Reuven Yosef

Avian migrants adjust migration in response to environmental conditions en route

The onset of migration in birds is assumed to be primarily under endogenous control in long-distance migrants. Recently, climate changes appear to have been driving a rapid change in breeding area arrival. However, little is known about the climatic factors affecting migratory birds during the migration cycle, or whether recently reported phenological changes are caused by plastic behavioural responses or evolutionary change. Here, we investigate how environmental conditions in the wintering areas as well as en route towards breeding areas affect timing of migration. Using data from 1984 to 2004 covering the entire migration period every year from observatories located in the Middle East and northern Europe, we show that passage of the Sahara Desert is delayed and correlated with improved conditions in the wintering areas. By contrast, migrants travel more rapidly through Europe, and adjust their breeding area arrival time in response to improved environmental conditions en route. Previous studies have reported opposing results from a different migration route through the Mediterranean region (Italy). We argue that the simplest explanation for different phenological patterns at different latitudes and between migratory routes appears to be phenotypic responses to spatial variability in conditions en route.

Resource dependence and territory size in loggerhead shrikes (Lanius ludovicianus)

Population declines of Loggerhead Shrikes (Lanius ludovicianus) have not been strongly correlated with the loss of preferred habitat (Bohall-Wood 1987, Brooks and Temple 1990). These findings suggest that factors other than breeding habitat are at least partially responsible and although, from time to time, researchers have searched for these other factors, no remedies for the situation have been found. U.S. Fish and Wildlife Service Breeding Bird Surveys (1966-1989) demonstrate that the Loggerhead Shrike is declining at a rate of about 5% per year continentwide. Of the 43 states censused during this period, shrike populations declined in 37 (86%; Robbins et al. 1986, Droege and Sauer 1990, Peterjohn and Sauer 1993). Although peninsular Florida, Oklahoma and New Mexico have been thought to be strongholds, Tyler (1992) and Yosef et al. (1993) have demonstrated that in both Oklahoma and peninsular Florida the species is declining at 5 to 9% per year. A number of factors have been implicated in the Loggerhead Shrikes decline (Busbee 1977, Anderson and Duzan 1978, Bystrak 1983, Lymn and Temple 1991). However, in the last decade of the 20th century, the continuing loss of foraging habitat and hunting perches to modern agricultural practices seems the most likely explanation (Novak 1989, Pittaway 1991). Shrikes are primarily sit-and-wait predators that hunt from conspicuous perch sites (Bohall-Wood 1987), and hunt on the wing only during adverse conditions (e.g. seasonal growth of annuals; Yosef and Grubb 1993), or when substantial numbers of prey are flying and can be caught easily. Loggerhead Shrike territories must include suitable hunting perches and a nest site. Although trees and bushes may be important for other activities (e.g. hiding from predators, roosting), their importance as hunting perches has been demonstrated in Northern Shrikes (L. excubitor) by Askham (1990) and Yosef (1993), and in Loggerhead Shrikes by Yosef and Grubb (1992). In the latter study, areas with sparse trees and fence posts were devoid of shrike territories. Furthermore, with the use of ptilochronology (Grubb 1989), Yosef and Grubb (1992) illustrated that territory size influenced the nutritional condition of the defending bird. Shrikes defending smaller territories were in better condition.

Conservation Commentary: Evaluation of the Global Decline in the True Shrikes (Family Laniidae)

The first International Shrike Symposium was held at the Archbold Biological Station, Lake Placid, Florida, from 11-15 January 1993. The symposium was attended by 71 participants from 23 countries (45% North America, 32% Europe, 21% Asia, and 2% Africa). The most exciting participation was that of a strong contingent of ornithologists from eastern Europe. In this commentary I present the points stressed at the Symposium and illustrate them with several examples as presented by the authors. The Symposium was convened to focus attention on, evaluate, and possibly recommend methods to reverse the worldwide decline of shrike populations. Many of the 30 species are declining, or have become extinct locally. Studies have focused mainly on the five species found closest to places where ornithologists live: Northern/Great Grey Shrike (Lanius excubitor); Red-backed Shrike (L. collurio); Loggerhead Shrike (L. ludovicianus); Lesser Grey Shrike (L. minor); and Woodchat Shrike (L. senator). Other than anecdotal observations, little is known about the other 25 species. A common theme of the Symposium was that most long-term studies of previously robust populations of shrikes have documented drastic population declines, mostly in the late 20th century. For example, Rothhaupt (1993) estimated that most central European countries have lost over 50% of their breeding populations of shrikes within the last 15 years. Regional status and declines.-In post-war Switzerland, shrike populations plummeted as human population increased from 4,700,000 in the 1950s to almost 6,500,000 today (Bassin 1993). Rapid economic development paralleled this demographic growth. In 1950, rural agricultural landscapes were still dominated by traditional livestock-breeding and cultivation methods, but extensive land-use changes have occurred following the adoption of modern mechanized practices. These changes resulted in sweeping landscape modifications that prevented populations of resident Northern Shrikes from rebounding after the severe winters of 1962-1963, 1983, and 1985-1986. Breeding populations of all four species of shrikes found primarily in rural areas have declined, and today in Switzerland only the Red-backed and Woodchat shrikes nest. The last nest of the Lesser Grey

