At the heat of the day, the sun beats down on the ever-drying marsh, now just a series of vaguely connected shallow ponds surrounded by deeply cracked, dusty soil. Some of the waterfowl have already fled to wetter regions, but other species remain, among them the northern jacana (Jacana spinosa). This abnormally long-footed inhabitant of marshy areas is a resident of Palo Verde and seems perfectly relaxed in its little remnant of marsh. But this apparent tranquility is a façade. Wait around for about ten minutes and you’re likely to watch as, out of the blue, a jacana launches into the air while vocalizing shrilly, lands next to another individual with its characteristic wings-up display, chases its subordinate and assumes its place all within a few seconds’ time.
This kind of intraspecific aggression is common, quick, and mostly results in no physical harm, so what’s the motive behind it? The dry season is a tough time for these insect and aquatic invertebrate eaters—their food supply is shrinking and their population is becoming denser as everyone crowds around the water holes. Could these two factors—food availability and density—play a role in triggering aggression? This is the question that I along with two fellow classmates pursued for our four-day independent research project at Palo Verde National Park.
First, let’s get up to speed on a little jacana biology. During the wet (breeding) season, it’s clear to see why jacanas might act aggressively toward each other. Exhibiting a polyandrous mating system, the females hold large megaterritories and two to four males hold smaller territories within that of their breeding female. Females defend their territory against other females, and males exclude other males. Once chicks hatch, both males and females (but much moreso males) defend their offspring against predators. In short, territoriality and chick defense describe aggression during the wet season, but it’s still unclear as to why jacanas exhibit aggressive behavior during the dry season.
For our project, we observed 74 jacanas over four days, going out to the marsh for three hours each morning and afternoon. In the field, we would select a jacana arbitrarily to be our focal bird for that trial, and each trial lasted 10 minutes. Next, we assigned roles: one person to be the data recorder, one peck rate monitor, and one distance measurer. To measure perceived habitat quality, the peck rate monitor glued their eyes to binoculars and counted the number of times the focal bird pecked at the water/substrate over 30 seconds. This measurement was taken once every minute for the duration of the 10-minute trial, and it gave us an idea of how much that bird valued the habitat—lots of pecks implied there was a high chance of getting food, and few pecks meant that there wasn’t much there to peck at. To measure density, the distance measurer counted the number of other jacanas within a 3-meter radius of our focal bird, and also estimated the distance from the focal bird to the nearest jacana. After 10 minutes, we moved on to another focal bird and rotated jobs.
Note that the distance measurer had to estimate distances visually, as it would be highly disruptive to tromp out there and drag a tape measurer from bird to bird. This meant that the three of us had to undergo “distance training.” In the grassy airstrip adjacent to the marsh, one person would set up two flags and measure the distance between them using a tape measurer without the other two looking. Then, standing at a range of 5-20m back (to emulate different scenarios in the field), the other two people would guess the distance to the nearest 0.1m. By no surprise, we were all rather horrendous at first (especially when the flags were set up vertically, with one in a plane directly behind the other), but after many repeated trials, all of us were able to estimate the distance correctly on average to within 20cm!
After four days of observing jacanas, we got some interesting results. Perceived habitat quality did not seem to influence likelihood of aggression, but density was more informative. The average density immediately preceding an aggressive encounter was higher than the average density overall (for all birds at all times during their 10-minute trials). This suggests that crowding might be a trigger for aggression. However, if the end goal of aggression were to increase an individual jacana’s “personal space,” we would expect its subordinate to be farther away after the aggressive encounter. Instead, we found that the distance between two birds engaging in aggression decreased just as often as it increased. Therefore, jacanas might not be concerned with crowding per se, but rather they defend a particular foraging site (whether or not it presently has food). In other words, perhaps it doesn’t matter how close your neighbor is to you, so long as he is outside the bounds of where you want to look for food.
Throughout the course of the study, we got the chance to become acquainted with the neighborhood at the marsh. Spoonbills and jabirus frequented the deeper edges of the water while the ducks tended to hang around the shallower areas, along with our jacanas.
One of our favorite characters at the marsh was the not-so-friendly neighborhood caiman. Every once in a while, we’d see something log-like moving through the marsh just a little too quickly and with a few too uniform ridges to be a log. Then it would stop, raise its head straight out of the water, and make several chomps at the air before slowly submerging itself back underwater. We all jeered at this attempt at ferocity, given that the caiman’s mouth looked barely large enough to fit a small sparrow, let alone any of these waterbirds. However, we did once find it feeding rather clumsily on what appeared to be a duck carcass (whether the caiman actually killed the duck or found it dead already is unclear).
And the white-faced capuchins made lovely company, whether they were playing, jumping through the trees, or, quite often, taking a late-morning snooze.
We’ll be sad to leave this lively marsh, and all of Palo Verde, but new adventures at the Monteverde cloud forest await us. (We will be without internet during our stay at Monteverde, but be sure to look for updates later this month!)