Climatic factors acting in both breeding and wintering areas

Sometimes migratory bird populations are influenced throughout their range by widescale climatic fluctuations, such as the El Nino-Southern Oscillation (ENSO). Climatic shifts of this type can act simultaneously on local weather conditions in widely separated breeding, migration and wintering areas. Changes in some bird populations have been linked to such grand-scale climatic events.1 For example, in the Black-throated Blue Warbler Dendroica caerulescens, reproduction in New Hampshire nesting areas and survival in Jamaican wintering areas were both

1El Niño and La Niña represent the extreme phases of the Southern Oscillation, causing year-by-year variations in atmospheric and oceanic conditions in the equatorial Pacific. These conditions in turn have significant consequences for surface temperatures and rainfall worldwide, which in turn modulate the productivity of many terrestrial and aquatic ecosystems. ENSO conditions can be quantified by use of the standardised ENSO index, which is measured as the difference in sea surface temperatures between Darwin (Australia) and Tahiti. High values indicate La Niña conditions and low (negative) values indicate El Niño conditions. This index has proved to be a robust measure for tracking ENSO changes.

lower during El Niño periods than during the intervening La Niña periods. In this species, therefore, population fluctuations were linked to local conditions in both areas (and probably also on migration routes) that were in turn influenced by the same underlying large-scale climatic cycle, and its effects on food supplies (which were measured in breeding areas) (Sillett et al. 2000). Moreover, good breeding seasons were followed by increased recruitment of new individuals into the wintering and breeding populations. Similar associations were found in the annual survival and reproduction of the Yellow Warbler Dendroica petechia studied over 10 years in Manitoba breeding areas (Mazerolle et al. 2005). These birds wintered in Central America and northeastern South America.

The effects of ENSO on regional weather patterns vary across a continent. Some regions during El Niño conditions receive more-than-average rainfall, while other regions receive less-than-average rainfall. The effects of ENSO on bird food supplies, and on the population dynamics of the birds themselves, would therefore be expected to differ regionally. In contrast to the studies just mentioned, but in association with local weather patterns, Neotropical migrants breeding in the Pacific Northwest had lower (rather than higher) reproduction in El Niño years (Nott et al. 2002). Together these various studies demonstrate the role of ENSO in the population dynamics of Nearctic-Neotropical migrants, and highlight the geographical variation in bird responses, depending on the differing effects of ENSO on regional weather.

The same trends in weather do not necessarily have the same effects on both breeding and wintering areas. Among Sand Martins Riparia riparia in England, annual survival rates increased with increase in rainfall in the dry African wintering areas, and decreased with increase in rainfall in the wetter breeding areas (Cowley & Siriwardena 2005). Heavy rain in summer also led to greater-than-usual starvation among the young. In the arid wintering areas, relatively high rainfall was assumed to promote greater vegetation growth and insect populations at the time the martins were there, whereas in the English breeding areas rain reduced insect activity and prevented adults from foraging effectively, reducing their body condition. Their subsequent survival in migration or wintering areas was then reduced. Rainfall in both areas thus affected year-to-year population changes, but in opposite directions.

If we can assume that breeding rates were influenced primarily by conditions in breeding areas, and mortality rates primarily by conditions in wintering areas (unless otherwise specified), then 28 of the bird populations in Table 26.1 were winter-limited, another 35 or more were summer-limited, while four were influenced by both summer and winter conditions. However, because both the extent and carrying capacities of habitats vary from year to year, and from area to area, the same species might be winter-limited in some years or areas, and summer-limited in other years or areas, as in the different Willow Warbler Phylloscopus trochilus populations in Table 26.1. Each case must be judged on its particular circumstances.

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