Effects of Forest Fragmentation on the Physiology and Health Status of Wood Thrush (Hylocichla mustelina) in the Upper Midwest

Abstract

Recent population declines of neotropical migrant songbirds breeding in the eastern deciduous forests have been attributed to forest fragmentation. Although some research has been conducted on the demographic processes occurring in forest songbirds subjected to habitat fragmentation, little is known about the physiological and health-related ramifications of forest degradation. I used Wood Thrushes (Hylocichla mustelina) to investigate whether forest fragmentation has an impact on immunological condition and infection with ectoparasites and blood-borne pathogens. I compared Wood Thrushes occurring in small, fragmented forest patches to others living in unfragmented areas for differences in morphology, immunological and body condition, behavior, and infection with hemoparasites and ectoparasites. Wood Thrushes in unfragmented sites exhibited more aggressive behaviors against standardized experimental challenges, and were in better physiological condition, as evidenced by higher hematocrits, than those from fragmented sites. Tail and wing length were negatively correlated to Julian date while total fat deposits were positively correlated to Julian date, suggesting that birds in the late summer had abraded plumage but higher energetic reserves. Total parasitemia (intensity of hemoparasite infections) positively correlated to heterophil-lymphocyte ratio (H/L) indicating heavy infections precipitated a skew in normal immunological profiles. Ectoparasite burden decreased over the course of the summer, and was negatively correlated to fat deposits, suggesting a negative impact of ectoparasites on the energetic balance of the host. Fragmentation did not appear to have an effect on either ectoparasite or hemoparasite infections. My results suggest that fragmentation has impacts on some aspects of Wood Thrushes’ behavior physiology but that such effects may not be immediately observable. Furthermore, any studies should consider timing of measurements in addition to habitat condition as important factors shaping the physiological condition and infection status of forest migrants.