy to assess the fattening rate of the current site in a short time. This may result in larger departure fat loads than would be necessary to reach the next stopover site.

Stopover durations of three warbler species along their autumn migration route.

In migrating birds, the success of migration is determined by stopover duration, the most important factor determining overall speed of migration, and fuel deposition rate. However, very little is known about stopover durations of small migrant birds, because appropriate methods for data analysis were lacking until recently. We used a new capture-recapture analysis to estimate stopover durations of 1st-year reed warblers Acrocephalus scirpaceus, sedge warblers A. schoenobaenus and garden warblers Sylvia borin at 17 stopover sites in Europe and Africa during autumn. Average stopover duration of non-moulting reed warblers was 9.5 days while moulting conspecifics stayed about twice as long. Average stopover duration of sedge warblers was 9.1 days and, in contrast to the other two species, differed between years at several sites. Garden warblers stayed 7.7 days on average. The long stopover duration of the reed warbler, resulting in slow overall migration speed, is related to its low fuel deposition rate. It can be explained by low, but predictable, food resources and an early departure during moult. Compared to the reed warbler, the stopover duration of the sedge warbler varies more between sites and probably also between years, as the supply of its preferred diet (reed aphids) is spatially and temporally unpredictable but can be superabundant. The short stopover duration of the garden warbler, leading to high overall migration speed, can be related to high fuel deposition rates, probably brought about by a change to an abundant, predictable and long-lasting fruit diet. Within species, stopover duration did not change significantly along the migration route. Hence, an increase of migration speed along the migration route, as suggested in the literature, may be caused by longer flight bouts in the south. However, it remains largely unknown which environmental and possibly endogenous factors regulate stopover duration.

Detours in Bird Migration

Bird migration routes often follow detours where passages across ecological barriers are reduced in extent. This occurs in spite of the fact that long barrier crossings are within the birds' potential flight range capacity. Long-distance flights are associated with extra energy costs for transport of the heavy fuel loads required. This paper explores how important the fuel transport costs, estimated on the basis of flight mechanics, may be to explain detours for birds migrating by flapping flight. Maximum detours in relation to expanse of the barrier are predicted for cases where birds travel along the detour by numerous short flights and small fuel reserves, divide the detour into a limited number of flight steps, and where a reduced barrier passage is included in the detour. The principles for determining the optimum route, often involving a shortcut across part of the barrier, are derived. Furthermore, the effects of differences in fuel deposition rates and in transport costs for the profitability of detours are briefly considered. An evaluation of a number of observed and potential detours in relation to the general predictions of maximum detours, indicates that reduction of fuel transport costs may well be a factor of widespread importance for the evolution of detours in bird migration at wide ecological barriers.

Fuel deposition of three passerine bird species along the migration route.

The rate at which migrant birds replenish their energy stores at intermittent stopovers largely determines overall migration speed, the manner in which migration proceeds and success of migration. In this study, data on the fuel deposition rate (FDR) of three long-distance migrants from 17 ringing sites along their autumn migration route were used to examine: (1) effects of endogenous factors on FDR, and (2) how relationships between exogenous factors and FDR affect the organisation of migration. We developed a model to estimate FDR from retrapped birds which takes into account time of day and various other factors which might influence FDR. The two endogenous factors, moult and current energy stores, generally reduced FDR. This may result in lower departure energy loads and more stopovers than expected from optimal migration theory. Differences between species with respect to seasonal, year-to-year and geographical patterns of FDR could be related to differences in availability and predictability of food resources, and help to explain differences in the organisation of migration. A low FDR in northern and central Europe could be related to low, but predictable, food resources and an early departure during moult of the reed warbler (Acrocephalus scirpaceus); FDRs varying between years were related to large spatial and year-to-year variation in the density of the main prey of the sedge warbler (Acrocephalus schoenobaenus); and a high FDR in the garden warbler (Sylvia borin) was related to abundant food resources, due to a switch from a purely invertebrate diet to a mixed diet including fruits which are abundant over large areas of Europe and north Africa. This study demonstrated that the organisation of migration is the outcome of a complex interplay of the seasonal timing of moult, food availability and predictability and a seasonal switch in diet, and can be modified by individual birds in response to a limited amount of time in which to migrate.

Should I stay or should I go? Testing optimality models of stopover decisions in migrating birds

Time-minimizing migrants should leave a stopover site if the instantaneous speed of migration drops to the average speed of migration in the environment. This argument has been used in two different ways: either there is local variation in the fuel deposition rate and a fixed expected speed or there is global variation in the fuel deposition rate, i.e. locally experienced variation represents global variation along the route. The first case leads to a far steeper relationship between departure load and fuel deposition rate than the second case. So far, data on departure loads have mainly been analysed within the concept of local variation of the fuel deposition rate and the result that the observed slopes are much lower than predicted has been explained by changes in the expected speed along the route or by individual differences in the expected speed. We show here that the observed relationships generally fall close to the predictions for global variation. We propose that migrants use a behavioural rule which projects the current experience into the future and therefore interprets local variation as global variation.

