Flight speeds of migrating birds: a test of maximum range speed predictions from three aerodynamic equations

Many temperate birds invest considerable time and energy totravel the long distances between their breeding grounds andwintering areas. It has generally been assumed therefore thatto minimize the energy cost of migration (and thus maximizefuel economy) birds ought to fly at speeds that maximize thedistance travelled per unit of energy expended (termed the maximumrange speed, V_mr). I tested this idea by comparing literaturereports of flight speeds for 48 avian species on migration andcomparing them to predictions of V_mr derived from three aerodynamicequations (Tucker, Pennycuick, and Greenewalt). No single equationmade V_mr predictions that matched the full range of observedspeeds. Species weighing 0.3 kg–3 kg (Greenewalt equation)and 0.1 kg–1 kg (Pennycuick equation) generally migratedat V_mr, but this represents only 42% (20/48) and 40% (19/48)of the total number surveyed, respectively. Deviations fromV_mr outside these ranges varied systematically with mass. Lighterspecies almost always flew faster than V_mr, whereas heavierspecies showed the opposite trend. The latter group is likelyconstrained to fly below V_mr due to limits on metabolic performanceimposed by mass-specific scaling effects. The Tucker equationalmost always predicted V_mr values that were less than observedspeeds.     

Skylark optimal flight speeds for flying nowhere and somewhere

Flight is energetically very costly. For birds the mechanicalpower in relation to airspeed is characterized by a U-shapedfunction. From this function we can derive optimal flight speedsassociated with minimum power (V_mp), minimum cost of transport(V_mr) and minimum overall time of migration (V_mt). Since flightis energetically so costly, aerial displays and song flightcan potentially serve as signals reliably indicating the individualquality or resource potential of the signaler. In order to maximizethe amount of song flight produced, we expect V_mp during songflight, while during migration we rather expect V_mr or V_mv Wecompared flight speeds of skylarks (Alauda arvensis) duringsong flight and migration flight, respectively. In this speciespredicted V_mp = 5.5 m/s, V_mr = 10.5 m/s, and V_mt = 12.1 m/s.The preferred airspeed during song flight did not differ significantlyfrom the predicted V_mp, while airspeed during migration wassignificantly higher than V_mr and Vmp indicating that flightspeed is a flexible trait that birds adjust to different situations.Why the skylarks speed up so much on migration is still unclear,but it may be that due to the shape of the predicted power curve,variation in cost of transport at high speeds is relativelysmall.     

Soaring across continents: decision‐making of a soaring migrant under changing atmospheric conditions along an entire flyway

Thermal soaring birds reduce flight‐energy costs by alternatingly gaining altitude in thermals and gliding across the earth's surface. To find out how soaring migrants adjust their flight behaviour to dynamic atmospheric conditions across entire migration routes, we combined optimal soaring migration theory with high‐resolution GPS tracking data of migrating honey buzzards Pernis apivorus and wind data from a global numerical atmospheric model. We compared measurements of gliding air speeds to predictions based on two distinct behavioural benchmarks for thermal soaring flight. The first being a time‐optimal strategy whereby birds alter their gliding air speeds as a function of climb rates to maximize cross‐country air speed over a full climb– glide cycle (V_opt). The second a risk‐averse energy‐efficient strategy at which birds alter their gliding air speed in response to tailwinds/headwinds to maximize the distance travelled in the intended direction during each glide phase (V_bgw). Honey buzzards were gliding on average 2.05 ms^{– 1} slower than V_opt and 3.42 ms^{– 1} faster than V_bgw while they increased air speeds with climb rates and reduced air speeds in tailwinds. They adopted flexible flight strategies gliding mostly near V_bgw under poor soaring conditions and closer to V_opt in good soaring conditions. Honey buzzards most adopted a time‐optimal strategy when crossing the Sahara, and at the onset of spring migration, where and when they met with the best soaring conditions. The buzzards nevertheless glided slower than V_opt during most of their journeys, probably taking time to navigate, orientate and locate suitable thermals, especially in areas with poor thermal convection. Linking novel tracking techniques with optimal migration models clarifies the way birds balance different tradeoffs during migration.     

