Blog # 236 The Amazing Egg

The avian egg has all the embryo needs to survive and grow. 

Bird eggs are amazing: self-contained wonders, like tiny spaceships with life support  systems. Eggs sustain life while inside, rapidly dividing cells miraculously construct a  living embryonic bird, getting ready to break out into the world. It takes about 36 hours  to form a fertilized osprey egg, but there is more to the story than that. Our osprey eggs’ tale began months ago in late winter when Ophelia’s shifting estrogen levels triggered  her urge to migrate home. In non-migrant bird species, changing day length often  stimulates this hormone surge, but not in the case of migrant ospreys. The cause of  osprey “Zugenruhe” or the uncontrollable urge to migrate remains a true scientific  mystery.  

During her 2,000-4,000-mile, month-long flight north to her nest, three eggs began to  develop in her ovary and yolk was deposited in them. The lipid-rich yolk, filled with  energy and tinted yellow-orange by anti-oxidants, will nourish and protect future  embryos. The germinal disc of a developing yolk contains the single ovum cell which,  after fertilization develops into a chick.  

Once back at her nest, courtship with her mate Orpheus stimulates more estrogen,  helping to mature the eggs. Even the sight of the nest fuels her hormones. Nuptial gifts  of fish from Orpheus are transformed into more yolk for her eggs. The bird’s single  ovary enlarges during breeding season as much as fifty times its non-breeding weight. Meanwhile, Orpheus, whose testes are internal, has also come into reproductive  readiness and is eager to mate.  

Anyone watching ospreys in spring has seen pairs copulating, and frequently—some  hundreds of times over a span of 2-3 weeks—yet only a third of these attempts are  successful and only unions during the last 3-4 days before laying fertilize the eggs.  Here’s why—

When it comes to mating, female ospreys call the shots; in order to achieve a  successful fertilizing “cloacal kiss,” the female must raise her tail and tilt forward so that  the male can curl his tail under hers. If the female is not receptive, none of this happens,  and the male is thwarted. If she is amenable, both birds evert their cloacae allowing  sperm to pass into her cloaca and swim up her oviduct. 

During ovulation, an osprey egg is released from a mature follicle on the surface of the  ovary. If it is fertilized by sperm, the ovum or egg becomes a developing embryo travelling through the oviduct. Here it is coated with proteinaceous albumen in about  three hours before passing into the isthmus, where the shell membranes are deposited  over the course of another hour. The egg moves into the shell gland for about 20 hours,  where the calcareous shell, made of calcium carbonates, is added.  

Urogenital system of a pigeon, courtesy of BrainKart.com. 

But the egg is not complete yet. Tiny glands in the shell gland coat the osprey eggs with  porphyrin pigments turning the shell a creamy brown with an endless variety of random  spots and blotches from deep chocolate to cinnamon to burgundy, The completed egg,  which are about the size of extra-large chicken eggs, passes into the vagina and cloaca  for laying, which usually occurs in the morning. The finished egg is laid, weighing about  

2.0-2.5 ounces each, with the first egg being the largest and the later ones  subsequently slightly smaller. 

Self-contained embryonic bird in egg. 

The completed osprey egg’s energy-rich yolk is 21 – 36% lipids, 16 – 22% proteins, and  the rest water and is suspended in the center of the egg by twisted strands of protein  fibers called chalazae. Albumen, consisting of 10% protein and 90% water, supplies  water to the embryo and acts like a shock-absorber cushioning the embryo and  insulating the embryo from sudden temperature changes. The inner and outer shell  membranes protect the egg from bacterial invasion and help prevent rapid evaporation  from the egg. Finally, the hard, outer shell protects the embryo and contains thousands  of pores that permit gas exchange for embryonic respiration.  

The beautiful pigment patterns on osprey shells vary from egg to egg, but are unique to  each female. While each of Ophelia’s eggs look different, it would be possible to  distinguish her clutches from that of other ospreys. Besides being recognizable to the parents, the eggs are perfectly camouflaged in the nest when seen from above,  protecting them from marauding aerial egg thieves like gulls and crows and egg collecting human

Osprey eggs, courtesy of reclaimednj flikr. 

s. 

Colorful osprey eggs, courtesy of Chesapeake Bay Program.

Ophelia incubates her eggs as soon as they are laid and ovulates again a few hours  later until she has a clutch of three eggs. In this uneven-aged clutch, the first egg gets a  developmental head start on the other eggs with the third, smallest egg being laid 4-6  days after the first egg. It will take, on average, 37-43 days in the Northeast for an  osprey egg to hatch and they do so asynchronously, in the order they were  laid, creating age and size differences between the nestlings.  

Mother osprey feeds young nestling, courtesy of the Hellgate nest cam. 

In my book, eggs are absolutely amazing. 

Eyes to the sky! 

Candace 

Candace E. Cornell  

Friends of Salt Point  

Lansing, NY 

cec22@cornell.edu 

ALL EYES ON OSPREYS 

WATCH! 

Salt Point Osprey Nest Cam 

READ!

On Osprey Time 

Ospreys of Salt Point 

VISIT! 

Cayuga Lake Osprey Trail