One of the interesting things about Wood Thrushes and many other migratory species is that they have an amazing plasticity in diet. Baby songbirds grow up getting half-smooshed arthropods shoved down their throats from Mommy and Daddy. The high-protein and high-fat content of insects and spiders make these a good food source for growing nestlings. There are some exceptions to this, but in general, baby-bird food is animal-based. Adult songbirds have a lot of different strategies. The thrushes and many warblers also eat a lot of ‘bugs’ during the breeding season, when summer productivity is high and there are lots of juicy lepidopteran larvae around. In fall though, many switch to gorging on fruits. Many songbirds are key seed-dispersers actually, and the many red-berried fruits decorating fall forests are a testament to the co-evolution of birds and plants (see the Wood Thrush-American Ginseng example here).
During the overwintering period, many songbirds continue to eat fruit, and some even take on a nectarivorous diet, like the Cape May Warbler Setophaga tigrina (Latta and Faaborg 2002). When birds do eat a mixed diet of animal and plant-based foods, is there an optimal combination for staying healthy? Or are plant foods just a poor-quality place holder until more arthropods can be found? This is a question that has been looked at in a few species of overwintering migratory songbirds, but with mixed results. Hermit Thrushes (Catharus guttatus) seem to fatten up more easily on diet of arthropods (Long and Stouffer 2003). But in Costa Rica, migrant songbirds as a group seem to increase their consumption of fruits later in the nonbreeding period, a time when they should be starting to fatten up and pack on muscle for migration (Blake and Loiselle 1992). This got me wondering about Wood Thrushes. I found out that the forests at my study site in Belize dry out seasonally, and the abundance of arthropods declines. Maybe Wood Thrushes would switch to a more fruit-filled diet later in the winter, prior to migration. If so, would there be a cost? Could they still get ready for their spring sprint northwards, feeding on tropical figs instead of ants and beetles?
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To figure this out I analyzed the diet of Wood Thrushes from my study site in Belize. It can be challenging to figure out the diet of small songbirds, since they are not easy to watch in the wild. I don’t know how many times I saw a Wood Thrush foraging on a trail, then ran over as soon as it flushed to try to see what it had been eating. Usually there was no sign of anything! It was more obvious when they were eating fruit, since they would join up with dozens of other Wood Thrushes and many other species, like the tropical resident Clay-coloured Robins, Black-faced Tanagers, Gray Catbirds, and even toucans like the Collared Araçari, all in the tops of fruiting trees, generally making an obvious ruckus and spitting out seeds all over the ground under the trees. I saw this most frequently with Ramon or breadnut trees – Wood Thrushes and other birds loved these fruits, and I could catch Wood Thrushes all day in a single net at the base of the tree, as birds from all over came in for the bonanza.
To analyze the diet of Wood Thrushes, I decided to look at the stable isotope ratios of nitrogen and carbon in their blood. Basically, there are multiple isotopes of these common elements, and animals preferentially incorporate the lighter isotope into their tissues. This means that animals eating things higher up on the food chain will be consuming tissues with more of the heavier isotope. For example, a top predator, like a jaguar, would have a higher stable isotope ratio of Nitrogen compared to a herbivore like a tapir. Carbon works basically the same way, but also with some differences in the isotope ratio of different types of plants (e.g. grasses versus herbaceous). These isotopes are useful for distinguishing a diet of plant- vs. animal foods because the animals should always have higher stable nitrogen isotope ratios (and to a lesser extent carbon ratios) than the plants. That means a Wood Thrush eating more bugs will also have a higher stable nitrogen isotope ratio in its tissues. For this project, I was already collecting a tiny blood sample for genetic sexing so I used the remainder of the blood for the stable isotope analysis.
I also collected a bunch of potential food sources for comparison – beetles, ants, fruits, spiders, and grasshoppers. First I just looked at the isotope ratios of carbon and nitrogen, then I tried to reconstruct the actual proportions of different foods in the diet by using a mixing model.
Here’s the basic food web that I reconstructed:
Right away, the data seemed to suggest that Wood Thrushes were not eating that much fruit. Their stable nitrogen ratios were just too high! I analyzed the data to see if the highest stable isotope values were associated with the best body condition in Wood Thrushes. Did birds at the ‘top’ end of the food web have a payoff compared with birds slightly lower down? The answer is a resounding no, although, there wasn’t a lot of variability in the diets of the Wood Thrushes I analyzed. Most of them seemed to be eating primarily arthropods, and the slight variability across individuals didn’t predict their body condition, fat or muscle levels. But there were some interesting patterns in the data that were somewhat unexpected.
First, I did find some differences by habitat. Birds in the driest, early successional forest tended to eat more fruit mid-winter but then actually increased the amount of arthropods they consumed as spring approached. The opposite pattern was found in the more mature forest sites, with the end result that diets were actually very similar across all habitats in late winter. This tells me that Wood Thrushes in different habitats might be fine eating whatever is easiest and available all winter, but as spring approaches they seek out an optimal balance of fruit and arthropods. This habitat difference also likely explains a few differences I found between males and females, since females were more abundant in the dry habitat. The diet differences didn’t relate to body condition though, so it wasn’t that females or birds in the dry habitat were suffering because of their different diets. They just ate different stuff.
So seems like bugs with a side of fruit is what Wood Thrushes go for in the winter (and not fruit with a side of bugs). What exact types of bugs and fruits would be interesting to figure out – maybe something like DNA barcoding will give us more specific information on their diet in future.
Check out the full paper here:
McKinnon, EA, TK Kyser and BJM Stutchbury. 2017. Does the proportion of arthropods versus fruit in the diet influence overwintering condition of an omnivorous songbird? Journal of Field Ornithology. Early online. doi:10.1111/jofo.12187