Flight speed of seabirds in relation to wind speed and direction

We studied flight speed among all major seabird taxa. Our objectives were to provide further insight into dynamics of seabird flight and to develop allometric equations relating ground speed to wind speed and direction for use in adjusting seabird density estimates (calculated from surveys at sea) for the effect of bird movement. We used triangulation at sea to estimate ground speeds of 1562 individuals of 98 species. Species sorted into 25 “groups” based on similarity in ground speeds and taxonomy. After they were controlled for differences inground speed, the 25 groups sorted into eight major “types” on the basis of response to wind speed and wind direction. Wind speed and direction explained 1664% of the variation in ground speed among seabird types. For analyses on air speed (ground speed minus apparent wind speed), we divided the 25 groups according to four flight styles: gliding, flap-gliding, glide-flapping and flapping. Tailwind speed had little effect on air speed of gliders (albatrosses and large gadfly petrels), but species that more often used flapping decreased air speed with increase in tailwinds. All species increased air speeds significantly with increased headwinds. Gliders showed the greatest increase relative to increase in headwind speed and flappers the least. With tailwind flight, air speeds were greatest among species with highest wing loading for each flight style except gliders, which showed no relationship. For headwind flight, species with higher wing loading had higher air speeds; however, the relation was weaker in flappers compared with species using some amount of gliding. In contrast, analyses for air speed ratio (i.e. difference between air speed in acrosswinds [with no apparent wind] and speed flown into headwinds, or with tailwinds, divided by speed acrosswind) revealed that among species using some flapping, and with lower wing loading (surface-feeding shearwaters, small gadfly petrels, storm petrels, phalaropes, gulls and terns), adjusted air speeds more than those with higher wing loading (alcids, “diving shearwaters”, “Manx-type shearwaters”, pelicans, boobies and cormorants). As a result, most flappers of low wing loading flew much faster than V_mr (the most energy efficient air speed per distance flown) when flying into headwinds. We suggest that better-than-predicted gliding performance with acrosswinds and tailwinds of large gadfly petrels, compared with albatrosses, resulted from a different type of “soaring” not previously described in seabirds.     

Testing predictions from flight mechanical theory: a case study of Cory’s shearwater and Audouin’s gull

Predictions from flight mechanical theory concerning optimal flight speeds were tested in the field in two Mediterranean seabirds, the Cory’s shearwater Calonectris diomedea and the Audouin’s gull Larus audouinii. Both species were commuting off the coast of Isola di San Pietro, 6 km south-west of the coast of Sardinia. Heading and airspeed were obtained by vector calculation of flight tracks and measured wind. The Cory’s shearwater used a mixture of gliding and active flight. At low wind speeds the proportion of active flight was large but it decreased with increasing wind speed. The mean airspeed was 12.0 m s^–1, which is not significantly different from minimum power speed (V _mp) in active flight or the speed for best glide (V _bg) used in gliding flight. However, the shearwaters showed a significant response to wind increment/decrement, indicating that they were not flying at V _mp, which should be unaffected by head and tailwind. Furthermore, shearwaters can potentially reduce induced drag by the ground effect while flying close to the sea surface at weak winds, which leads to a reduction in characteristic flight speed. We suspect that the predictions for gliding flight are most valid for shearwaters at moderate to high wind speeds, when they should be maximising distance by using V _bg. Audouin’s gulls used active flight exclusively, with a mean airspeed of 11.3 m s^–1 that was significantly different from the predicted V _mp. Interestingly, though, the gulls did not show any significant wind response, indicating that they were flying close to their true V _mp when foraging along the coast. Received: 17 May 2000 / Received in revised form: 21 November 2000 / Accepted: 8 January 2001     

Flight speeds of swifts (Apus apus): seasonal differences smaller than expected

We have studied the nocturnal flight behaviour of the common swift (Apus apus L.), by the use of a tracking radar. Birds were tracked from Lund University in southern Sweden during spring migration, summer roosting flights and autumn migration. Flight speeds were compared with predictions from flight mechanical and optimal migration theories. During spring, flight speeds were predicted to be higher than during both summer and autumn due to time restriction. In such cases, birds fly at a flight speed that maximizes the overall speed of migration. For summer roosting flights, speeds were predicted to be lower than during both spring and autumn since the predicted flight speed is the minimum power speed that involves the lowest energy consumption per unit time. During autumn, we expected flight speeds to be higher than during summer but lower than during spring since the expected flight speed is the maximum range speed, which involves the lowest energy consumption per unit distance. Flight speeds during spring were indeed higher than during both summer and autumn, which indicates time-selected spring migration. Speeds during autumn migration were very similar to those recorded during summer roosting flights. The general result shows that swifts change their flight speed between different flight behaviours to a smaller extent than expected. Furthermore, the difference between flight speeds during migration and roosting among swifts was found to be less pronounced than previously recorded.     

