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August 2014


Spring has arrived and much of the vegetation is beginning to produce new foliage and flowers, as temperatures rise steadily and daylight hours lengthen. The onset of the summer rains may still be some way off, as we reputedly enter another el Niño period. Active weed growth and effective control will only commence once the rain arrives.

This month, we will look at the fascinating subject of bird reproduction - which is about to commence in earnest. All birds lay eggs, a puzzling phenomenon when one considers the reproduction of the two animal groups most closely related to birds, namely mammals and reptiles. Mammal reproduction can involve egg-laying (echidnas and platypus), but all others give live birth. Reptile reproduction can involve egg-laying, live birth after incubation, the hatching of eggs within the mother’s body, or live birth in much the same manner as most mammals.

Live birth has possibly never evolved in birds due to the adverse implications of the extra weight on flight ability. Another factor may involve temperature, as avian embryos, like those of mammals, need to be at 36-39 degrees celsius for normal development. Avian body temperatures are normally above 40 degrees - probably due to the intense muscle activity required for flight.

What is an egg? Essentially, it is a survival capsule for the developing embryo. As the embryo develops, it requires a supply of oxygen, water and nutrients as well as a way to dispose of carbon dioxide and waste products. The yolk consists primarily of proteins and fats and provides nutrition for the embryo. It also contains antibodies from the mother’s immune system to provide protection against pathogens. The albumen (egg white) serves as a source of water, as a shock absorber and a buffer against sudden temperature changes. Waste materials are handled by a structure called the allantoic sac. The uptake of oxygen and release of carbon dioxide occur through microscopic pores, in the shell. The shell is often pigmented to provide camouflage.

Shape and sizes. Egg size is generally relative to bird size, expressed as a percentage of adult body mass. The largest egg is that of an Ostrich, representing 2-3% of the mother’s body mass. However the Brown Kiwi’s single egg is a staggering 20% of the mother’s body mass! Eggs also vary considerably in shape from sharply pointed to almost spherical. Part of this variation is related to the nest site. Spherical eggs are common in hole-nesters. Pointed eggs are common in cliff nesters and other sites, where eggs could easily roll out. Their shape reduces the risk of this happening. Shape may also influence how eggs fit under an incubating adult. Spherical eggs are optimal for single-egg clutches, symmetrically oval eggs for clutch sizes of 2-3, pointed eggs for clutches of 4-7 and for clutches in excess of 7 eggs, spherical again.

Incubation. Most eggs are laid in locations where temperatures are cooler than the optimal range for embryonic development. Incubation is provided by one or both parents. In approximately half of all taxonomic families, both parents share incubation, 40% by female-only and about 6% by male-only incubation. The efficiency of heat transfer to eggs is often enhanced by the presence of one or more brood patches, (areas of bare skin in close contact with eggs). Incubation is a trade-off: time on the nest is good for eggs but bad for the adult whereas time off the nest is bad for the eggs but good for the adults. Birds breeding in extremely hot habitats have the challenge of keeping eggs cool enough to avoid lethal hyperthermia. Teamwork is critical with parents alternating incubation duties in shifts as short as 10 minutes so they can drink frequently and avoid dehydration. Other species cool their eggs by visiting water and soaking their belly feathers before returning to incubate.

African Birdlife magazine - Sept/Oct edition (Volume 2 no. 6) article by Andrew McKechnie titled “Cracking the secrets of eggs”.

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