Date Published: May 29, 2019
Publisher: Public Library of Science
Author(s): Irith Aloni, Shai Markman, Yaron Ziv, Michelangelo Morganti.
Most papers on the physical condition of birds during spring migration focused on food availability preceding migratory take-off. Only a few studies examined the effect of climate conditions at the wintering grounds upon autumn arrival on bird physical condition later on. Here, we hypothesized that environmental conditions upon arrival at the wintering grounds, and not necessarily upon departure, have a crucial carry-over effect on bird spring migration. Using 29,000 observations of the lesser whitethroat, Sylvia curruca, and the eastern Bonelli’s warbler, Phylloscopus orientalis, we found temperatures upon arrival at the African wintering grounds to be the only climatic variable correlated with birds’ body state upon spring stopover in Israel, six months later. Two different mechanisms could explain these results. One possibility is that high temperatures create favorable conditions for insect activity, which allows rapid recovery from autumn migration and hence successful winter survival and maintenance. Another possible scenario is that harsh conditions, due to the heat and dry environment, cause high mortality, permitting survival of larger individuals which, then, enjoy reduced inter- and intra-specific competition. Whatever the mechanism is, our findings suggest that conditions upon autumn arrival, and not necessarily at the end of winter as traditionally thought, may have a major impact on migrating birds.
In an extensive review of the reasons for recent declines in migratory bird populations, Vickery et al.  point out habitat loss and degradation and climate change as the most influential factors. Many studies in recent years have focused on the effect of changing climate on bird migration and survival [1–8]. Migrating birds, particularly long-distance migrants, are more susceptible to environmental changes than their resident counterparts due to their complicated life cycle, which relies on various habitats and locations at different times along the year [1,9–10]. Food availability at wintering grounds has been pointed out by several researchers as a key factor affecting the birds’ physical condition and their ability to complete spring migration successfully and timely [3,6,9,11–17]. Additionally, many studies suggest that favorable conditions during winter induce advanced migration in spring, which potentially increases the chances to breed successfully [2–3,11,17–20]. Moreau  pointed out that numerous Afro-Palaearctic migrants wintering in the Sahel experience deteriorating ecological conditions along their stay due to the semiarid climate. The rainy season in the eastern Sahel takes place in summer, just before the arrival of the migrating birds, and the area dries up during the birds’ overwintering period. Moreau emphasized the difficulty that birds face as they have to prepare for spring migration when conditions are worst. This phenomenon is known as the Moreau’s Paradox .
We obtained all bird data from the International Birding and Research Centre in Eilat (IBRCE), Israel. The IBRCE is managed by the Israel Nature and Parks Authority, the very same authority that provides bird ringing licenses. Therefore, all people who ring birds at the IBRCE must hold a valid ringing license. The data we used was all from regular ringing procedures, in which birds are captured, ringed, measured, and immediately released. There is no requirement for an ethic committee approval for regular bird ringing.
Correlation analysis of climate conditions at the African wintering grounds and bird body state and weight upon spring arrival in Eilat, Israel, demonstrated a similar pattern for both S. curruca and P. orientalis. We found statistically significant and positive correlations at the 0.01 and 0.05 levels between autumn temperatures at the wintering grounds and both bird body state and weight (Table 1). For S. curruca, correlations at these levels were observed for September, October and November, whereas for P. orientalis we found such correlations only in October. Maximal daily temperature showed the highest correlations in most cases. Neither of the temperature variables during later months, nor any of the precipitation variables showed any significant correlation with body state or weight (Tables 1 & 2).
For both the lesser whitethroat and the eastern Bonelli’s warbler, we found a highly significant positive relationship between autumn temperatures at the wintering grounds and bird body state and weight upon spring arrival in Eilat, Israel. These findings strongly support our carry-over hypothesis stating that environmental conditions upon autumn arrival at the wintering grounds can have a crucial impact on bird physical condition throughout the wintering season and into spring migration. The correlations between November temperatures and body state and weight for the lesser whitethroat (Table 1) further support this notion, as autumn migration of this species lingers into November . This idea of carry-over effect of autumn temperatures is further sustained by the absence of any correlations between either temperatures of later months or any of the precipitation variables with body state or weight for any of the species (Tables 1 & 2). These results contradict the traditional presumption that conditions immediately preceding departure from wintering grounds are of upmost importance [1,3,13,15], at least for these two species.