Flight speeds and climb rates of Brent Geese: mass-dependent differences between spring and autumn migration

Aerodynamic theories of bird flight predict that horizontal flight speed will increase with increasing load whereas vertical flight speed will decrease. Horizontal flight speed for birds minimizing overall time on migration is predicted to be higher than flight speed for birds minimizing energy expenditure. In this study we compare flight speeds of Brent Geese Branta b. bernicla recorded by tracking radar and optical range finder during spring and autumn migration in southernmost Sweden, testing the above-mentioned predictions. Geese passing Sweden in spring are substantially heavier than in autumn and there might also be a stronger element of time-selection in spring than in autumn. Recorded airspeeds were significantly higher in spring (mean 19.0 m s⁻¹) than in autumn (mean 17.3 m s⁻¹), the average difference being slightly larger than predicted due to the mass difference alone. The effects on airspeed of wind, vertical speed, flock size and altitude were also analysed, but none of these factors could explain the seasonal difference in airspeed. Hence, the results support the hypothesis of mass-dependent flight speed adjustment. The difference between the two seasons was not large enough to corroborate the hypothesis of a stronger element of time-selection in spring, but this hypothesis cannot be rejected. Vertical flight speeds were lower in spring than in autumn, supporting a negative effect of load on birds' flight power margin.     

Flight speeds among bird species: allometric and phylogenetic effects

Flight speed is expected to increase with mass and wing loading among flying animals and aircraft for fundamental aerodynamic reasons. Assuming geometrical and dynamical similarity, cruising flight speed is predicted to vary as (body mass)^1/6 and (wing loading)^1/2 among bird species. To test these scaling rules and the general importance of mass and wing loading for bird flight speeds, we used tracking radar to measure flapping flight speeds of individuals or flocks of migrating birds visually identified to species as well as their altitude and winds at the altitudes where the birds were flying. Equivalent airspeeds (airspeeds corrected to sea level air density, U_e) of 138 species, ranging 0.01–10 kg in mass, were analysed in relation to biometry and phylogeny. Scaling exponents in relation to mass and wing loading were significantly smaller than predicted (about 0.12 and 0.32, respectively, with similar results for analyses based on species and independent phylogenetic contrasts). These low scaling exponents may be the result of evolutionary restrictions on bird flight-speed range, counteracting too slow flight speeds among species with low wing loading and too fast speeds among species with high wing loading. This compression of speed range is partly attained through geometric differences, with aspect ratio showing a positive relationship with body mass and wing loading, but additional factors are required to fully explain the small scaling exponent of U_e in relation to wing loading. Furthermore, mass and wing loading accounted for only a limited proportion of the variation in U_e. Phylogeny was a powerful factor, in combination with wing loading, to account for the variation in U_e. These results demonstrate that functional flight adaptations and constraints associated with different evolutionary lineages have an important influence on cruising flapping flight speed that goes beyond the general aerodynamic scaling effects of mass and wing loading.     

Pop up satellite tags impair swimming performance and energetics of the European eel (Anguilla anguilla)

Pop-up satellite archival tags (PSATs) have recently been applied in attempts to follow the oceanic spawning migration of the European eel. PSATs are quite large, and in all likelihood their hydraulic drag constitutes an additional cost during swimming, which remains to be quantified, as does the potential implication for successful migration. Silver eels (L_T = 598.6±29 mm SD, N = 9) were subjected to swimming trials in a Steffensen-type swim tunnel at increasing speeds of 0.3–0.9 body lengths s⁻¹, first without and subsequently with, a scaled down PSAT dummy attached. The tag significantly increased oxygen consumption (MO₂) during swimming and elevated minimum cost of transport (COT_min) by 26%. Standard (SMR) and active metabolic rate (AMR) as well as metabolic scope remained unaffected, suggesting that the observed effects were caused by increased drag. Optimal swimming speed (U _opt) was unchanged, whereas critical swimming speed (U _crit) decreased significantly. Swimming with a PSAT altered swimming kinematics as verified by significant changes to tail beat frequency (f), body wave speed (v) and Strouhal number (St). The results demonstrate that energy expenditure, swimming performance and efficiency all are significantly affected in migrating eels with external tags.     

