I’ve been watching the pair of peregrine falcons nesting on Charing Cross Hospital a few times in person when I’ve been passing recently and remotely via the wonderful Twitter account @FaBPeregrines as well as on their website (http://www.fabperegrines.org.uk/). It has fantastic CCTV footage showing their nest and breeding efforts, where in recent days there has been some unusual activity. The female, Charlie has been worrying everyone as she has been laying eggs many days apart rather than on a regular schedule as in previous years. It is unknown whether this years breeding attempt will be successful with the eggs hatching, but the pair are currently incubating the clutch of three eggs with the male falcon or tiercel, Tom, providing food and allowing Charlie off the eggs to take breaks from incubation. Have a look yourselves! It should be an interesting story if we get to see the chicks grow and take flight later in summer.
Speaking of flight, watching Charlie caring for the eggs and leaving the nest earlier, it got me thinking about the evolution of flight and the vulnerability of those eggs in the nest when left unattended. My first thought was that laying eggs allows the female bird to maintain her ability to fly as carrying foetuses in gestation until fully developed would weigh her down. But perhaps it is more accurate to say that the evolutionary step the reptiles developed of laying hard, shelled eggs to come out of the water onto land allowed them to later evolve the advantage of flight and turn into birds by taking to the skies! Virtually everything about a bird’s anatomy and design is aimed at maintaining a light body weight capable of flying through the air. Even today’s flightless birds such as penguins, ostriches and Kiwis demonstrate specific adaptations of this order of animals which makes them unique from other orders such as reptiles and mammals. The defining characteristics of birds in general relate to the ability to fly. Other animals however can fly to varying degrees. Bats for instance are the only mammals capable of true flight. Various other species such as lizards, frogs, flying squirrels and so on glide rather than truly fly. So aerial mobility is not unique to birds. Flight is not unique to birds. The distinctive or unique characteristic of birds is the evolution of the feather as a flight aid.
Feathers allow insulation and aerodynamic contouring as well as providing lift to allow flight. Feathers evolved in ancient reptilian ancestors of birds as can be seen in the fossil Archaeopteryx, the earliest documented bird. Some other adaptations to flight include the following:
- Hollow but strong pneumatised bones supported by a network of internal cross struts to minimise bodyweight but power strong flight
- The development of a stongly keeled sternum or breast bone to allow a large area for attachment of well developed flight muscles for wing propulsion
- The unique anatomical adaptations of the wrist or carpometacarpal bones forming the avian wing
- A fused clavicle or collarbone to act as an anchor or fixed point from which to drive the flight stroke
- A unique respiratory system comprising paired lungs and a series of air sacs within the body through which air passes in a unidirectional flow. These air sacs contribute to the lightness and buoyancy of birds
- Fusion and elimination of some bones, again to maintain a strong yet lightweight frame
- Reproductive organs remain tiny therefore lightweight during non active periods, with ovaries, oviducts and testicles enlarging only during the breeding season