Hands-On Learning

     The Eastern Bluebird is a common species of open and semi-open habitats. Easily recognized by their brilliant blue plumage, bluebirds are a familiar sight in my Canton neighborhood, perch-hunting for insects from tree snags, mailbox poles and stonewalls.  

     My own yard not only provides excellent feeding opportunities, it provides bluebirds with a place to nest and raise young. In a typical breeding season, our lone nest box will produce two clutches of eggs.   

     After a successful first nesting this summer, a second attempt was soon underway. As activity inside the box increased, I confirmed that a new clutch of five eggs were placed. However, in the days to follow, the lively activity that had preceded the egg laying had all but ceased. Suspicious, I decided to check the box one more time. What I discovered was truly upsetting. The bluebirds had abandoned the nest box and five eggs due to a large infestation of sugar ants.

     Disappointed, I removed the nest and prepared the box for another possible nesting attempt. I brought the infested nest down to the woods to discard it properly and bury the eggs. As I was carefully picking out each individual bluebird egg to set it down on the ground, one of them had broken in my hand. The photograph below is what I found inside the broken egg.

     Admittedly knowing nothing about the development of an egg, I turned to the book; Manual of Ornithology (Noble Proctor and Patrick Lynch, 1993) for some enlightenment. I’d like to share with you some information from the book which helped me better understand the structure and development of an egg.

 

     The avian egg is a miracle of natural engineering. Light and strong, it provides everything a developing bird embryo needs from just after the ovum is fertilized until the chick hatches.

     The avian egg is a self-contained womb in which the embryo is protected and provided with the food and nutrients it needs to develop and hatch. But the egg is not a completely closed system, shutting off all outside contact. Birds, even as they are developing in the egg, are warm-blooded animals, with all of the metabolic activity that homoiothermy demands. The chick within the egg must be able to respire, to exchange gases and water vapor with the outside world, or it will suffocate.

     The bird egg must allow an efficient exchange of gases to support the growing chick, allowing waste gases out and oxygen in. The avian eggshell is not simply a coating of calcium carbonate but a complex laminate of mineral crystals embedded within a web-like matrix of material similar to collagen fibers. Reptiles, birds and mammals are called amniotes because the eggs of all three groups develop within an amniotic membrane. The amniotic membrane forms an enclosed, watery environment that protects the developing fetus and allows it to exchange gases with the surrounding world (Carey 1980, 1983; Romanoff and Romanoff 1949).

      The yolk of the egg supplies the embryo with nutrients, storing food until the chick hatches. The yolk sac surrounding the yolk is another embryonic membrane. The yolk is approximately 30 percent lipids (fats), 15 percent protein, and 55 percent water. Fat, not protein, is the primary food in bird eggs. This gives bird eggs a significant advantage over reptile eggs, in which the primary nutrients are stored as protein, because when broken down, fats yield more metabolic energy and water per unit than do proteins. They can thus survive in much drier environments than reptile eggs.   

     The age of the embryo, from what information I could gather, is anywhere between eight to twelve days old, given the size of the formed digit and toe plates.

     Despite the loss of our five bluebird eggs, I feel there is always a way to take a bad situation and extract something positive from it.  Learning about egg structure and the developing embryo was certainly an eye opening experience, as was the actual embryo in my hand.

     I also realized that I needed to be a better steward to the birds I provide for. Whether it be a hummingbird feeder or a nest box, periodic inspections and cleaning helps ensure a birds health and nest productivity.

     Finally, I would like to thank Noble Proctor and Patrick Lynch for publishing this outstanding book. The writing is excellent and easy to understand, and the drawings are exceptional.  Roger Tory Peterson’s foreword to the book says it best; “A gold mine of facts…Every library and biology department, as well as every birder, should have a copy close at hand.”  I couldn’t agree more.

Mob Mentality

     Red-tailed Hawks are one of the largest and most commonly seen species in Connecticut. The other morning while driving to the Riverside Park Sewage Pond in Hartford, I caught sight of one of these large raptors resting on a city lamp post. Perched beside the hawk was a Northern Mockingbird who was highly agitated and scolding loudly from a safe distance. Having failed to elicit a response, the mockingbird then embarked on an aerial assault which involved multiple pecks to the back of the hawks head and rump. In the end, the Red-tailed Hawk gave up and flew over to a nearby woodland, but not before enduring another strike from an angry Eastern Kingbird.

     So why are avian predators like hawks and owls always the recipient of another bird’s aggression? First and foremost, predatory birds provide a major threat to most other bird species, either because there is a good chance of being eaten or an even greater chance of having their young eaten. The behavior they exhibit is called "mobbing" and is done singly or in groups, depending on the species. For those who are not familiar with a bird’s mobbing behavior, it can be recognized by an array of harsh calls, wing fluttering and tail flicking.

     Mobbing occurs mostly during the height of the breeding season as a way to distract and encourage a predator to relocate to another area. Also referred to as the ‘move on’ hypothesis (Curio 1978), previous studies have confirmed that predators are profoundly distressed by mobbing and avoid the areas where they have been harassed. The ‘move on’ hypothesis appears to be most clearly linked to territoriality (Pavey and Smyth1998).  Learn more about territorial mobbing here;

http://www.fsl.orst.edu/flel/pdfs/Betts%20et%20al%202005%20Ethology.pdf

     As you can see from the photographs above, it can appear that predator harassment is risky business.  But in reality, birds who participate in mobbing are usually in very little danger. Species that are chosen to be mobbed are done so very carefully. For example, slow moving raptors like Red-tailed and Red-shouldered Hawks are less likely to strike back at an attacker, preferring to move on and save energy. Rarely would you see the same situation involve a Peregrine Falcon or Cooper’s Hawk.

Adult Cooper's Hawk in the same area two days later.
Where's the mockingbird now?

     Birders should also be aware of mobbing behavior during the winter months. On past Hartford Christmas Bird Counts, I have discovered Northern Saw-whet Owls in woodlands by following the scolding calls of Black-capped Chickadees and Tufted Titmice. David Allen Sibley writes in his book, The Sibley Guide to Bird Life and Behavior; Parids are social, bold, and inquisitive, traits that endear them to humans but make them among the most tenacious and aggressive songbirds when mobbing owls or other predators. Chickadees and titmice usually take the lead in announcing the presence of a small owl.

     Mobbing, on a rare occasion, can lead to dire consequences for the intended target. Winter flocks of American Crows often chase Red-tailed Hawks through city streets, forcing them into near-miss situations with telephone wires, buildings and even vehicles. Some, without a doubt, get injured or die as a result of these relentless attacks.

     Late last winter, I saw a Great Horned Owl get chased through the Cedar Hill Cemetery by a flock of very angry, noisy crows. When I finally caught up with the flock, the owl hung motionless by its neck, wedged between two branches. Did the owl make a mistake and stay out to long for another chance at a squirrel? If it did, it ended up costing the owl its life.  

Photo courtesy of Marcel Guerard