Filmed and edited by by Jean and Phil Slosberg in 2010 the sequence of events captured provides a nice basis for telling the natural history of chestnut-mandibled toucans. Slosbergs were able to record the nest preparation, parental care and nest maintenance phases from their home in Costa Rica. They note that after a sucessful breeding season the pair tried to nest in the same tree again in 2011 but were not successful because meliponid stingless bees have taken over the cavity. They attempted to clean the nest but couldn’t keep up with the bees. You can watch a short clip of a Meliponid bee nest below shot in a different location at La Selva Biological Field Station in 2004. The tube entrance made of bees wax is especially quite noticeable:
This is where the story gets interesting. Toucans nest in tree cavities and few trees provide that opportunity. Therefore they are limited by the availability of nest sites. A great majority of the trees with suitable cavities are found in old-growth forests. Pairs breeding in old-growth forests are expected to be the ones higher up in pecking order and defend their territories aggressively. Pairs ranking lower in hierarchy are expected to be pushed towards lower quality habitats such as forest edges or younger second-growth forests. As Slosbergs reported nests can be rather hard to maintain and even can be occupied by other species.
Social pressures coming from residents of higher quality habitats lead to formation of a behaviorally different group of birds that ornithologists call floaters which are adapted to survive in lower quality habitats. Floaters have no territories therefore can travel much longer distances than resident birds who spend a major part of their energy for defense. To give an example, floaters of the great horned owl can have home ranges 5 times the average territory of a resident. Toucan floaters can forage as a flock and invade resident birds’ territories overwhelming by numbers.
Movements of toucans have been measured using radio telemetry. Researchers caught toucans using mist nets and attached small radio transmitters that enable tracking of individual birds for several days allowing us to learn about their home ranges and territories. Using the technique researchers have tracked many toucans in Ecuador over the course of 4 years and estimated a home range of 86 ha with a maximum travel distance of a little over 3 km.
As we observe in Slosbergs’ video toucans can be predators. When the opportunity strikes they can take lizards, snakes, frogs and even the chicks of other birds. However a great portion of toucan diet consists of fruits. They are famed as fearless frugivores for they can fly over open spaces among forest fragments. They are very good in tracking resources. They remember locations of fruiting trees and consume large amounts of fruits. Trees with large seeds benefit from this in particular. Toucans can fly large distances away from the fruiting trees and disperse seeds by defecation or regurgitation.
Floaters could be especially important in seed dispersal and forest regeneration. Since almost all of the radio-tracked individuals were resident birds (it is easier to catch and track residents) we do not know much about floater movement patterns. One way of understanding floater movements comes from genetic studies of the trees whose seeds they disperse. Toucans play a quite special role for many trees since they do not chew and kill seeds like other dispersers such as agoutis, peccaries or monkeys. Therefore seeds dispersed by toucans have higher chance to germinate.
A study carried out on the black palm (Astrocaryum standleyanum) however painted a quite interesting picture about agouti seed dispersal by hoarding. Seeds tagged and tracked by miniature radio transmitters over a year have shown that agoutis steal seeds cached by their neighbours and more than a third of the seeds burried this way moved more than 100 m.
A compilation of night time camera trap footage below provides a good summary of most potential seed dispersers (as well as seed predators) other than toucans in a Neotropical forest including collared peccaries, white-lipped peccaries, Amazon red squirrel, agouti, paca, crab-eating racoon, coatis, pale-winged trumpeter, chachalaca and ocelot.
Scientists can identify parents of seedlings and saplings growing on forest floor through analysis of their DNA profiles. A genetic study of an abundant and geographically widespread tropical palm tree Iriartea deltoidea perhaps is providing clues about floater behavior. Researchers from the University of Connecticut matched the genetic profiles of the Iriartea palms growing in old-growth/second-growth boundary. Palms are structurally and functionally important components of tropical rainforests worldwide and therefore are particularly informative group to understand recolonization in second-growth forests. One in every ten tree-sized stems in a Costa Rican lowland rainforest is an Iriartea palm. Researchers have found that more than half of the trees in a young second-growth forest came from only two trees located in adjacent old-growth forest. Why seeds from only a very few trees germinated and survived while there were many other trees fruiting at the same time within the study area? Could this be due to a group of trespassing floaters raiding fruiting trees near the old-growth forest edge and returning back to second-growth repeatedly to avoid confrontation with residents? We do not yet know but science will tell.
Toucan behavior can be quite influential in restoration, maintenance of species diversity and genetic quality of the Neotropical forests. There is a growing body of evidence coming from other tropical forests that frugivores akin to toucans such as hornbills of Africa and Asia also behave and function similarly.
Finally, how toucans have evolved a beak so exaggerated in size has always been a curious subject. In his 1871 book titled The Descent of Man: And Selection in Relation to Sex, Charles Darwin was impressed. He stated on Chapter XVI “…that toucans may owe the enormous size of their beaks to sexual selection, for the sake of displaying the diversified and vivid stripes of colour with which these organs are ornamented”. Thermal imaging of a toco toucan (Ramphastos toco) while sleeping shows that the beak is very efficient in dissipating heat. The beak functions as a thermoregulatory structure just like the elephants’ ears and therefore selected to solve a physiological problem. However, Darwin may still be right about the beak color of these iconic Neotropical birds.