Before going to Belize I thought of hummingbirds as docile little creatures who went through life serenely sipping nectar from flowers, but upon arriving at La Milpa Ecolodge I was quickly shown how aggressive and territorial these little guys can be. There were three species clamoring for a location at one of the many feeders around the dining area, and it amused me to no end to see them give chase to each other. Of the three species found two were identified as the White Necked Jacobin, and the Buff Bellied Hummingbird, while the third, given its very common coloration and tendency to not sit still, went unidentified. These interactions sparked an interest in how these feeders could potentially cause conflict between species that normally would never come into contact with each other, and how these conflicts can have impacts on the energy expenditures for them.
But first it may be pertinent to start with why animals, with such similar life histories, would actually never meet, or at the very least never fight. Even though all hummingbirds drink nectar, not all hummingbirds use the same flowers due to physical constraints like beak size, or perhaps they use them at different times of the day, or even they forage in different habitats, as is the case for the White Necked Jacobin, and the Buff Bellied hummingbird, two of the three species found at La Milpa’s feeders. The Jacobin is primarily a canopy dweller, except when there are feeders to be found. And the Buff Bellied prefers open woodlands, and second growth clearings.
These hummingbirds under normal circumstances would never come into contact with each other due to these differences in habitat preference, but the feeders draw them in together. Feeders lack some of the things that can exclude certain species from capitalizing on certain flowers. The Sword Billed Hummingbird, whose bill is longer than the rest of its body, can capitalize on flowers such as the Passiflora mixta, which has an extremely long corolla. While this species was not observed at La Milpa feeders, it does serve to illustrate the specificity that hummingbirds exhibit with their food resources. Birds with much shorter bills will not be able to get to the nectar at the bottom of the corolla, which would mean that those short billed hummingbirds would not compete with the Sword Billed for that food source. Feeders do away with this kind of exclusion by providing easily accessible sugar water at all times. Any kind of hummingbird can use it, not just ones with extraordinarily long bills or ones that feed high in the trees as is the case with the Jacobin.
All hummingbirds walk a razor thin line between energy gained and lost. They must enter a hibernation like state called torpor every night just to survive without eating. If they are unable to do this, or go too long without eating they will die because their metabolism is so fast, and they do not store much energy because it hampers flight. It was intriguing to think about how the constant availability of a high quality food source such as the La Milpa sugar water would alter the number of defense activities of the hummingbirds, and how that might alter their energy requirements.
Research has shown that when the resource value for a territory increases, the territory defenses will increase as well. This was shown in the the Dearborn 1998 study, and the Camfield 2006 study. In the Dearborn study increased resource value by injecting extant flowers with sucrose solution increasing the yield. The other study had three different solution concentrations, 10%, 20%, and 30%. Both of these studies all looked at territorial species just like the Jacobin.So it stands to reason that the Jacobin would behave in much the same way when presented with a high value resource such as a near infinite supply of food from a feeder. That should mean that Jacobins choosing to utilize the feeders at La Milpa are spending more time chasing away rivals, of which there were many, than Jacobins that do not have access to feeders which are easily defensible, and are a limitless resource when compared to flowers that can only provide a small amount of nectar at a time. This would cause them to increase their energy output due to burning calories by chasing or fighting with other hummingbirds.
For the Dearborn study, the intruder’s size was also examined, and it was found that the size of the intruder had a negative correlation with the number of territory defenses. However, this negative correlation lessened when the resource value of the territory was increased by injecting extant flowers with sugar solution to increase their yield.
The Powers and McKee 1994 study, the territoriality was investigated in response to changing food availabilities for a Blue Throated Hummingbird’s territory. Sucrose solution in syringes was left out consistently without limit to the amount or the territory was only given 32mL of solution per day. On restricted days 48% of interspecific intruders were chased, while on unrestricted days only 11% were chased. Intraspecific intruders were chased 81% and 80% of the time on unrestricted and restricted days respectively, giving a new element to the intricacies of hummingbird interactions where intraspecific and interspecific intruders will be treated differently.
In the Tiebout 1993 study two species of hummingbirds were evaluated to see the costs of competition between them. One was the territorial Amizilia saucerottei, and the other was the trapline feeding Chlorostilbon canivetii. They were monitored for many variables that can indicate dominance, and energy expenditure. These birds were paired in heterospecific and conspecific pairs, and the activities of these pairs was compared to individuals that served as controls. For paired birds there was a 25% increase in hovering, and an overall 13% increase in energy expenditure, which illustrates that the presence of rivals has a marked impact on the energy balance of these birds. These costs can be offset by taking in larger meals, which is afforded to the bird who wins control of the feeder. Normally the winner is the territorial species, which, in the case of La Milpa is the Jacobin.
Another study focusing on the habits of a territorial hummingbird species was the Lyon, Crandall, and McKone 1977 study. A territory was artificially increased to an area of 85m2 by moving feeders out further away from one another making it more difficult for the resident male Blue Throated Hummingbird to defend the boundaries of his territory. As the territory got larger the male spent more time chasing, and foraging to make up for the increased energy expenditure. Another interesting factor to this study was that as the territory size increased the number of different species allowed to forage increased as well, mostly due to the fact that it was impossible for the male to chase all of them off. I suspect that this is one of the reasons we see multiple species at the La Milpa feeders. For the very few territorial hummingbirds that reside in the area, there are too many feeders spaced too far apart to adequately defend all of them.
Hummingbird feeders are a great way to draw in many types of hummingbirds, but they do cause conflict between species that may never come into contact with one another. These conflicts can have significant effects on the energy allocation of the birds, and will result in altered behavior be it increased agonistic activities, increased food intake, or both. For endangered species having access to feeders like these has the potential to increase their survivorship, and reproductive output because it lessens the burden of finding enough food, but there is a downside to these feeders as we have seen.
Camfield, A. F., 2006. Resource Value Affects Territorial Defense by Broad-Tailed and Rufous Hummingbirds. Journal of Field Ornithology. 77:120-125.
Dearborn, D. C. 1998. Interspecific Territoriality by a Rufous-Tailed Hummingbird ( Amazilia tzacatl ): Effects of Intruder Size and Resource Value. Biotropica. 30:306–313.
Lyon, D. L., Crandall, J., and McKone, M. 1977. A Test of the Adaptiveness of Interspecific Territoriality in the Blue-Throated Hummingbird. The Auk. 94:448–454.
Powers, D. R., and T. McKee. 1994. The Effect of Food Availability on Time and Energy Expenditures of Territorial and Non-Territorial Hummingbirds. The Condor. 96:1064-1075.
Tiebout, H. M. 1993. Mechanisms of Competition in Tropical Hummingbirds : Metabolic Costs for Losers and Winners. Ecology. 74:405–418.