Sunday, December 7, 2008

Nature Fight!

The life of a Green-rumped Parrotlet (Forpus passerinus) is not an easy one. Adults face a constant risk of death by tooth and talon, and their nests are easy targets for all manner of raiding predators. The long-running Forpus project has generated empirical knowledge about mortality rates, predation risks, competition, and the life-history strategies parrotlets have adapted to survive them. I’ve broken the struggles of the parrotlet into three rounds:

Round 1: Finch vs Parrot! (Interspecific competition)

Many other species attempt to make use of the parrotlet nestboxes for themselves, providing competition for a limited resource – nesting sites. Several different types of ants frequently colonize the woodchips in the bottom of the nest box. Various small mammals including rats and mice build grass nests in the boxes. Wasps and bees use the PVC boxes as anchor points to build colonies (we abandon any hope of checking those boxes until they clear out on their own!). Several bird species also think the parrotlet nest boxes make good homes – Myiarchus flycatchers, the Straight-billed Woodcreeper (Xiphorhynhus picus), and the Saffron Finch (Sicalis flaveola) being the most common. We try to empty out the nest material of these species before they get an egg laid (we are here to study parrotlets, not finches!), but sometimes they beat us. This Saffron Finch nest was particularly crafty and had us stymied for a while, so we let them breed. This pair, instead of lining the bottom of the chamber with plant fibers, somehow built a platform and a nest suspended right up by the entrance of the box.

The woodcreepers have also snuck a few nests in this year. They line the boxes with woodchips, making for a very distinctive nest.

In September I saw some parrotlets decide they liked a box occupied by woodcreepers. A woodcreeper was brooding three chicks unseen inside the box while her partner was off foraging. The parrotlet pair, in typical prospecting behavior, approached the box, chattering. They sat on the top, looking down at the hole, then moved down on the perch and looked nervously inside. At this the woodcreeper came up and sat in the hole, with a chick’s fecal sac in her beak. The parrotlets retreated to the top of the box and the fence, chattering. It was something of a standoff for several minutes – the woodcreeper in the box, not leaving because of the parrotlet overhead, and the other woodcreeper on a nearby tree trunk, calling, bill full of invertebrate prey. Eventually the parrotlets gave in and flew off, allowing the woodcreepers to reunite and feed their nest.

Generally, this kind of interspecific competition for nest sites doesn’t lead to violence. How about something with a bit more blood?

Round 2: Snake vs Bird! (Predation)

The most obvious mortality risk to a parrotlet of any age is predation. Parrotlet nestlings and eggs fall victim to snakes (including the boids Boa constrictor, Epicrates cenchria, Corallus ruschenbergii, and the colubrid Spilotes pullatus), mammals (including the lowly Rat (Rattus rattus) and the mouse opossum (Marmosa robinsoni)), and even nasty carnivorous ants (any entomologists want to join the project?).

On average, a little less than 40% of the parrotlet nests monitored by the project failed without fledging young. Of these, the proportion of failures due to predation varies year to year, but somewhere between 10 and 30% of the nest failures are due to predation, which is concentrated mostly when the parrotlets have nestlings and are frequently away collecting food. The impact of snake predation on the Forpus project can be significant when nests under observation get hit, so we catch and relocate any potential snake predator we can. This means I get to play with more snakes! The most common snake I found in the parrotlet boxes is Leptodeira annulata, all of which have been too small to pose much of a threat to the parrotlet nests except maybe to the eggs. I’ve even found them residing in the bottom of a box underneath an active nest.

The next most common snake I found is the tree boa, Corallus ruschenbergerii. I recovered three of these lovely guys from boxes (none with active nests) and relocated them to distant forest patches.

The third snake I caught in the act. While approaching an active nest, I noticed both adults sitting on the surrounding fenceposts, quiet. Thinking this was odd, I stopped and realized there was a snake crawling up the fencepost and across the top of the nestbox. It was a gorgeous adult Chironius carinatus. I chased it off the box, but it kept the fence between us and escaped up a tree. Later I cornered it in the tree and got some pictures of it, before it again saw fit to flee.

