Let us step out onto the front porch of the schoolhouse, it’s another beautiful day in Gamboa!
Front porch, Gamboa Schoolhouse, picture by Elise
Say, what’s that? Look up there!
See that clay colored robin up there, sitting on a nest? Gosh, how cool! This brown bird is the national bird of Costa Rica, chosen in part because it is common and familiar to the majority of the population.
It does seem unusually early for this species to start nesting, as previous studies in the area (over at Summit Gardens) found the species breeding from about late February to mid-June (Morton 1971, Dyrcz 1983). Still, let’s check the nest!
Eggs in the clay colored robin nest on Gamboa School House, picture by Elise
Why, it’s incubating!
(My feet never left the ground, this was all a matter of attaching things together to give me enough reach, safety was a top priority here)
Four eggs, which is in the range for the species although on the high end and a good deal higher than the modal clutch size in the tropics (2 eggs). One egg looks a little small, if it survives to hatching, it may still wind up starving as smallest nestlings sometimes do… Perhaps the fruiting trees provided sufficient resources for the parents or, perhaps, because we’re in a human-dominated area, there may be other food sources that these birds can count on (but IF food for adults sufficient for growing nestlings is another matter altogether).
It makes one wonder.
We live in a rapidly changing world. Even if temperatures don’t change rapidly in the tropics, rainfall patterns are expected to shift. Morton (1971) suggested that high rates of nest predation during the rainy season drives these thrushes to breed in the dry season. Will those predators track changes in the rainfall regime? Depending on the direction of change, it could go either way for these common birds. What might happen for rarer (and thus potentially more vulnerable to extirpation) species if nest predation pressures change?
This nest is right here, on the Schoolhouse, right above the door where there’s frequent (but not necessarily predictable) human presence. The human population on this is boiling, bubbling, burgeoning up past the 7 billion mark, predicted to race towards 8 billion by the next decade. The tropics are predicted to experience dramatic growth in human population (with rapid and unplanned urban expansion occurring as a consequence). That’s going to make it (even more than it already is) very, very difficult to address big problems like climate change (and it really won’t be fixed JUST with individual action. Do your best, do your part to help, but the commons are pretty tragic, huh?), urbanization, food security, etc.
Now, Gamboa sure isn’t a big city, but the habitat has been altered by and for human habitations (and, well, the giant canal with the gigantic boats going through right over there was a pretty major alteration to the area too…) and small towns can also alter wildlife populations. Avian populations in more urban habitats, where there are concentrated sources of food and potentially reduced rates of nest predation, may remain viable due to high rates of survival and higher numbers of offspring produced per female(Stracy and Robinson 2012). However urban areas may become ecological traps in which immigrants settle into lower quality habitat and face novel sources of mortality, such as from collisions with vehicles (Stracy and Robinson 2012).
The combination of climate change and urbanization may be just a bit too much instability for species to deal with, perhaps a breakdown of biotic interactions will occur when one species is less able to tolerate human-induced changes, perhaps populations persist in certain landscapes only because adults become sedentary once a territory is established even if they are unable to generate offspring, perhaps I’m thinking too much about one early-breeding pair of thrushes that are probably pretty tired of the fool with a camera on a stick pestering them for photos.
Dyrcz, A. 1983. Ibis 125: 287-304
Morton, ES. 1971. Science 171(3974):920-1
Stracey & Robinson. 2012. Journal of Avian Biology. 43:50-60.