Time for an update on the seed and diet study -- one in which may explain why blog posts have been so infrequent over the past year.
Recap of RRBO's research trajectory
When RRBO was founded in 1992, there was very little research being done on how birds used urban natural areas. Our first task was to establish solid baseline data on the species, relative abundance, and seasonal distribution of birds using the UM-Dearborn campus utilizing historical data, standardized regular surveys, and bird banding. This has resulted in a variety of publications, including a book, documenting the 261 species of birds found in the city of Dearborn, mostly on campus.
Once we understood that many birds used the area, our next question was: How? Were migratory birds that stopped here able to find the resources that would enable them to continue their migratory journey? Based on our banding data, we looked at 15 years of recaptures of thrushes during fall migration, and found that all three species for which we had sufficient data -- Gray-cheeked, Swainson's, and Hermit Thrushes -- did gain enough mass to make the next leg of their migratory flight. This research was published in 2009 in The Wilson Journal of Ornithology.
That led us to another question: What were these birds eating that helped them gain weight? Fortunately, there is a reliable and low-tech way to find out: identify the seeds in their poop. Over 300 samples consisting of over 1200 seeds were analyzed from Hermit and Swainson's Thrushes, and a paper summarizing the results is in progress. The majority of samples were from two non-native plant species, Amur Honeysuckle and Common Buckthorn. This was a little surprising, because these species were said to have little nutritional value to birds.
Every "answer" leads to more questions!
Our thrush work led me to begin working on the following...
Were fruits being eaten in proportion to their abundance in the landscape?
Last year I finished up censusing fruit in a large plot to provide us with a sense of the rank abundance of all the ripe fruit available to birds to eat throughout the fall season. The end of this post describes that work.
Were there some intrinsic characteristics (size, number of seeds, etc.)
of the fruits that made some more apt to be consumed than others?
This fall and winter I will be finishing up the fruit morphology work, which I first described in detail in this post.
I will have detailed data on many physical characteristics of the fruit
of 40 species of fall-fruiting plants, based on measurements of over
6500 fruits and over 9000 seeds (so far!). I'll be preparing a paper on this data for The Michigan Botanist.
Were migratory thrushes eating non-native fruits because resident birds had already eaten the "better" native fruits before the thrushes arrived?
In 2009, we started collecting fecal samples from all bird other species we banded. Two things became immediately clear. First, other birds began eating honeysuckles and buckthorn as soon as they became ripe, even when native species like pokeweed and dogwoods were available. Second, we could collect A LOT of samples from American Robins.
Robins have been RRBO's most commonly banded species, although they are poor subjects for mass gain studies. We rarely recapture them and we cannot distinguish which nested in the area and which were migrants (thus making it impossible to estimate stopover duration and obscuring mass gain patterns).
Since we don't need to rely on mass gain data, we don't need to capture robins and can use "seed traps" to collect robin samples. Often, seed trap arrays are plastic trays with drainage and screen tops placed throughout an area frequented by birds. This is a costly method in an area like ours where field equipment is frequently vandalized. Fortunately, robins have the habit of gathering at drinking and bath sites, during which time they nearly always poop. We began collecting samples from robins when we were able to observe a flock around puddles or foraging in a specific area within our study plot. When a small retention pond was constructed right outside the banding lab, we set out boards along the margins -- these made excellent seed traps! In 2013, we also began collecting samples at a Washtenaw County site which has more native fruiting plants and fewer non-natives (especially honeysuckle) to use as a comparison, using similar methods.
This fall, 949 samples were collected from robins, bringing the 2009-2015 fall total of samples to 2207 consisting of 16931 seeds, of which only one was unidentified! And why stop in fall? Robins are here nearly year-round, so we have continued to collect in winter, and have 350 samples (2252 seeds). At the Washtenaw County site the total number of robin samples is 1135 (6327 seeds) for fall and 792 (4515 seeds) for winter just over the past two years. Summer samples are being collected as well,
although fewer plants fruit in summer. Samples are also collected from
all species banded at the Washtenaw site.
So far, altogether, from both sites for all species throughout the year, RRBO has collected and compiled data on over 5300 samples consisting of over 35000 seeds.
This very robust data set will show us what fruits robins eat (and
therefore disperse in the landscape) throughout the year in both an
urban and more rural setting.
Because fruit crops vary from year to year, I want to continue the collection (especially in winter) for at least another year. I'll have to draw the line at some point in order to dedicate some large blocks of time dedicated to data analysis, rather than data collection!