Evolutionary ecology of senescence: a case study using Tree Swallows, Tachycineta bicolor

Tree Swallows, Tachycineta bicolor, have been studied at nest box colonies at sites throughout North America. They can be easily aged, captured, repeatedly sampled, and their physiology and reproductive success measured across their lifetime. These traits have made Tree Swallows an important model species for many ecological and evolutionary studies. We have studied aging in free-living Tree Swallows colonies located in Iowa and New York, USA, for several years, and here we review what these studies are revealing about physiological attributes of senescence and their ecological and evolutionary consequences. Telomeres, the DNA "caps" on each chromosome, shorten with age, which may contribute to the senescing phenotype, and birds with longer telomeres at 1 year of age have a significantly longer predicted life span than birds with shorter telomeres. Tree Swallows display decreased cell-mediated immune function with age, but other components of the immune system do not decline with age. Older, nesting females who display immunosenescence also show exaggerated sickness behavior compared to younger birds in response to a simulated bacterial infection. Sickness behavior in old birds leads to loss of body condition and decreased parental behavior, suggesting that immunosenescence carries fitness costs in the wild. We are also carrying out an experimental test of the leading theory for aging-oxidative stress. Preliminary data from metabolically-handicapped Tree Swallows carrying an extra 5% of their body mass suggest birds may suffer increased oxidative damage and telomere shortening, reduced immune function, and lower reproductive success. Continued cross-sectional and longitudinal studying of Tree Swallows will enable us to distinguish between intrinsic change with age common to all birds and the results of selection on traits that affect survivorship and differ between individuals. Our results suggest continued work with Tree Swallows holds great promise to understand the evolutionary ecology of senescence in birds and other animals.