We know less about adult parrotlet survival, since it is impossible to separate emigration from the study area from death, but we can make some estimates. Sandercock et al. (2000) estimated an average local survival rate on the parrotlets at Masaguaral of 0.56, meaning a little more than half of the adult population persists locally each year. As you might guess from these data, Green-rumped Parrotlets are not very long-lived, with the oldest individuals monitored by the project reaching an old age of around ten years.

Adult parrotlets have plenty to fear from the predators that also attack their nests, but they also face aerial predators outside of the box. Aplomado Falcons (Falco femoralis), Savannah Hawks (Buteogallus meridionalis), and Roadside Hawks (Buteo magnirostris) are known to attack adults or recently fledged parrotlets. I myself witnessed an unsuccessful Aplomado Falcon attack on a parrotlet, and I’ve seen plenty of strafing runs from the resident pair.

Round 3: Parrot vs Parrot! (Intraspecific competition)

Even if a parrotlet succeeds in claiming a nestbox for its own, even if it avoids the attention of any predators as they raise their young, they may still fall to their most vicious foe: another parrotlet’s beak. Competition for nesting sites is fierce, and these tiny, compact little birds are actually quite vicious to one another, putting those powerful parrot jaws to good use (as my band-aid covered fingers can attest). Prospecting pairs looking for a suitable nestbox will destroy any unattended nests they happen across, breaking and burying the eggs. An experiment (Beissinger et al 1998) placed extra boxes containing eggs along the fences to monitor the rate of nest destruction. 40% of these unguarded nests were destroyed by prospected birds within 72 hours, a third of those were destroyed on the same day the nest was placed in the field.

Beissinger et al. believe this rate of destruction is one of the factors influencing parrotlet breeding biology. Parrotlets begin incubation on laying the first egg, and space their eggs out every 1-3 days. With large clutch sizes (7-10 eggs), this creates a huge asynchrony in hatching. The first egg hatches not that long after the last egg is laid. It is believed that limited nesting opportunities (rotten stumps) drives this fierce competition for nests and frequent nest destruction, which in turn drives females to incubate earlier, because Beissinger’s studies have also shown that a female’s presence significantly reduces the rate of nest destruction.

Even worse than destruction of eggs is what happens when nestlings are targeted. Sometimes prospecting birds can actually stage a takeover of a box, killing of the nest and driving away the parents. Infanticide also happens when a member of a pair is killed or otherwise goes missing, and the remaining parent cannot defend the nest. Somehow the other parrotlets in the neighborhood quickly discover these undefended nests, and flocks of lone males and prospecting pairs can bring a whirlwind of chaos down on them. Sometimes the lone bird persists in defending the box and raising the young, or selects a new partner and starts over. The bloodiest result, as I observed at B106, is when the parent is driven off and the whole nest killed. Infanticide is a really nasty business, as the following pictures of a massacre at Box 106 show. All of the nestlings were killed by bites on the head and cleaving off the beak, the mark of infanticide in parrotlets.

Well, I think that is a good note to end this post on. I hope you enjoyed a glimpse of the dark side of parrotlet study.


Beissinger SR, Tygielski S, Elderd B (1998) Social constraints on the onset of incubation in a neotropical parrot: a nestbox addition experiment. Animal Behavior 55:21-32

Sandercock BK, Beissinger SR, Stoleson SH, Melland RR, Hughes CR (2000) Survival rates of a neotropical parrot: implications for latitudinal comparisons of avian demography. Ecology 81(5):1351-1370

Stoleson SH, Beissinger SR (2001) Does risk of nest failure or adult predation influence hatching patterns of the green-rumped parrotlet? Condor 103:85-97

Waltman JR, Beissinger SR (1992) Breeding behavior of the green-rumped parrotlet. Wilson Bulletin 104(1):65-84

1 comment:

  1. That's really interesting work especially the infanticide. I love those herp photos.