Residents Versus Migrants in a Shade Coffee Farm
By Gregory Gough
Every fall, millions of migratory birds leave their nesting grounds and head south to the tropics. Scientists wondered how this might affect the resident tropical birds.
Researchers set up a study site on a shade coffee farm in Mexico. Shade coffee, coffee shrubs grown under a canopy of trees, is excellent habitat for both resident and migratory birds.
A small resident bird, the rufous-capped warbler (pictured at right), was followed year-round to see if its behavior changed when migratory birds moved in.
When migrants were absent, during the wet season, the rufous-capped warbler spent equal time in the tree canopy and in the coffee shrub layer, although it was more successful in catching its insect prey in the canopy layer.
But during the dry season, when the migrants moved in, the rufous-capped warblers spent 80 percent of their time in the shrub layer while the migrants spent the majority of their time in the canopy.
This article summarizes the information in this publication:
Jedlicka, J., Greenberg, Russell S., Perfecto, I., Philpott, S. and Dietsch, Thomas Victor 2006. Seasonal foraging niche shifts of tropical avian residents: resource competition at work? Journal of Tropical Ecology, 22: 385-395.
This study examined the foraging behaviour of a resident bird species, the rufous-capped warbler (RCWA, Basileuterus rufifrons), in a shaded-coffee farm in Chiapas,Mexico. Unlikemany resident species that use shaded-coffee agroecosystems seasonally, RCWAs do not move to other habitats when migrants are present. RCWA foraging was compared when migrant birds were present (dry season) and absent (wet season). It was hypothesized that RCWAs would exhibit a seasonal foraging niche shift because of resource competition with migrants. Observations from both the canopy and coffee understorey show that RCWAs foraged almost equally in both vegetative layers during the wet season although they were more successful foraging in the canopy. In the dry season, migrants foraged primarily in the canopy and RCWAs shifted so that 80% of RCWA foraging manoeuvres were in the understorey. At that time RCWAs foraged less successfully in both vegetative layers. Avian predation in the dry season was found to reduce densities of arthropods by 47–79% in the canopy, as opposed to 4–5% in the understorey. In the canopy, availability of large (>5mm in length) arthropods decreased by 58% from the wet to dry season. Such resource reductions could have caused the RCWA foraging niche shift yet other alternative or additional hypotheses are discussed. Shifts in foraging niche may be a widespread mechanism for some small insectivorous residents to avoid seasonal competition with abundant migrant species.
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