The Effect of Variability in Microclimate and Regional Climate on the Productivity of Neotropical Migrant Landbirds in Midwestern Forests: Application to Climate-Change Scenarios
Donald R. Whitehead, Susan B. Grimmond and Scott M. Robeson
Indiana University
Objectives: The impacts of climatic change and variability on four abundant Neotropical migrant birds (NTMB) are being investigated in a variety of forest contexts in south-central Indiana. Our focus is on two understory species (Acadian Flycatcher, Wood Thrush) and two ground nesting species (Worm-eating Warbler, Ovenbird). Understanding climatic impacts on NTMB is critical since a variety of factors -- such as landscape fragmentation, loss of wintering habitat -- may lead to fluctuations in NTMB populations. Quantifying the role of climatic variability in Midwestern forests will provide important information for a variety of scientific and policy questions.
Products: A variety of unique datasets are being developed, including extensive data on NTMB productivity. Information on brood size, number of Cowbird eggs, date of hatching, number of nestlings, the occurrence of predation or date of fledgling have been recorded. NTMB data from south-central Indiana now are available from 1991-1995. Climatic data include measurements of air temperature, humidity, solar radiation, net all-wave radiation, wind speed, soil heat flux, soil temperature, and soil moisture at a variety of sites in Hoosier National Forest during 1994 and 1995. Other key products from this research are the models that relate climatic variability to insect abundance and NTMB productivity.
Approach: Intensive nest monitoring has been used to estimate a variety of productivity measures. Comprehensive micrometeorological data also are being collected at a number of forest sites. The impacts of climatic variability on NTMB may be both direct (e.g., thermal and moisture stresses) and indirect (e.g., insect abundance, loss of habitat); therefore, our data collection and modeling strategies consider both types of impacts. Together with variables derived from landscape pattern, climatic variability on a variety of spatial scales are being used to develop models that explain observed variability in NTMB productivity.
Results to Date:

Regional climatic data from National Weather Service stations and field data from within-forest sites are being used in conjunction with data from intensive nest monitoring to assess the role of climatic variability. To assess the role of regional climatic variability, transfer functions that relate regional climatic data to within-forest variability are being developed. Preliminary analysis of climatic data suggests that moisture availability during late spring may be a key variable in NTMB productivity. Data from 1991-1994 indicate that lower overall reproduction rates are associated with dry conditions.

The four-year data set for the interior sites shows appreciable variation in reproductive success for the Acadian Flycatcher. A higher proportion of nests fledged in 1992 and 1993 (50.0% and 57.6% respectively) as compared to 1991 and 1994 (in which fledging rates were 32.7% and 36.4% respectively). The overall survival rate for Acadian Flycatchers averaged across all nest stages was correspondingly higher in 1992 and 1993 (0.447 and 0.506 respectively) than 1991 (0.286) or 1994 (0.359). Consequently the average number of young produced per pair was higher in 1992 and 1993 (1.89 for both years) than in 1991 and 1994 (0.96 and 1.50 young per pair respectively). Although there is higher productivity in 1992 and 1993, none of the between-year differences are statistically significant.

A preliminary analysis of historical climate data for the region (1951-1990) indicates some patterns that may relate to the observed variation in nest success. For example, the June monthly maximum temperatures were higher than mean by about 5°C in 1991 and 1994; May was also warmer in 1991. In addition, June daily average precipitation was higher than average in 1993 by about 6mm/day, while July daily average precipitation in 1992 was higher by 5mm/day. this suggests that cooler, moister conditions in late spring and early summer may favor higher productivity.

One explanation that is being explored is the relationship between climatic conditions and insect abundance. Insect samples taken using Malaise traps during 1993 and 1994 show that the pattern of insect abundance differed between and within years. In 1993, there was no significant difference between months (May-July) in overall insect abundance. However, when the insects were subdivided by body length, the quantity in the class 7.5-10 mm was significantly lower in July than in June (p=0.012; ANOVA, pairwise comparison of means). The pattern was quite different in 1994. The count of insects from all traps dropped significantly from 13,389 in May to 8,103 in June (p=0.016). There was also a further significant drop between June and July (p<0.01), although this was not evident in the largest size category (body length >10.5 mm).

Acadian Flycatcher nests were observed throughout the summers of 1993 and 1994 in order to assess the type and quantity of food items brought to the nestlings. The nestlings were fed exclusively on arthropods in both years. The nestling provisioning rate was significantly lower in 1994 than in 1993 (4.31 and 7.94 arthropods/nestling/hour, respectively; p<0.01, T--test). Further nest observations are being conducted in 1995.

Within-forest climatic variability also has been observed, with ridgetop sites being warmer and drier than ravine sites. The differences in air temperature and water vapor pressure between ridgetop and ravine locations exhibit strong diurnal patterns. These climatic differences help to explain a variety of key variables, such as patterns of nesting locations and insect abundance.

Publications and Presentations: None as yet.
Student Participation: A large number of graduate and undergraduate students from the Departments of Biology and Geography at Indiana University have participated in field work and data analysis. The project has provided a great deal of valuable data, but has also acted as a unique vehicle for instructing students in experimental design, sampling methods, the use of instrumentation, etc. Listed below are the participating students and the degree they are seeking .

Lisa Angelos (M.S.), Linda Evers (M.S.), John Gano (M.S), Elizabeth Geils (Ph.D), Mark Hubble (Ph.D.), Michael Janis (Ph.D.), Donnelle Keech (M.S.), Thomas King (Ph.D.), Mark McKee (B.A.), Neal Schroeder (B.A.), Karen Seeh (B.A.), Karsten Shein (M.A.), Bernadette Slusher (M.S.), Jane Southworth (M.A.), Andrew Struck (M.S.), Kevin Willsey (B.A.), Amanda Zidek (B.A.).