Predator exaptations and defensive adaptations in evolutionary balance: No defence is perfect

SummaryThe lubber grasshopper,Romalea guttata, is large, aposematic, and extremely toxic. In feeding trials with 21 bird and lizard species, none were able to consume this chemically defended prey. Predators that attempted to eat lubbers, often gagged, regurgitated, and sometimes died. Loggerhead shrikes,Lanius ludovicianus, regularly impale this toxic prey in peninsular Florida. They, like other bird species, are unable to consume fresh lubbers. However, our tests show that they are able to consume lubbers if the prey are allowed to ‘age’ for 1–2 days. This suggests that lubber toxins degrade following death and that shrike impaling behaviour serves as a preadaptation for overcoming the toxic defences of this large and abundant prey. These results also imply that counter adaptations against chemical defences need not involve major morphological or metabolic specializations, but that simple behavioural traits can enable a predator to utilize toxic prey.

Stopover ecology of migratory Sedge Warblers (Acrocephalus schoenobaenus) at Eilat, Israel

Migrating Sedge Warblers Acrocephalus schoenobaenus were captured in Eilat, Israel, during their spring and autumn migration. Data on spring and autumn body masses, their interannual variation and the pattern of fuel accumulation were analysed. Body mass and body condition index significantly varied between the years of study in spring but not in autumn. This may be due to birds over-flying the area, but loop migration might also be involved. Sedge Warblers gain mass in Eilat, both in spring and in autumn. Birds in poor initial condition and those stopping over for a longer period gained more body mass faster. In spring, but not in autumn, the progress of the season was another important factor: late birds gained more body mass. The average rate of body mass gain was 0.179g day-1 ± SE = 0.026. This suggests that Eilat is an important staging area for Sedge Warblers in spring and to a smaller extent in autumn.

Longer is fatter: body mass changes of migrant Reed Warblers ( Acrocephalus scirpaceus ) staging at Eilat, Israel

Ecological barriers are the riskiest phases of the annual migrations for migratory birds. Comparatively little field data exists pertaining to the ability of migratory birds to prepare for the challenges of crossing ecological barriers, or their ability to recuperate afterward. Migrating Reed Warblers (Acrocephalus scirpaceus) were captured in Eilat, Israel, during their spring and autumn migrations. Data on spring and autumn body masses, their inter-annual variation, and the pattern of body mass increase were analysed. The birds show a significant inter-annual variation in their body mass and body condition index in both seasons, which is consistent with the data from other sites and for other passerine species. During stopovers, mass gain occurred in both seasons. Birds in poor initial condition, and those that stop over for a longer period of time, gained more body mass faster. In spring, but not in autumn, the progress of the season was also an important factor; late-arriving birds gained more fuel faster. The average rate of fuel gain was 0,157g·day−1 ± 0.018 SE.

Differences Between the Spring and Autumn Migration of the Red-Backed Shrike Lanius collurio: Record from the Eilat Stopover (Israel)

Abstract. Migratory Red-backed Shrikes were mist-netted during the spring and autumn migrations (n = 1031 individuals) from 1984 to 2001 in Eilat (Israel). In a similar pattern of trapping, more than four times as many shrikes were caught during autumn than in spring. Males migrated significantly earlier than females in spring but not during the autumn migration, which suggests that in males there is a stronger drive to reach their breeding territories early. In both seasons and between both sexes we did not find any significant relations between the body measurements of individuals and the time of passage. The spring migration was much shorter time than the autumn migration. This was expressed by the minimum stopover duration, as well as by the time when Red-backed Shrike occurred in Eilat. There are significant differences between wing chord length, body mass and fat scores between seasons. In autumn males had longer wings, and both sexes were heavier and in better condition than in spring. The data suggest that the differences are an adaptation to their having to cross the Sahara Desert. The birds spend a statistically significantly shorter period of time at the Eilat stopover site in spring than in autumn (median 1 ± 1.5 days vs 5 ± 6.5 days). In autumn, recaptured birds were 6.3% heavier than during the first capture. The change in body mass was significantly correlated to the duration of the stopover period. The results suggest that migration over desert is energetically costly and that Eilat is a very important stopover site for migrating Red-backed Shrikes.

