The rock pigeon (Columba livia) is one of the most common sights in cities around America. Toward the end of February, I sat in the shuttle pick-up area of the Orlando International Airport in Florida for what seemed like hours, waiting for the shuttle that would transport me to my hotel. But what was trying for my patience was fortunate for my bird-watching. While the airport is slightly removed from the more urban areas of Orlando, there were several pigeons to observe. As shown by this and the Rice campus, pigeons thrive in places other than the urban downtown areas with which people usually connect them. I saw approximately ten in the half an hour I sat waiting for the van.
Pigeons are considered by many to be rats with wings, but I have never really understood why. In high school, my history teacher and I developed an ongoing dispute over his insistence on shooting pigeons at his house. I remember that he sent me an article about a bridge on the verge of collapse. The hypothesized reason? The acidity and ammonia of pigeon excrement had caused the steel of the bridge to rust! I must admit, that is pretty bad, and the regularity of pigeon droppings is a nuisance. But other than small annoyance toward the ubiquity of pigeon waste, I have never felt the need for animosity toward pigeons that others have. (And apparently Mike Tyson agrees, although these aren’t necessarily rock pigeons in particular.)
Pigeons are such a common sight that we rarely feel the need to stop and watch them, which is a large part of the reason others find them so annoying, I can only presume. I realized, though, after consciously watching them and inspecting their physical characteristics, that they are actually quite attractive birds! And the cooing sound they make is fairly pleasant. The markings on each pigeon vary more than in other bird species, but in general their simple gray feathers are complemented by extremely iridescent teal and purple colors that are more extravagant than most birds so common. With their beady orange eyes, bright orange feet, and distinctive black bars on the wings, they are pretty colorful birds, although the eye is usually dominated by the majority color, gray. The pigeon isn’t underappreciated by everyone; however, they definitely have their devotees, although the most pigeon-crazy people are still not making a big fuss about the wild variety. There seems to be quite the dichotomy between opinions of feral pigeons and domesticated ones.
The pigeons matched the hustle and bustle of the airport, rarely staying still and almost constantly walking somewhere while characteristically bobbing their heads. They frequently made movements of scuttling sort, as well, as I noted in one bird that landed on a rafter and side-stepped until it decided it would rather be somewhere else and flew to the ground. I observed how they responded to external stimuli, mainly humans. There was little noticeable interaction between the birds. When headed in a general direction, they tried to maintain their course as long as possible despite looming interference from a human passerby. If it was absolutely necessary to avoid being kicked, they would hasten their steps and scurry to the side slightly. Only rarely would they agitatedly flap their wings and quickly move away. They generally displayed a lack of interest in attempting to significantly distance themselves from humans, which is interesting. It demonstrates that they do not need or desire to move far away from humans as quickly as possible. Few animals are as successful as pigeons are at adapting to human change in the environment and coexisting with us.
One interesting adaptation of pigeons, although not related to living with humans, is beak shape. Most pigeon beak tips feature a small hook, but why is it there? Turns out, it actually evolved as a mechanism for combating feather-eating lice infestations, which frequently afflict rock pigeons. The ancestors of modern pigeons were better able to damage, kill, or remove the lice while preening if they had this tiny hook on their beak, and since their infestation would be consequently less severe, they maintained better feathers than non-hooked conspecifics. Feathers are vitally important to a bird for many things, including maintaining warmth, evading predators, and attracting mates. Dale Clayton found that females significantly prefer clean males to males with high parasite loads, even though plumage damage is not visible and time spent grooming did not vary much between the two groups. So, birds with better feathers are better able to survive and reproduce, and therefore they have more offspring. Clayton and his colleagues decided to demonstrate the validity of this hypothesis for the existence of hook-tipped beaks. In order to test this, they caught 26 pigeons and trimmed the tips of the beaks. Over the course of the next 18 weeks, these birds developed lice infestations of a higher concentration than they had been prior to the beak-trimming. 13 of the birds, chosen at random, regrew their hooks. The feeding efficiency between both groups was not significantly different. The parasite loads, however, of these newly-hooked birds grew smaller (Figure 1), while they remained at similar levels for the hookless birds, indicating that the hooks are indeed useful in removing lice.
