Autumn temperatures at African wintering grounds affect body condition of two passerine species during spring migration

Abstract

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.

Fig 1. Boxplots of weight upon spring arrival in Eilat, Israel, of Sylvia curruca.

1998 weight range, marked in shaded gray, was excluded from the analysis. The central box represents the central 50% of observations for each year, where the central line of the box represents the median, and the box lower and upper edges represent the first and third quartiles respectively. The whiskers represent the length of 1.5 times the interquartile range (IQR).

Fig 2. Boxplots of weight upon spring arrival in Eilat, Israel, of Phylloscopus orientalis.

1998 weight range, marked in shaded gray, was excluded from the analysis. The central box represents the central 50% of observations for each year, where the central line of the box represents the median, and the box lower and upper edges represent the first and third quartiles respectively. The whiskers represent the length of 1.5 times the interquartile range (IQR).

Fig 3. Mean yearly body state of the lesser whitethroat, Sylvia curruca, upon spring arrival in Eilat, Israel, as a function of autumn maximum temperature at African wintering grounds, 1984–2013 (1998 excluded).

Fig 4. Mean yearly body state of the eastern Bonelli’s warbler, Phylloscopus orientalis, upon spring arrival in Eilat, Israel, as a function of autumn maximum temperature at African wintering grounds, 1983–2013 (1998 excluded).