Adult vultures outperform juveniles in challenging thermal soaring conditions.

Roi Harel,Nir Horvitz,Ran Nathan

doi:10.1038/srep27865

Abstract

Due to the potentially detrimental consequences of low performance in basic functional tasks, individuals are expected to improve performance with age and show the most marked changes during early stages of life. Soaring-gliding birds use rising-air columns (thermals) to reduce energy expenditure allocated to flight. We offer a framework to evaluate thermal soaring performance, and use GPS-tracking to study movements of Eurasian griffon vultures (Gyps fulvus). Because the location and intensity of thermals are variable, we hypothesized that soaring performance would improve with experience and predicted that the performance of inexperienced individuals (<2 months) would be inferior to that of experienced ones (>5 years). No differences were found in body characteristics, climb rates under low wind shear, and thermal selection, presumably due to vultures’ tendency to forage in mixed-age groups. Adults, however, outperformed juveniles in their ability to adjust fine-scale movements under challenging conditions, as juveniles had lower climb rates under intermediate wind shear, particularly on the lee-side of thermal columns. Juveniles were also less efficient along the route both in terms of time and energy. The consequences of these handicaps are probably exacerbated if juveniles lag behind adults in finding and approaching food.

Highlights

Low performance in basic functional tasks can have detrimental consequences for individuals and might explain the relatively high mortality rates in juveniles often seen in nature[1,2]
We consider three basic components required for mastering this art: (1) Thermal selection – Since thermals vary in their intensity, duration, and shape across time and space, it is important for the bird to select an efficient path in terms of climbing in thermals, and decide which thermals to utilize
Soaring-gliding efficiency is defined as the inter-thermal displacement gained per given climbing time while soaring in thermals[25,26], whereas, flapping flight has a dramatic effect on energy balance because it requires high energy expenditure compared to soaring-gliding flight[27]

Summary

Introduction

Low performance in basic functional tasks can have detrimental consequences for individuals and might explain the relatively high mortality rates in juveniles often seen in nature[1,2]. Despite a large and constantly increasing body of research on both interspecific[15,16,17,18,19] and intraspecific[11,20] differences in flight performance among soaring-gliding birds, only few studies have explored the role of age-related experience Even those that do[3,21] have drawn indirect inference on age-related experience effects based on relatively coarse movement data and without information on bird behavior, energy expenditure or environmental factors. The Risk-Averse Flight Index (RAFI) which is the ratio of actual to theoretical risk-averse gliding airspeed in inter-thermal gliding measures the level of risk aversion, more risk-prone flight with faster sink are indicted by lower RAFI values[17] These components may have consequences on larger scale properties of movement and behavior, such as the efficiency of soaring-gliding flight and the tendency of individuals to use flapping flight. To investigate the potential consequences of age-related differences in thermal soaring performance, we examined age-related differences in larger-scale properties of the observed foraging trips and predicted that adults will exhibit higher soaring-gliding efficiency, less flapping flight, and lower energy expenditure during flight

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Results

Conclusion