LEVIN, R.N.*; FREUND, K.; FUXJAGER, M.; Pomona College, Claremont, CA: Understanding Patterns of Variation in Avian Life History Traits: Comparisons of Temperate & Tropical House Wrens

Food availability, nest predation and adult mortality are three factors commonly proposed to explain latitudinal and altitudinal variation in avian life history traits. We report here on our ongoing studies of four populations of house wrens, northern house wrens (Troglodytes aedon) at low (34&deg N, 480m) and high (38&deg N, 2164m) elevations, and low (10&deg N, 35m) and high (10&deg N, 1342m) elevation populations of southern house wrens (Troglodytes musculus). At each site, we collected data on clutch size, the duration of incubation and nestling periods, female incubation bouts and their impact on nest vs. ambient temperature, nestling growth rate, male and female feeding rates, parental response to artificial snake presentations and nesting success. Our data confirm previously assumed patterns of variation in life history traits. That is, as latitude decreases or altitude increases, clutch size decreases and the duration of incubation and nestling stages increases. Nestlings grew at similar rates at all sites, but were larger at lower latitudes and higher altitudes. Preliminary analyses suggest that in the temperate zone, male participation in feeding decreased as nestlings matured; this was not true in the tropics. Males fed nestlings more frequently at higher elevations. Nest predation rates varied between years but were higher in the tropics than in the temperate zone. Nests at higher elevations in the tropics were more likely to lose eggs, while nests in lowland tropical areas were more likely to lose nestlings. This finding is consistent with our observation that birds in the lowlands were more likely to defend nestlings during snake presentations. Together, these data provide insights into the selective pressures that influence life history strategies.