Optimal Fat Loads in Migrating Birds: A Test of the Time-Minimization Hypothesis Author(s): Ake Lindstrom and Thomas Alerstam Reviewed work(s): Source: The American Naturalist, Vol. 140, No. 3 (Sep., 1992), pp. 477-491 Published by: The University of Chicago Press for The American Society of Naturalists Stable URL: http://www.jstor.org/stable/2462777 . Accessed: 18/01/2012 05:48 Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at . http://www.jstor.org/page/info/about/policies/terms.jsp JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. The University of Chicago Press and The American Society of Naturalists are collaborating with JSTOR to digitize, preserve and extend access to The American Naturalist. http://www.jstor.org

Vol. 140, No. 3 The American Naturalist September 1992 OPTIMAL FAT LOADS IN MIGRATING BIRDS: A TEST OF THE TIME-MINIMIZATION HYPOTHESIS 0 AKE LINDSTROM AND THOMAS ALERSTAM Department of Ecology, Animal Ecology, University of Lund, Ecology Building, S-223 62 Lund, Sweden Slubmnitted Jul/v 2, 1990 Revised October 9, 1991 Accepted October 18, 1991 Abstract.-We tested the hypothesis that birds are selected to minimize the time spent on migration, that is, to migrate as fast as possible. Optimal fat loads in time-selected migration were predicted for different rates of fat accumulation at stopover sites. We analyzed departure fat loads of migrating bluethroats Lluscinia sivecica sivecica, experimentally provided with extra food at a stopover site, and of migrating rufous hummingbirds Selasphorlls rulfius, which showed considerable individual variation in fat-deposition rate, in relation to these predictions. We found qualitative agreement with the time-minimization hypothesis. However, quantitative agreement requires that specific assumptions be fulfilled for both species: (1) consistent differences in expected speed of migration should exist between different individuals of the same species and/ or (2) the expected speed of migration should increase along the route. Both of these assumptions are probably valid, and ringing data suggest an increase in bluethroat autumn migration speed along the route. Physiological and flight mechanical constraints will prevent birds from depos- iting excessively large amounts of fuel. These assumptions and constraints should be taken into account in future critical tests of the hypothesis that natural selection operates to maximize the speed of migration. The optimal way for a migrating bird to safely reach its destination within the appropriate time differs depending on the selective forces that act on the birds (Alerstam and Lindstrom 1990). Selection may have favored birds that minimize energy expenditure they keep flight costs low by storing only as much fat as is needed to reach the next fat-deposition site. However, birds may also be selected to minimize the time spent on migration, that is, to migrate as fast as possible (Carpenter et al. 1983). It may be advantageous to arrive at the destination before competitors, so as to obtain good territories, both for breeding (von Haartman 1968) and for wintering (Price 1981), or to spend as little time as possible in unknown areas along the migration route. A third possible selective force in bird migration is predation. If there is a significant predation pressure during migration (Rudebeck 1950-1951 Walter 1979 Lindstrom 1989), the optimal be- havior may be to minimize the overall predation risk during the migratory journey (cf. Lindstrom 1990). Numerous studies of migrating birds have focused on fat-deposition rates, stop- over durations, and fat loads at departure (reviews by Odum 1960 Berthold 1975 Blem 1980 Lindstrom 1986). Using an optimality approach, we (Alerstam and Lindstrom 1990) predicted patterns of stopover length, fat load at departure, Am. Nat. 1992. Vol. 140, pp. 477-491. ? 1992 by The University of Chicago. 0003-()147/92/4003-0006$02.00. All rights reserve(d.

478 THE AMERICAN NATURALIST y a) Sinst 0 0)~~~~~~~~~~ 0)~~~~~~~~~~~~~~0 0)-~~~~~~~~ ~0 f f Fat load Fat load FIG. 1.-a, Because of higher flight costs with added body mass, the flight range, Y, of a bird is a negatively accelerated function of its fat load, f (eq. [1]). b, A bird depositing fat reserves for a migratory flight can be considered to achieve a certain speed of migration since its potential flight range increases with the time spent on fattening. The instantaneous speed of migration, Sinst (the marginal rate of gain in potential distance during fat deposition), decreases with increasing fat load (eq. [2]) because the value of additional fat decreases with increasing body mass (cf. eq. [1]). response to different fat-deposition rates, and habitat selection in migrating birds. Different predictions emerged when assuming different optimization criteria, that is, minimization of time, energy expenditure, or predation risk. In this study we set out to test the time-minimization hypothesis. A crucial term, which we will use frequently in this article, is "speed of migration" (S), measured in km/d, the total time it takes to cover the migration distance, including the time used for the necessary buildup of energy reserves before and between the successive migra- tory flights. PREDICTIONS OF OPTIMAL FAT LOADS From flight-mechanical theory (Pennycuick 1975) it can be derived that the potential flight range, Y, is a concave function of a bird's fat load, f (ratio of fat mass to lean body mass [LBM]), according to the relationship y = c[l - (1 + f)- 1/2], (1) where c is a constant that differs between species and populations depending on the birds' flight morphometrics and the energy value of the fuel. Hence, our general reasoning and predictions apply equally well for all types of fuel, not only fat, used by migrating birds as long as different types of fuel are deposited in the same proportions throughout the relevant part of the stopover period (see below). Because the major part of birds' fuel reserves usually consists of fat, we use "fat" as a synonym for "fuel" throughout this article. The concave relationship of equation (1) is due to the decreasing utility of additional fat with increasing fat loads because of the higher flight costs connected with added load (fig. la). It should be noted that the predicted effect of increasing fat load on expected migration range (fig. la) depends on the assumption that the bird always flies at the optimal speed, which increases as the total mass increases (Pennycuick 1975).