Blissful Ignorance? Departure Rules for Migrants in a Spatially Heterogeneous Environment

Incomplete knowledge of the variation in site quality to be encountered during migration may make it necessary for birds to base their departure decisions on behavioural rules of thumb. Here I analyse four such rules for time-minimizers and show how they perform in spatially heterogeneous, autocorrelated environments. The first rule projects the currently experienced fuelling rate onto all the future sites, the second rule is based on a fixed expected rate along the route, the third rule assumes that the expected rate at sites nearby is close to the value currently experienced and the fourth rule uses the experienced values to form an expectation during migration. Simulations demonstrate that a global update rule which uses the current experience and interprets it as the global value to be expected at all the future sites is outperforming the second rule, which is based on fixed expectations, in nearly all circumstances and that it is very robust with respect to spatial autocorrelation and the degree of variability in fuelling rates. This rule leads to the repeatedly observed pattern of a shallow relationship between departure fuel load and fuel deposition rate. A third rule shows its best performance under the assumption that the quality of sites only changes little with distance and therefore comes close to the global update rule. The fourth departure rule, a modified version of the global update rule, which allows some adjustments of the expected value through all the experienced values so far, leads only to small increases in performance.

Stopover Decisions under Wind Influence

Despite evidence that wind conditions are an important factor in determining stopover decisions, models of time-minimizing bird migration have up to now emphasized the optimal response of the migrants to variations in fuel acquisition rates. We present a simple model of a time-minimizing migrant faced with two potential wind conditions on each day, which occur with a fixed probability. Wind assistance is modelled as a multiplicative factor in the flight range equation. We identify conditions under which birds leave the stopover site even with no tailwinds and conditions where the birds leave only with tailwinds in cases with global and local variation of the fuel deposition rate. The optimal policy depends on the probability and amount of wind assistance. In all cases there is an initial period at a stopover site when the bird should stay and build up its initially small fuel reserves irrespective of wind. After this initial time, there is a period when the optimal departure decision is to leave when tailwinds occur but stay and continue fuel deposition in other winds. If the probability of tailwinds is low the bird should at some later time change its policy to leave even in unfavourable winds. However, if a certain threshold value of the probability of favourable winds is reached the birds should never leave without wind assistance. These patterns lead to a complex relationship between departure load and fuel deposition rate. We compare our predictions with a null-model where the birds simply leave as soon as favourable winds occur. We further show that the inclusion of wind assistance cannot explain the discrepancy between observed and predicted values of departure loads under local variation in fuelling rates.

Optimal avian migration: A dynamic model of fuel stores and site use

Birds migrating between widely separated wintering and breeding grounds may choose among a number of potential stopover sites by using different itineraries. Our aim is to predict the optimal migration schedule in terms of (1) rates of fuel deposition(2) departure fuel loads and (3) stopover site use, when only a handful of such sites are available. We assume that reproductive success depends on the date and fuel load at arrival on the breeding grounds. On migration, the birds face a trade-off between gaining fuel and avoiding predation. To allow the optimal decision at any given moment to depend on the fuel load and the location of the bird, as well as on unpredictability in conditions, we employed stochastic dynamic programming. This technique assumes that the birds know the probability distribution of conditions in all sites, but not the particular realization they will encounter. We examined the consequences of varying aspects of the model, like (1) the shape of the relationship between arrival date and fitness, (2) the presence of stochasticity in fuel deposition rates and wind conditions, and (3) the nature of predation (i.e. whether predation risk depends on the fuel load of the birds or their feeding intensity). Optimal fuel deposition rates are predicted to be constant if there are either only predation risks of maintaining stores or only risks of acquiring fuel stores. If only fuel acquisition is risky, fuel deposition rates can be below maximum, especially if there also is an intermediate best arrival time at the breeding ground. The fuel deposition rate at a site then depends not just on the site's quality but on the qualities of all visited sites. In contrast, rates of fuel deposition are not constant if both the acquisition and the maintenance of fuel stores carry risk. Optimal departure fuel loads are just enough to reach the next site if the environment is deterministic and are simply set by the energetic cost of covering the distance. As with time-minimizing models, more fuel than necessary to reach a site is only deposited under very restricted circumstances. Such overloads are more likely to be deposited if either fuel gains or expenditure are stochastic. The size of overloads is then determined by the variance in fuel gainat the target site and the worst possible conditions during flight. Site use is modified by differences in predation risk between sites and differences in fuel deposition rates. An expression derived to predict site use under time minimization provides agood approximation in state-dependent models. In some cases, the possibility of starvation may influence optimal decisions, even when the probability of starvation under the optimal policy is low. This effect of starvation has also been found in other contexts. VA:IBN

Migratory fuelling in blackcaps ( Sylvia atricapilla ) under perceived risk of predation

It has been argued that the body mass levels achieved by birds are determined by the trade-off between risks of starvation and predation. Birds have also been found to reduce body mass in response to an increased predation risk. During migration, the need of extra fuel for flights is obvious and crucial. In this study, migratory blackcaps (Sylvia atricapilla) were subject to an experimental stopover situation where the predation risk was manipulated by exposure to a stuffed predator. Blackcaps that perceived an imminent risk of predation increased their food intake and fuel deposition rate during the first period of stopover compared with a control group. The pattern of night activity indicates that birds that were exposed to the predator also chose to leave earlier than birds in the control group. Since there was no cover present at the stopover site, birds might have perceived the risk of predation as higher regardless of whether they were foraging or not. Under such circumstances it has been predicted that birds should increase their foraging activity. The findings in this study clearly indicate that birds are able to adjust their stopover behaviour to perceived predation risk.