Rock pigeons may have evolved another way to combat lice. Some bird species have the uropygial gland, also known as a preen gland because it produces oil that the birds spread throughout their feathers during preening. The oil is important in maintaining plumage condition, preventing breakage and brittleness. The reduction of feather quality in the absence of this oil is potentially due to increased ectoparasite loads because the oil may have an insecticidal quality. A study performed by Clayton and his colleagues at the University of Utah was somewhat inconclusive, as captive birds with the gland removed did not demonstrate noticeable change in parasite loads, but lice raised in an incubator died more quickly on feathers with oil than on feathers without. The authors recommend further research on the matter. It would be quite interesting if something as seemingly mundane as feather oil served as the pigeons’ own personal insecticide!
Another curious feature of pigeons is their characteristic and consistent head-bobbing, which I alluded to above. (Although it actually occurs in 8 out of 27 orders of birds as well). What could they possibly get out of bobbing their heads so dramatically whenever they moved? There have been theories that the movement aids balance and depth perception, but Barrie J. Frost found in a study that it is primarily a visual response, rather than an equilibratory response. The motion was studied using high-speed motion photography. It was found to consist of two phases: one where the head is locked in space but moves backward relative to its body, and one in which it is quickly thrown forward (Figure 2). Interestingly, Frost had the pigeons walk on a treadmill, and head-bobbing stops! Since the treadmill acts to keep the pigeon in place, its environment remains relatively stable. The birds are still moving, but the scenery does not change, hence why it is perceived to be a visual matter as opposed to equilibratory.
Domesticating rock pigeons may have occurred as long ago as five thousand years. One of the things that they have become most renowned for is their homing ability, and so pigeons have been used to deliver messages during wars. They are even lauded as the “thoroughbreds of the air” for this characteristic. It might be easy to assume that they use visual clues from familiar landscapes to orient themselves, like humans and other animals. However, they also have a navigation system that is independent of learned landmarks, and so these visual clues are somewhat redundant, so it is difficult to determine what exactly the pigeon is responding to during its flight. One study found that birds allowed to preview the surrounding scenery before release returned more quickly, indicating that visual cues do in fact aid the pigeon’s homing system. Overall, exactly how this system works is somewhat of a mystery. Other potential cues include the Earth’s magnetic field, the sun used as a compass, stars, infrasound, a polarized light compass, and olfactory stimuli.
While watching the pigeons at the airport, I observed many physical attributes and behaviors of the birds. For most, I did not realize the underlying significance. Now I know pigeon’s beaks are hooked to help defend against lice, and they bob their heads to aid depth perception and balance. Despite being such a common species, there is still a lot to learn about rock pigeons!
This article explained how pigeon parasite load affects sexual selection.
I used this article to learn about adaptations of beak shape in response to lice.
This article discussed the reasons why pigeons bob their heads when they walk.
This article discussed the role of oil in the feathers of pigeons as a possible insecticide.
This article discussed the roles of visual cues in homing pigeons.
Figure 1 (from Clayton et al 2005)
This demonstrates the impact of the hooked tip on lice.
a) Rock pigeon before trimming.
b) Rock pigeon after trimming.
c) Young pigeons with trimmed beaks (black squares) had more lice on average than young pigeons with untrimmed beaks (grey triangles).
d) Mean number of lice on adult pigeons. The hooks of all birds were trimmed for 17 weeks and at week 18, half the birds (gray triangles) were allowed to regrow them, while the remaining half (black squares) continued to be trimmed weekly. The birds with regrown hooks had fewer lice than the birds who continued to be trimmed.
e) Birds allowed to regrow their hooks (grey bars) tended to have a greater mass of feathers than birds trimmed throughout the rescue experiment (black bars).
Figure 2 (from Frost 1978). This demonstrates that, while the body is moving fairly evenly, the head moves in a pattern of quick movement forward followed by a subsequent “hold” position, where the head remains locked in space.
Figure 3. Distribution map for the rock pigeon. The dark red represents the approximate native range while the pink represents introduced non-native populations. From:
Rock pigeons in their natural habitat. From: http://en.wikipedia.org/wiki/File:Rock_pigeons_on_cliffs.jpg
Adult rock pigeon. From: http://en.wikipedia.org/wiki/File:Blue_Rock_Pigeon_(Columba_livia)_in_Kolkata_I_IMG_9762.jpg