Energetics and growth rate of northern shrike (Lanius excubitor) nestlings

Northern Shrikes (Lanius excubitor) breed in a variety of habitats, including deserts. Deserts are characterized by unpredictable food supplies, which can lead to a slow growth rate of nestlings. However, given that Northern Shrike males use prey from their caches to augment freshly caught prey in providing food for their mates and nestlings, we hypothesized that their nestlings do not have a slow growth rate, but one that is equivalent to that in other passerine nestlings from temperate areas. To test this hypothesis, we measured growth rates and energy use in Northern Shrike nestlings and fledglings. We also measured energy expenditure in two adult males that were attending nests. Growth rate of Northern Shrike nestlings was similar to that predicted for passerines in temperate areas and therefore our hypothesis was supported. However, metabolizable energy available in the cache amounted to only 7.2% of the total energy requirements of the nestlings or 4.2% of the total energy requirements of parents and nestlings during the nestling period. This suggested that other factors in addition to the cache were important in determining growth rate. These included (1) an extremely low maintenance energy requirement of the nestling; 30% of that predicted for a bird of its body mass when it weighed 10 g, which gradually increased to 70% at 50 g. This allowed for more of the energy intake to be used for growth and also reduced foraging costs of males; (2) the relatively low amount of body energy retained as a fraction of metabolizable energy intake, 0.15 to 0.16, indicating that more water per unit growth was incorporated than in other passerines. Energy expenditure of adult males was substantially lower than that predicted for a passerine of similar body mass. Males collected 4.83 times the energy that they consumed in order to provide their mates and offspring with food. Yet, their ratio of field metabolic rate to basal metabolic rate was only 2.33, the lowest reported value for adults feeding altricial young.

Differential sex- and age-related migration of Bluethroats Luscinia svecica at Eilat, Israel

This paper examines the phenology and biometrics of Bluethroats staging in the Eilat region. This is of special interest because of the extreme conditions with which this temperate zone breeding species has to contend because Eilat is a desert habitat and is the last green area before the crossing of the deserts in autumn or after it in spring. Data were collected during 20 spring and 18 autumn migration seasons in the years 1984–2003, and a total of 7,464 Bluethroat were recorded. The number of trapped birds was much higher in autumn than in spring. The majority of Bluethroats caught in both the autumn and spring migrations were juveniles. We found differences in sex ratio in the individual age classes only in the autumn wherein among both adults and juveniles, males were in greater numbers. We also found significant differences in the dates of ringed birds from different sex–age classes in the spring and in autumn migrations. In spring, males from both age classes were caught earlier than females. In autumn, adult birds arrived earlier than juveniles. We think that it is important to identify and conserve the high quality stopover habitats such as Eilat wherein not only Bluethroats have been shown to stopover but also several hundred other species.

Measuring Fat Mass in Small Birds by Dual‐Energy X‐Ray Absorptiometry

In numerous studies, body condition of animals trapped in the wild has been expressed either as a function of body mass or as a ratio of body mass to some linear body measurement, such as snout-vent length in mammals, or simply by visual assessment of stored fat (Brown 1996). However, using body mass or a body mass : length ratio is problematic in that neither measure truly accounts for body composition and, depending on the source of additional body mass, be it water, fat, carbohydrates, or protein, this method may yield different, even spurious, results. Visual estimates of fat reserves are qualitative at best. Despite these limitations, biologists rely on “body condition,” so measured, as a variable in many analyses, including animal decisions in game theory and other models (Elissa et al. 1999; Pasquet et al. 1999), which may lead to cascades of incorrect conclusions with serious theoretical and applied implications. Different direct and nondestructive methods have been used to measure body condition. These methods include the ratio of total body water to body mass (Karasov and Pinshow 1998), total body electrical conductivity (TOBEC; Walsberg 1988), or bioelectrical impedance analysis (BIA; Nyboer 1991). Each