Avoidance of headwinds or exploitation of ground effect—why do birds fly low?

ABSTRACT Birds often fly close to the ground or water. Wind shear theory predicts that wind speeds decline with proximity to the substratum, so birds might be expected to fly lower when flying upwind than when flying downwind. We tested this prediction and found that the wind shear equation is valid at heights below 4 m, with wind speed over a smooth surface ～40% lower at a height of 0.08 m than at 4 m. Birds that fly close enough to smooth substrata can also benefit energetically from ground effect, where vortices generated by their flight interact with the ground or water. This suggests that birds should use ground effect more when flying upwind than when flying downwind. We determined the percent time spent flying in ground effect by 21 species of passerine and non‐passerine birds flying in sheltered coastal aquatic and nearby terrestrial areas of County Cork, Ireland. We found that use of ground effect was uncommon for passerines, but common for a variety of non‐passerine waterbirds. However, phylogenetic analysis indicates no linkage between phylogeny and incidence of ground effect use and it is probable that incidence of use is determined by ecology rather than phylogeny. Great Cormorants (Phalacrocorax carbo) used ground effect most frequently over water (59.4% of time in flight). Over land, Barn Swallows (Hirundo rustica) used ground effect most often (19.8% of time). Phylogenetic contrasts regression analysis showed no significant relationship between use of ground effect and either wing aspect ratio or wing loading for 18 of our focal species, though simple linear regression analysis indicated that birds with greater wing loading used ground effect slightly (but significantly) more often. We found that 95% of Great Cormorants flying upwind used ground effect whereas only 35% did so when flying downwind. Few Black‐headed Gulls (Chroicocephalus ridibundus) used ground effect (probably because they fly high to locate prey), but still showed greater use when flying upwind (25%) than downwind (2.5%). When flying upwind in ground effect at the wind speeds encountered in our study, the velocity for minimum power (V_mp) for Great Cormorants was exceeded, suggesting theoretical benefits of about 14.3%. Our study indicates that several species exploit both wind shear and ground effect to minimize energy expenditure during commuting and foraging, but that others do not, because of either complexity of habitat morphology or the demands of their foraging ecology.     

Energetics of free existence in swallows and martins (hirundinidae) during breeding: a comparative study using doubly labeled water

Energy metabolism of three sympatric swallows (Hirundinidae) was investigated during the breeding season using doubly labeled water (²H₂ ¹⁸O). Interspecific and intraspecific differences in energy metabolism were examined in relation to the habits, size and environment of the birds. To facilitate comparisons we expressed energy metabolism (M) as the ratio of average daily metabolic rate (ADMR, cm³CO_2g ^-1h^-1) to basal metabolic rate (BMR). We observed adults during incubation and when feeding nestlings. Then, both sexes of Sand Martins Riparia riparia and House Martins Delinchon urbica were either at the nest or on the wing. Incubation reduced activity levels during the day resulting in M (incubation) being 17–26% lower than during rearing. Differences in energy costs for rearing chicks depended mainly on flight behaviour, the smaller Sand Martin doing nearly twice as much flapping during flight as the House Martin, giving higher values for M. In Swallows Hirundo rustica the female incubates alone, alternating between short feeding trips and incubating in daytime. This pattern was linked with a relatively high value for M in the only individual behaving like our controls. Both sexes of Swallows feed the chicks, and they showed similar values of M. They also closely resembled House Martins, despite contrasts in the time spent flying and their behaviour during flight. Feeding conditions affected activity, and thereby M, in a species specific way. The House Martin did more gliding in poor weather, taking less mobile prey, reducing M. Swallows reduced foraging costs further by using body reserves, as in the House Martin. The smaller Sand Martin, in contrast, showed a high expenditure in poor weather. Over two breeding seasons ADMR reached values around 5 BMR for all three species.     

Flight modes in migrating European bee-eaters: heart rate may indicate low metabolic rate during soaring and gliding

Background

Many avian species soar and glide over land. Evidence from large birds (m _b>0.9 kg) suggests that soaring-gliding is considerably cheaper in terms of energy than flapping flight, and costs about two to three times the basal metabolic rate (BMR). Yet, soaring-gliding is considered unfavorable for small birds because migration speed in small birds during soaring-gliding is believed to be lower than that of flapping flight. Nevertheless, several small bird species routinely soar and glide.

