Abstract
Migratory bird populations are often limited by food during the non-breeding season. Correlative evidence suggests that food abundance on territories varies among years in relation to rainfall, which affects plant productivity and arthropod biomass. At the Font Hill Nature Preserve in Jamaica, we used an irrigation experiment to test the hypothesis that rainfall affects the condition of wintering American redstarts (Setophaga ruticilla) via intermediate effects on plant productivity and arthropod abundance. Experimental plots were irrigated in late February and early March to simulate a mid-season pulse of 200 mm of rain. Irrigation maintained soil moisture levels near saturation and had immediate effects on plant productivity. Cumulative leaf abscission over the dry season was 50% lower on experimental plots resulting in greater canopy cover, and we observed significantly higher ground level shoot growth and the flushing of new leaves on about 58% of logwood (Haematoxylon campechianum) individuals. Arthropod biomass was 1.5 times higher on irrigated plots, but there was considerable inter-plot variability within a treatment and a strong seasonal decline in biomass. Consequently, we found no significant effect of irrigation on arthropod abundance or redstart condition. We suspect that the lack of an irrigation effect for taxa higher on the trophic chain was due to the small spatial scale of the treatment relative to the scale at which these taxa operate. Although redstart condition was not affected, we did observe turnover from subordinate to dominant territorial individuals on experimental plots suggesting a perceived difference in habitat quality that influenced individual behavior.
Highlights
Populations of Nearctic-Neotropical migratory songbirds are influenced by a multitude of factors that operate throughout the annual cycle [1], [2], [3], [4]
Irrigation ceased in mid-March and soil dryness continued to increase on both experimental and control plots until rainy conditions started in late April (Figure 1)
Effects of Irrigation on Plant Productivity Irrigation increased soil moisture levels on the experimental plots, which were approximately maintained at saturation, whereas soil moisture continued to decline as the dry season progressed on control plots
Summary
Populations of Nearctic-Neotropical migratory songbirds are influenced by a multitude of factors that operate throughout the annual cycle [1], [2], [3], [4]. The quality of a particular habitat can vary temporally among years, due in large part to the effects of precipitation on plant productivity and insect abundance. In Jamaica, arthropods can be up to five times more abundant in wet years and this has direct effects on winter body condition and migratory timing of insectivorous bird species such as the American redstart Setophaga ruticilla [15], [16]. Annual variability in conditions across the Caribbean wintering range translate into effects on population abundance for Neotropical migrants on the breeding grounds [17]
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