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.

A General Model for Time-minimising Avian Migration

We develop an optimality model for time-minimising bird migration for any distribution of feeding habitats and qualities along the migratory route. If the fuel deposition rate is constant along the route and feeding is possible everywhere, it is optimal to divide the journey into steps of equal length. The relationship between fuel deposition rate and departure fuel load is not continuous. If the fuel deposition rate is changing monotonically along the route, step lengths are not constant. Step lengths increase if fuel deposition rates increase along the route and decrease if fuel deposition rates decrease along the route. In both cases the relationship between fuel deposition rate and departure fuel load is also not continuous. These qualitative patterns remain unaffected if different flight range equations are used and if the dependence of time spent flying on departure fuel load is included. If ecological barriers are introduced where fuel deposition is impossible it is in most cases optimal to make a stop at an edge of the barrier.

Composition of fuel stores and digestive limitations to fuel deposition rate in the long-distance migratory thrush nightingale, Luscinia luscinia.

During their autumn migratory phase, thrush nightingales (Luscinia luscinia) previously starved for 2 d were allowed to refuel under three different ambient temperature conditions (-7 degrees, 7 degrees, and 22 degrees C). During the refueling period, as well as during the preceding control and starvation periods, food intake, body mass, and feces production were monitored. In addition, daily energy expenditure was measured during the refueling period. The compilation of the energy balance during the refueling period revealed an energy density of the deposited tissue of 33.6 kJ g-1. Assuming that the deposited tissue consists of fat and protein exclusively, with energy densities of 39.6 and 5.5 kJ g-1 wet mass, respectively, we estimated the deposited tissue to consist of 82% fat and 18% wet protein (6% dry protein and 12% water). Nitrogen balances during control, starvation, and refueling phases and during a period of prolonged and complete starvation indicated that 5% of the nutrient stores consisted of dry protein. Our results support recent findings that nutrient stores for migration often contain protein in addition to fat and consequently are 15%-25% less energy rich than pure fat stores. These proteins might be stored as muscle or other functional tissue and may be required to support the extra mass of the stores and/or reflect an incapacity of the metabolic machinery to catabolize far exclusively. Fuel deposition rate was positively related with ambient temperature, whereas food intake rate was unaffected by temperature. These results indicate that the rate of fuel deposition is limited by a ceiling in food intake rate; when this ceiling is reached, fuel deposition rate is negatively affected by daily energy expenditure rate. To a certain extent, the ceiling in food intake rate varies depending on feeding conditions over the previous days. These variations in food intake capacity probably reflect the building and breakdown of gut tissues and/or gut enzyme systems and might be insensible and not evolutionary adaptive. Significant energetic costs, however, are probably associated with the maintenance of gut tissues. It is therefore feasible that changes in digestive capacity are regulated and are directed at energy economization.

The timing of post-juvenile moult and fuel deposition in relation to the onset of autumn migration in Reed Warblers Acrocephalus scirpaceus and Sedge Warblers Acrocephalus schoenobaenus

The extent of post-juvenile moult and fuel deposition prior to migration in Reed Warblers Acrocephalus scirpaceus and Sedge Warblers A. schoenobaenus were analysed using 11 years ringing data from Lake Kvismaren in South Central Sweden. Re-trap data showed that Reed Warblers acquired their first winter plumage in 34 days, compared to 22 days in Sedge Warblers. The moult pattern of the two species differed due to Sedge Warblers only undertaking a fill-in moult of bare body areas. Fuel deposition rates for the two species were similar, 0.38 g/day for Reed Warblers and 0.34 g/day for Sedge Warblers. The estimated average fuel stores at departure from Lake Kvismaren differed, with Reed Warblers found to leave with a fuel store of 21% (in relation to lean body mass) compared to 6.2% in Sedge Warblers. The median date of departure from Lake Kvismaren was estimated to 30 August for Reed Warblers and 12 August for Sedge Warblers. The earlier departure of Sedge Warblers, with smaller fuel stores, may reflect different migration strategies between the species.

Thermal Decomposition of Aircraft Fuel

A test program has been conducted to determine the thermal stability and heat transfer characteristics of JP-5 and several other kerosene-type fuels which vary in composition and tendency to form deposits. Tests were conducted in small-diameter, resistively heated tubes at typical aircraft engine operating conditions. A detailed mapping of the thermal decomposition characteristics of JP-5 was performed to evaluate the importance of key environmental factors—such as temperature, pressure, flowrate, and test duration—and to establish a data base for estimating the relative performance of new fuels. Tests were conducted over the temperature range 425 to 870 K for durations up to 32 hr. Fuel deposition rates varied among fuels, and the relationship between the “breakpoint temperature” (determined according to ASTM D3241) and the rate of fuel deposition was investigated.