Methodology/Principal Findings

To estimate the energetic cost of soaring-gliding flight in small birds, we measured heart beat frequencies of free-ranging migrating European bee-eaters (Merops apiaster, m _b∼55 g) using radio telemetry, and established the relationship between heart beat frequency and metabolic rate (by indirect calorimetry) in the laboratory. Heart beat frequency during sustained soaring-gliding was 2.2 to 2.5 times lower than during flapping flight, but similar to, and not significantly different from, that measured in resting birds. We estimated that soaring-gliding metabolic rate of European bee-eaters is about twice their basal metabolic rate (BMR), which is similar to the value estimated in the black-browed albatross Thalassarche (previously Diomedea) melanophrys, m _b∼4 kg). We found that soaring-gliding migration speed is not significantly different from flapping migration speed.

Conclusions/Significance

We found no evidence that soaring-gliding speed is slower than flapping flight in bee-eaters, contradicting earlier estimates that implied a migration speed penalty for using soaring-gliding rather than flapping flight. Moreover, we suggest that small birds soar and glide during migration, breeding, dispersal, and other stages in their annual cycle because it may entail a low energy cost of transport. We propose that the energy cost of soaring-gliding may be proportional to BMR regardless of bird size, as theoretically deduced by earlier studies.     

Perch-hunting in insectivorous Rhinolophus bats is related to the high energy costs of manoeuvring in flight

Foraging behaviour of bats is supposedly largely influenced by the high costs of flapping flight. Yet our understanding of flight energetics focuses mostly on continuous horizontal forward flight at intermediate speeds. Many bats, however, perform manoeuvring flights at suboptimal speeds when foraging. For example, members of the genus Rhinolophus hunt insects during short sallying flights from a perch. Such flights include many descents and ascents below minimum power speed and are therefore considered energetically more expensive than flying at intermediate speed. To test this idea, we quantified the energy costs of short manoeuvring flights (<2 min) using the Na-bicarbonate technique in two Rhinolophus species that differ in body mass but have similar wing shapes. First, we hypothesized that, similar to birds, energy costs of short flights should be higher than predicted by an equation derived for bats at intermediate speeds. Second, we predicted that R. mehelyi encounters higher flight costs than R. euryale, because of its higher wing loading. Although wing loading of R. mehelyi was only 20% larger than that of R. euryale, its flight costs (2.61 ± 0.75 W; mean ± 1 SD) exceeded that of R. euryale (1.71 ± 0.37 W) by 50%. Measured flight costs were higher than predicted for R. mehelyi, but not for R. euryale. We conclude that R. mehelyi face elevated energy costs during short manoeuvring flights due to high wing loading and thus may optimize foraging efficiency by energy-conserving perch-hunting.     

Ectoparasites and host energetics: house martin bugs and house martin nestlings

We measured the energy cost of ectoparasitism in an experimental study of the house martin bug Oeciacus hirundinis and its main host, nestlings of the house martin Delichon urbica. Nests were randomly assigned to inoculation with 0 (control) 10, or 100 bugs during egg laying, and this resulted in statistically significant differences in parasite loads following fledging of nestlings. Parasite loads negatively affected house martin nestlings as estimated from their body mass at age 16 days and from mass loss estimated over 1 day late in the nestling period. Daily energy expenditure (KJ/d), average daily metabolic rate (ml CO₂/g h), and mass independent daily energy expenditure (kJ/mass^0.67d) did not differ significantly between experimental treatments. However, average daily metabolic rate increased with increasing intensities of ectoparasitism. Mass independent daily energy expenditure also increased with higher levels of parasite infestation. These results demonstrate that the bug imposes an energy cost on its host by elevating the level of metabolism.     

Effects of fasting and feeding on the fast‐start swimming performance of southern catfish Silurus meridionalis

This study investigated the effects of fasting and feeding on the fast‐start escape swimming performance of juvenile southern catfish Silurus meridionalis, a sit‐and‐wait forager that encounters extreme fasting and famine frequently during its lifespan. Ten to 30 days of fasting resulted in no significant change in most of the variables measured in the fast‐start response except a 20–30% decrease in the escape distance during the first 120 ms (D_120ms) relative to the control group (48 h after feeding). The ratio of the single‐bend (SB) response (lower energetic expenditure) to the double‐bend (DB) response increased significantly from 0% in the control group to 75 and 82·5% in the 20 and 30 day fasting groups, respectively. Satiated feeding (25% of body mass) resulted in a significantly lower (36·6%) maximum linear velocity (V_max) and a significantly lower (43·3%) D_120ms than in non‐fed fish (control group, 48 h after feeding). Half‐satiated feeding (12·5% of body mass), however, showed no significant effects on any of the measured variables of the fast‐start response relative to control fish. It is suggested that the increase in the ratio of SB:DB responses with fasting in S. meridionalis may reflect a trade‐off between energy conservation and maintaining high V_max, while variables of fast‐start performance were more sensitive to feeding than fasting might be an adaptive strategy to their foraging mode and food availability in their habitat.     

