Examining the effects of behavior, landscape fragmentation, and climate change on avian distributions.
Abstract: I examined the patterns and processes that determine Bobolink
(Dolichonyx oryzivorus) distribution and abundance. First, I
tested the hypothesis that visual openness could explain area sensitivity, the pattern of
species being disproportionately absent from smaller habitat patches. I compared Bobolink
density and occupancy to a novel openness index, patch area, and edge effects. Our ... read moreresults
supported a seasonally consistent openness threshold in occupancy (i.e. patches are either
suitable or not). Once occupied, however, I found no relationships between patch openness
or area and population density or measures of body condition (body mass, body size,
circulating corticosterone levels). Despite individuals differing in response to simulated
predators (flight initiation distance), differences were not related to patch area or
openness. The openness response also was detected within fields, as both Bobolinks and
Savannah Sparrows (Passerculus sandwichensis) placed nests
both away from edges and in more open habitat when compared to expectations based on random
placement. (Openness and edge were only moderately correlated.) However, I found no strong
relationships between either openness or edges and reproductive success (numbers of eggs
and fledglings, % of eggs producing fledglings, and nest survival), although there may be
an openness effect on timing of reproduction (clutch completion date). Second, at the
landscape scale, I found that climate and land use variables explained Bobolink
distribution and abundance, but did a poor job of explaining the observed range shift in
abundance from 1970-2008. I propose that some apparently area-sensitive species are
actually responding to how open a habitat patch is, rather than to patch size. Our findings
have implications for studies of area sensitivity, especially with regards to
inconsistencies reported within species: specifically, (1) whether or not a study finds a
species to be area sensitive may depend on whether small, open sites were sampled, and (2)
area regressions were sensitive to observed densities at the largest sites, suggesting that
variation in these fields could lead to inconsistent area sensitivity responses. Finally,
our landscape results suggest that bioclimate models might do a poor job in predicting
species range shifts due to climate change.
Thesis (Ph.D.)--Tufts University, 2012.
Submitted to the Dept. of Biology.
Advisor: J. Michael Reed.
Committee: Colin Orians, L. Michael Romero, and Christopher Elphick.
Keywords: Ecology, Conservation biology, and Wildlife conservation.read less