Photo by Patrick Emerson*
The full moon is pregnant with foreboding interpretations, from the legendary werewolves who are supposed to transform under its malevolently shining face to a recent article about November’s upcoming full “supermoon” that faux-reassures, “despite all the rumors… there is no evidence linking supermoons to natural disasters.”
If you look at a full moon and shiver, you aren’t alone—but you are a bit of a mystery. In humanity’s past, the full moon should have been the safest time of the month, since our nocturnal predators tend to attack most on dark nights. The new moon should be spooky, as your hindbrain—unaware that you no longer live on the African savannah (unless you do!)—looks out for predators slinking in the shadows. This is presumably why the fear of darkness is such a common and instinctual one. But the full moon is bright: it should be comforting.
We all know about food webs—or we think we do. Herbivores eat plants, then predators eat herbivores, and if one part of the web is affected, other parts are impacted too. Seems pretty simple—except that the threads in those webs sometimes connect things you would never expect.
For example: trout and a songbird, the Gray-crowned Rosy-Finch, in an alpine habitat. The fish are in the water and the birds are on land—how connected can they be? If the birds were Bald Eagles or Ospreys or Great Blue Herons, sure, they would be connected because the birds eat the fish. If the birds were ducks, maybe the trout would be an occasional threat to the ducklings. But this is a Gray-crowned Rosy-Finch:
Photo by Blake Matheson*
They’re not going to be eating fish, and their babies definitely don’t float about on the water.
The reason that we need to worry about what threads on the food web those trout might be tugging at is that the trout are introduced, nonnative species. Alpine lakes often don’t have any fish in them naturally. In the Sierra Nevada and many other mountain habitats, however, people have stocked these lakes with fish so that people can come and fish them for fun. This has been a problem for aquatic species such as frogs, which get gobbled up quite happily by the new fish, but nobody was particularly worried about the effects on songbirds.
It turns out that we should have been.
We were searching for junco nests when I heard the unmistakable tic-tic-tic of junco alarm chipping. We followed the sound a ways and found a pair of juncos perched on a low branch, alarm chipping for all they were worth. Strange of the juncos to be alarm chipping at us when we were so far away, before, I thought. I wouldn’t have thought they’d see us as a threat from that far away. Odd birds. Directly below the branch with the agitated juncos was a small shrub. “The nest will be in there,” I predicted, showing off for my new field assistants.
I parted the prickly branches, and tiny pink beaks gaped hungrily at me. “There they are,” I said, pleased with myself. “Three chicks.”
And just as I said that, I saw the snake.
Recently I was lucky enough to spend some time in Spain, where the land is dotted with hilltop castles and the winding narrow streets of the old cities are encircled by sturdy stone walls. I crouched behind crenellations, pretending to be a bowman awaiting attack, and climbed dark winding staircases glad that no defending army waited at the top.
Winding staircase in the Olvera castle.
Photo by Q. Stedman
It’s exciting and romantic to imagine castles and walled cities in the flush of functionality, but it’s hard to ignore that the motivating force for those structures was real, unromantic, gut-knotting peril and fear. The people who lived in those cities put up thick stone walls with their hands because they thought other people were going to come and unromantically kill them—which they sometimes did.
Gulls standing guard over Tarifa.
When birds build nests, they’re responding to that same threat. Eggs and baby birds are easy targets for anything from mice to snakes to deer to toucans. To keep them safe, birds too rely on stout walls, secret passageways, and defending armies.
Melanic color-dependent antipredator behavior strategies in barn owl nestlings. By Valentin van den Brink, Vassilissa Dolivo, Xavier Falourd, Amélie N. Dreiss, and Alexandre Roulin. Behavioral Ecology, 2011.
I’ve been slacking off on the Featured Papers, since it’s the field season and I’ve been reading
almost nothing less than usual; but this paper is too crazy not to mention.
Poor ROAR. He has good subcutaneous fat and a decent mass (18.6 g), but he looks like he’s having a rough summer.
Corey Tarwater. 2008. Predators at nests of the Western Slaty Antshrike (Thamnophilus atrinucha). The Wilson Journal of Ornithology, volume 120(3): pp 620-624.
Nest predation is a major worry for most birds. Chicks in the nest are essentially helpless, and parents usually can’t do much to stop predators, although they will try. Losing broods to predation is common. In this study, 79% of nests were lost to predation. If you’re a Western Slaty Antshrike, you can expect 4 out of every 5 broods of chicks you have to get eaten!
Yet actually seeing these—or any—predation events is extremely rare. Dr. Tarwater describes five nest predation events on Western Slaty Antshrikes (small, cryptic brownish birds) in Panama, one she observed in person and the rest of which she caught on video.