Energetic limitation of avian parental effort: Field experiments in the kestrel (Falco tinnunculus)

We studied the limiting factors for brood size in the kestrel, Falco tinnunculus, by measuring parental effort in natural broods of different size and parental response to manipulation of food satiation of the brood. Parental effort was quantified as total daily time spent in flight, and total daily energy expenditure, from all-day observations. During nestling care males with different natural brood sizes (4 to 7 chicks), spent an average of 4.75 h · d^?1 in flight independent of brood size, and expended an average total daily energy of 382 kJ · d^?1. Due to a higher flight-hunting yield (mammal-prey caught per hour hunting), males with larger natural broods were able to provision their broods with the same amount of food (mainly Microtus arvalis) per chick (62.6 g · d^?1), with the same effort as males with smaller broods. This provisioning rate was close to the mean feeding rate of hand-raised chicks in the laboratory, that were fed ad libitum, (66.8 g · d^?1 · chick^?1). Our food deprivation experiments revealed that male kestrels strongly respond to food shortage in the nest. In the older nestling phase males on average increased their daily rate of food delivery to the nest as a response to experimental food deprivation by almost three times to 646.4 g · d^?1, by increasing their flight activity level from 4.46 to 8.41 h · d^?1. This increased energy expenditure was sustained, for as long as eleven days, by increasing the metabolizable energy intake up to what is presumed to be the maximum rate. Even under considerable experimental food stress (chicks not being satiated due to continuous removal of delivered food by the observers) about half of the available daylight time remained unused for foraging. We conclude 1) that the mean daily energy expenditure of males during nestling care — to which clutch size is apparently initially adjusted — is well below the maximum they are able to sustain and 2) that the energy expenditure they can sustain under extremely high nestling demand is not set by the available time for foraging or the available energy in the environment. Thus the birds normally operate well below their presumed maximum, and only during food shortage, e.g., as caused by our experiments, do they increase activity up to this maximum. Therefore we conclude that the kestrels have costs other than energy expenditure, such as parental survival, that are involved in the increased “cost” of parental effort. We discuss possible generalisations about existing energetic limitations during parental care in altricial birds. From published estimates of daily energy expenditure during parental care (DEE_par) in 30 different bird species we derived the equation: DEE_par = 14.26 kg^0.65 Watt. This relationship differs significantly in slope (T = 2.49; p > 0.02) from the allometric equation for the maximum rate of energy assimilation (DME_max) as provided by Kirkwood (1983): DME_max = 19.82 kg^0.72 Watt. In smaller species (ca. 25 g) DEE_par about equals DME_max, while in the larger species (ca. 10 kg) DEE_par represents only about 60% of the predicted DME_max. This suggests that limitations in parental effort are more frequently set by the maximum sustainable energy intake in the smaller species than in larger species. Our allometric equations for DEE_par suggests that the relation between BMR, estimated using the equations of Aschoff and Pohl (1970), and the observed parental energy expenditure, is such that on average bird parents work at a daily level somewhere between 3 and 4 times BMR.     

Rush and grab strategies in foraging marine endotherms: the case for haste in penguins

The speed at which air-breathing marine predators that forage by diving should swim is likely to depend on a variety of factors that differ substantially from those relevant in animals for which access to oxygen is unlimited. We used loggers attached to free-living penguins to examine the speed at which three species swam during periods searching for prey and compared this to their speeds during actual prey pursuit. All penguin species appeared to travel at similar speeds around 2 m/s during normal commuting between the surface and feeding depths, which accords closely with minimum costs of transport. However, Adélie penguins, Pygoscelis adeliae, slowed down to feed, Magellanic penguins, Spheniscus magellanicus, speeded up and king penguins, Aptenodytes patagonicus, travelled at a variety of speeds, although mean speed did not change from normal commuting. Since energy expenditure, and therefore oxygen usage, in swimming animals increases with the cube of the speed, we hypothesized that prey escape speed (a function of prey size) and prey density would prove critical in determining optimum pursuit speeds in predators. Simple models of this type help explain why it is that some penguin species apparently benefit by increasing speed to capture prey while others benefit by decreasing speed.     

