Thomas P. Weber

Optimal moult strategies in migratory birds

Avian migration, which involves billions of birds flying vast distances, is known to influence all aspects of avian life. Here we investigate how birds fit moult into an annual cycle determined by the need to migrate. Large variation exists in moulting patterns in relation to migration: for instance, moult can occur after breeding in the summer or after arrival in the wintering quarters. Here we use an optimal annual routine model to investigate why this variation exists. The modelled birds decisions depend on the time of year, its energy reserves, breeding status, experience, flight feather quality and location. Our results suggest that the temporal and spatial variations in food are an important influence on a migratory birds annual cycle. Summer moult occurs when food has a high peak on the breeding site in the summer, but it is less seasonal elsewhere. Winter moult occurs if there is a short period of high food availability in summer and a strong winter peak at different locations (i.e. the food is very seasonal but in opposite phase on these areas). This finding might explain why only long-distance migrants have a winter moult.

Basal metabolic rate and the mass of tissues differing in metabolic scope: migration-related covariation between individual Knots Calidris canutus

To examine whether variability in the basal metabolic rate (BMR) of migrant shorebirds is a function of a variably sized metabolic machinery or of temporal changes in metabolic intensities at the tissue level, BMR, body composition and activity of cytochrome-c oxidase (CCO, a marker for maximum tissue respiration) were measured in 14 captive Knots Calidris canutus islandica in late spring, during the period of mass loss after the migratory body mass peak. Although the body mass cycle of captive birds closely followed the changes of free-living conspecifics, their fat-free mass of muscles and organs was somewhat lower and their fat content higher. BMR significantly declined during mass loss, as did the fat-free dry mass. BMR was an allometric function of both body mass (exponent=0.687) and lean dry mass (exponent=1.132). Fat-free dry mass of heart sind flight muscle decreased with the loss of fat. CCO-activity was determined in heart, flight muscle, leg muscle, liver and kidney. It was highest in heart and flight muscle and low in the other tissues. CCO-activity was not correlated with total fat mass. Intraspecific migration-related variation in BMR seems better explained by variation in the mass of organs with a high metabolic scope (as indicated by high CCO-activity), than by variation in the intensity of tissue metabolism.

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.

Consequences of habitat loss at migratory stopover sites: a theoretical investigation

We use a dynamic optimization model to assess the consequences of habitat loss at migratory stopover sites. We emphasize costs birds face during stopover (e.g. costs of gaining energy), the timing of site use and the behavioural rules birds might use to implement migratory strategies. Behavioural rules may be flexible enough that birds can still produce optimal behaviour in the changed environment, or the rules may result in suboptimal behaviour. If birds behave optimally in the altered environment, habitat loss on the wintering ground has the highest impact, because this site, unlike the intermediate stopover sites, cannot be skipped if the quality drops below a threshold. If birds continue to use the old behavioural rules that now result in suboptimal behaviour, we can distinguish two cases. Birds can continue to use a constant foraging behaviour that was optimal in the unaltered environment under many circumstances. Then the effects of habitat loss are proportional to the length of stay before habitat loss and the departure fuel load from a site. However, the effects of habitat loss do not depend on the location of the site within the network. In some circumstances birds are expected to forage with intensities that are below maximum. If birds use the foraging behaviour that is appropriate for a given fuel load, time and site but inappropriate for the altered fuel gain, then changes at sites close to the breeding ground have a greater impact than more distant sites. Finally we discuss the importance of sites that are not used before habitat loss. If birds behave optimally these sites may be used in an altered environment and can buffer against habitat loss at other sites.

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.

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

Abstract 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.

Resistance of flight feathers to mechanical fatigue covaries with moult strategy in two warbler species

Flight feather moult in small passerines is realized in several ways. Some species moult once after breeding or once on their wintering grounds; others even moult twice. The adaptive significance of this diversity is still largely unknown. We compared the resistance to mechanical fatigue of flight feathers from the chiffchaff Phylloscopus collybita, a migratory species moulting once on its breeding grounds, with feathers from the willow warbler Phylloscopus trochilus, a migratory species moulting in both its breeding and wintering grounds. We found that flight feathers of willow warblers, which have a shaft with a comparatively large diameter, become fatigued much faster than feathers of chiffchaffs under an artificial cyclic bending regime. We propose that willow warblers may strengthen their flight feathers by increasing the diameter of the shaft, which may lead to a more rapid accumulation of damage in willow warblers than in chiffchaffs.

A Plea for a Diversity of Scientific Styles in Ecology

1 1 Opinion is intended to facilitate communication between reader and author and reader and N reader. Comments, viewpoints or suggestions arising from published papers are welcome. I I Discussion and debate about important issues in ecology, e.g. theory or terminology, may P ( also be included. Contributions should be as precise as possible and references should be 0 N kept to a minimum. A summary is not required.

Gone with the Wind? A Comment on Butler et al. (1997)

Because wind speed and flight speed of migratory birds generally are of the same magnitude, wind direction has a significant effect on the ground speed of birds. Consequently, migrants should prefer days of favorable tailwinds for commencing migratory flights, or at least they should avoid strong headwinds. These predictions generally are the case as revealed by radar studies (Richardson 1990). Based on estimated flight costs of shorebirds, Butler et al. (1997) concluded that tailwinds are necessary for successful spring migration of Western Sandpipers (Calidris mauri). Butler et al. (1997) proposed that the migration strategy for Western Sandpipers consists of maintaining relatively large fuel reserves and departing on migratory flights when tailwinds are present. This strategy was put forth as an alternative to the strategy of time minimization of migration (sensu Alerstam and Lindstrom 1990). We think it is premature to reject time minimization as a migration strategy for Western Sandpiper on the basis of the data presented by Butler et al. (1997), and we outline our arguments below. First, we show that new information concerning the aerodynamic drag of bird bodies indicates that Western Sandpipers may complete their spring migration without wind assistance. Next, we provide a critical discussion of the assumptions, methods, and conclusions put forward by Butler et al. (1997). Flight costs: They can do it.-Butler et al. (1997) calculated that when using Pennycuicks (1989) model of bird flight performance, the arrival mass will be 17.5 g for a male Western Sandpiper in the absence of wind assistance. Two amendments to the theory of bird flight may change this conclusion. First, the default value for the body-drag coefficient (Cpar), which determines the magnitude of parasite drag (i.e. drag of the body), has been overestimated previously. The old default value (Cpar= 0.4) was based on measurements using frozen bird bodies in a wind tunnel (Pennycuick et al. 1988), although the experiments were suspected to overestimate Cpar. Based on new wind-tunnel experiments on live birds, Pennycuick et al. (1996) found that CP,, is much lower than previously measured on frozen birds, and they rec-