Another aspect of our work this fall was describing various morphological characteristics of the common species of fruit available at our site. Many factors go into fruit choice by birds. Among them are the size of the fruit, how many seeds it contains, and the size of the seeds.
Some of these data are available in the literature. As I searched for these metrics, I found that for some species different sources reported quite different numbers, some sources had data on one characteristic but not others, while measurements for some species were not to be found. The best, though not the easiest, solution was to take measurements ourselves. Weekly, we collected around 10 fruits from each of around 30 species, measured the fresh fruit, counted the seeds per fruit, and measured the seeds. Dana Wloch started this project last year, and it was expanded this year. I'll be finishing up with some of the late fruiting species next week, but so far we have measured about 3300 fruits and over 7000 seeds!
Some of our results are basic. For example, the average diameter of the five most common seeds in fecal samples (Common Buckthorn, Amur Honeysuckle, American Pokeweed, Riverbank Grape, and crabapple) is 8.2 mm. This agrees with other studies that have shown bird-dispersed fruits typically average about 8 mm.
Some results are much more intriguing. Glossy Buckthorn is a non-native species that can be quite invasive, especially in wetlands. A recent study from its native range in Sweden stated the average diameter of the fruits was 8.7 mm (458 fruits) with an average of 1.7 seeds per fruit. We measured 134 fruits here over two years, and the average both years was 7.7 mm. The average number of seeds for 168 fruits was 2.5.
|Glossy Buckthorn fruit does not ripen simultaneously, so red,
unripe fruit are often on the same branch as black, ripe fruit.
Glossy Buckthorn has been shown to have evolved different morphological characteristics in different parts of its native range. This species has been present in North America for over 200 years. If our data is truly representative of the local population, it might suggest adaptation to a different suite of dispersers, exposure to more or better pollinators, and the higher seed set may be a factor in its success as an invader. All speculative at this point, but no doubt it will prompt me to continue measuring Glossy Buckthorn!
Meanwhile, during the fall season, we collected 453 fecal samples from 6 bird species. However, over 80% of the samples were from robins that we did not band. Several years ago, the University began cleaning off all sidewalks and roads on a daily basis in fall. In a number of areas, low spots in the pavement hold water and attract robins, which often then "leave a deposit." Collecting these samples in communal bathing and drinking areas, as well as along other paths where we observed robins foraging on the ground, is a convenient way to acquire a lot of data.
For catbirds, the top four species in our 41 samples this fall were:
- American Pokeweed (native)
- Riverbank Grape (native)
- Amur Honeysuckle (non-native)
- shrub dogwoods (native)
Sample sizes for robins are much higher. This fall the 453 samples revealed the top four species as:
- Amur Honeysuckle
- Common Buckthorn (non-native)
- crabapples (non-native)
- Riverbank Grape
Two other species in robin samples were notable. Previously, Multiflora Rose (non-native) was found in 4% of samples; this year it increased to 6%. This is interesting considering a number of Multiflora Rose has been reduced in the past year or so both by removal and from infection by rose rosette disease. We also have the native Illinois or Climbing Rose (Rosa setigera) here. There is overlap in the appearance of seeds of these two species, but there are fewer R. setigera, their hips are larger and "ripen" later than multiflora. I believe most of the seeds found in fall samples are probably multiflora.
|A comparison of the large hips of the native Rosa setigera (left) and the
small hips of non-native Rosa multiflora (right).
|Asiatic Bittersweet is especially conspicuous when there are no leaves on trees.
One possible explanation for the increase in these two species is that robins are eating more of them because a great deal of their top-ranked species (buckthorn and honeysuckle) have been removed in the larger landscape. However, annual fluctuations due to weather and crop size can be large, so it will take more years of sampling to see if this year was just a quirk, or if trends will start to appear.
I haven't done much digging through the numbers yet, but even this quick look is pretty interesting.