Ragin' cagin' birding: Reverse migration....aka my thesis!

Well after almost 2 years working towards my Master's and another year before that for my undergrad thesis I have finally finished my thesis on reverse migration!

Brandon has done a nice photo essay capturing reverse migrants with his photography, so check out his blog for more pics.

I've pasted my abstract below for your enjoyment....

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The spring reverse migration of songbirds (i.e., birds flying in the direction opposite to be expected), within the Great Lakes region is controversial because it is not understood if the extent or reversal of flight direction in spring is truly a change in migratory patterns or a brief anomaly. It is also not clear what the fitness and community level impacts are. My objective was to determine what and how weather influences reverse migration and to determine which species and families of birds participate the most frequently in this form of flight. I examined species which are participatory (and those that are not) as well as the impacts of specific weather covariates on the abundance of songbirds and focused on the putative reverse migration of passerines.

Field sites were located at the extreme southern tip of Fish Point, Pelee Island (2010-2012) and Point Pelee National Park (2012), where my field assistant and I visually recorded the total number of birds observed to be reverse migrating, while identifying all birds to species or family as best possible. This study was conducted over 97 days during April 26 – May 20, in 2010-2012. Information pertaining to potential reverse migration has only been formally documented twice in the Great Lakes region, most recently in 1951.

I undertook a descriptive analysis to compare the numbers of individuals of bird species and families. Temperate and neotropical migrants were examined, compared and divided into sub-sets based on their geographic ranges. I identified species at risk which I observed during reverse migrations as well as vagrants. Based on provincial population estimates, I determined the proportion of all reverse migrants where ≥200 individuals were observed. A descriptive analysis was undertaken to determine differences between sites (i.e., Point Pelee and Fish Point) in the final year of surveys (2012). Species and abundance were comparatively differentiated between each site location and subsequently compared.

A total of 61,677 birds of 80 species was documented. My results indicate temperate migrants vastly
outnumbered neotropical migrants (as much as 4:1) and numbers of birds varied between study sites. Temperate migrants were noted to be more common (in the final study year) at Point Pelee compared to Fish Point, while neotropical migrants were more numerous at Fish Point than Point Pelee. Despite most migrant species participating in reverse migrations (i.e., of the species regularly occurring in the Pelee region at this time of year), complete absences were noted most notably in Catharus thrushes, while species such as Rose-breasted Grosbeak, and Scarlet Tanager, and families such as Tyrant Flycatchers, Vireos and Sparrows were observed to be in less abundance than anticipated. Species at risk and vagrants were noted relatively frequently during this study, suggesting that these surveys are an efficient and important tool for migration monitoring in this region. Disproportionally diurnal migrants, most notably Blackbirds, were observed to engage in reverse migration in higher numbers than nocturnal migrants, such as Wood Warblers.

Seven weather covariates were measured and modeled with the total number of birds detected using R to
determine which covariates explain the most amount of variation of the total number of birds during my surveys. I used an AICc approach to select the best model for each hypothesis. After selecting the top weather covariates with time lags according to the best (lowest) AICc values, I built general models by comparing all possible combinations of the covariates identified in the top models for each hypothesis. I included a random effect intercept for study site to discern any site difference or correlations between Point Pelee and Fish Point and specified a Poisson distribution (log link function as implemented in the LMER package) due to the data set being continuous (time-series) and count oriented.

My adjusted time lag results show that most migrants tend to migrate during and ahead of inclement weather. I also found that all identified covariates influence reverse migration to some degree. Wind direction and barometric pressure were the most significant of the covariates examined (β = 0.718 and -0.213, respectively). Specifically, wind direction is the most important covariate in explaining reverse migration, with days of south winds dramatically increasing the probability of higher numbers of birds during surveys. Low barometric pressure is also important for explaining the number of observed reverse migrants; therefore days with lower barometric pressure have a greater likelihood of increased bird observations.

Based on my observations and results I theorized that while reverse migration pertains to a distinct form of flight, it is likely not an actual form of migration. This form of flight in its simplest is likely a form of re-orientation, whereby migratory passerines take advantage of local weather conditions by flying south extended distances. I anticipate this form of flight has serious repercussions on the fitness levels and life cycles of migratory passerines.

Studies looking at reverse migration provide a useful tool for migration monitoring, particularly on an underexplored phenomenon. Observations of thousands of birds, many of which are either species at risk or vagrants, in an efficient manner are vital for determining population trends related to migratory birds. Continuing this study will allow for a continual monitoring program to assess songbird populations passing through the lower Great Lakes region. Understanding the impacts of climate and climate change on migratory songbirds is another aspect this study will help to address in the future.
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And for shits and giggles, I've put some of the pics I really like from Pelee Island....