Testing optimal foraging models for air-breathing divers

Models of diving optimality qualitatively predict diving behaviours of aquatic birds and mammals. However, none of them has been empirically tested. We examined the quantitative predictions of optimal diving models by combining cumulative oxygen uptake curves with estimates of power costs during the dives of six tufted ducks, Aythya fuligula. The effects of differing foraging costs on dive duration and rate of oxygen uptake (V_O2up) at the surface were measured during bouts of voluntary dives to a food tray. The birds were trained to surface into a respirometer after each dive, so that changes in V_O2up over time could be measured. The tray held either just food or closely packed stones on top of the food to make foraging energetically more costly. In contrast to predictions from the Houston & Carbone model, foraging time (t_f) increased after dives incorporating higher foraging energy costs but surface time (t_s) remained the same. While optimal diving models have assumed that the cumulative oxygen uptake curve is fixed, V_O2up increased when the energy cost of the dive increased. The optimal breathing model quantitatively predicted ts in both conditions and oxygen consumption during foraging (m₂t_f) in the control condition, for the mean of all ducks. This offers evidence that the ducks were diving optimally and supports the fundamentals of optimal diving theory. However, the model did not consistently predictt_s or m₂t_f for individual birds. We discuss the limits of optimal foraging models for air-breathing divers caused by individual variation. Copyright 2003 Published by Elsevier Science Ltd on behalf of The Association for the Study of Animal Behaviour.      

Effect of altitude on male parental expenditure in Mountain Bluebirds ( Sialia currucoides ): are higher-altitude males more attentive fathers?

Male investment of time and energy in caring for offspring varies substantially both between and within bird species. Explaining this variation is of long-standing interest to ornithologists. One factor that may affect male care is breeding site altitude, through its effects on climate. The harsher, less predictable abiotic conditions at higher altitudes are hypothesized to favour increased male investment of time and energy in offspring care. We tested this hypothesis by comparing male parental behaviour in Mountain Bluebirds (Sialia currucoides) nesting at 1500 and 2500 m a.s.l. in the Bighorn Mountains of Wyoming, USA. We compared rates of prey delivery to nestlings at these two altitudes at two times: 1–2 days after hatching, when females spend much of their time brooding young, and 12–13 days later, when brooding has ended and nestling energy demands are peaking. High-altitude males fed nestlings 18 and 28% more often than low-altitude males early and later in the nestling stage, respectively, but only the difference in late-stage feeding rates were significant. Like males, females at the high site also fed nestlings significantly more often than females at the low site later in the nestling stage (45% difference in feeding rates). Consequently, the proportion of all feeding trips made by males at the high site (40%) did not differ significantly from that at the low site (44%). Parents at the high altitude may feed nestlings more often to compensate for their greater thermoregulatory costs. Parents may also be attempting to assist nestlings in storing fat and/or attaining a large size and effective homeothermy as quickly as possible to enhance nestling ability to survive bouts of severe weather which are common at high altitudes.     

The metabolic cost of changing walking speeds is significant,implies lower optimal speeds for shorter distances,and increases daily energy estimates

Humans do not generally walk at constant speed, except perhaps on a treadmill. Normal walking involves starting, stopping and changing speeds, in addition to roughly steady locomotion. Here, we measure the metabolic energy cost of walking when changing speed. Subjects (healthy adults) walked with oscillating speeds on a constant-speed treadmill, alternating between walking slower and faster than the treadmill belt, moving back and forth in the laboratory frame. The metabolic rate for oscillating-speed walking was significantly higher than that for constant-speed walking (6–20% cost increase for ±0.13–0.27 m s⁻¹ speed fluctuations). The metabolic rate increase was correlated with two models: a model based on kinetic energy fluctuations and an inverted pendulum walking model, optimized for oscillating-speed constraints. The cost of changing speeds may have behavioural implications: we predicted that the energy-optimal walking speed is lower for shorter distances. We measured preferred human walking speeds for different walking distances and found people preferred lower walking speeds for shorter distances as predicted. Further, analysing published daily walking-bout distributions, we estimate that the cost of changing speeds is 4–8% of daily walking energy budget.