Avian eggshell formation

Many birds, more specifically passerines (order Passeriformes), rely on different sources of calcium for skeletal growth and store calcium in their bodies, a necessity for their ability to form strong eggs during the eggshell formation process. Near human settlements birds and in areas with high qualities soils, birds tend to have more access to the calcium that they need. Because of the importance of calcium passerines often ingest snail shells in areas in which the soil is calcium deficient. The most common source of calcium is snail shells followed by calcareous grit, bones, eggshells, and other sources.  Soil can be calcium deficient can be caused by acid precipitation which removes calcium from the soil. Most passerines are unable to ingest their much-needed amounts of calcium due to the scare amounts of nutrients in the habitats that they live in. This results in the development of thin fragile eggs that are unable to hatch thus affecting the bird population in various habitats.

 

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 Burgundy snail (Helix pomatia): a common European species

 

Dr. Jaap Graveland studied the impact of calcium on these birds. Great Tits (Parus Major) are a common Netherland forest bird. They are in the same family as the black-capped chickadee (Poecile atricapillus) and look like a fancy chickadee. Greats tits are native to 76 countries, are widespread, and have increasing populations. In Europe, it lays 6 to 11 eggs in the breeding season that occurs between January and September. Dr. Graveland studied the birds by comparing data obtained from the great tits gathered in the areas that were nutrient-rich with those that were scarce in calcium. Four of the seven study sites were typical Dutch forest and had soils that had been acidified therefore have reduced calcium content. The trees at these sites were Scots pine (Pinus sylvestris) and oak. 80 percent of Dutch forests are comparable to these sites. Three sites had nutrient-rich clay and differ tree populations. Some of these sites had only oak trees while others also had Corsican pine (Pinus nigra). All sites were 1-2 square kilometer. 100 nest boxes were placed at each site. The data showed that the habitat played an influential role in the calcium intake of great tits and demonstrated a correlation with eggshell defects in the area. Dr. Graveland’s data gives reason to believe that eggshell defects occur in nutrient scarce areas across the globe and not just the Netherlands. High-quality soils had almost a gram more of calcium in them per square kilometer.

 

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A pair of great tits. 

 

All sites the abnormal eggs were marked. It was found that only 10 percent of the abnormal eggs hatched. In comparison, as much as 95 percent of the regular eggs hatched. The defective eggs faced desiccation, or dying out, 55 percent of the time and breaking 32 percent of the time. Desiccation was a result of too thin of an eggshell.Nests were also collected after the breeding season. Samples were also taken at 14 days when the babies were weighed. Calcium was extracted from samples by using hot water. Collected nests were heated to kill insects, nests with babies more than a week old were used. Many nests with defective eggs where left before the eggs hatched therefore not included. Ten females were killed after they laid their eggs. Five females came from forests that laid normal eggs, others did not. All sites the abnormal eggs were marked. It was found that only 10 percent of the abnormal eggs hatched. In comparison, as much as 95 percent of the regular eggs hatched. The defective eggs faced desiccation, or dying out, 55 percent of the time and breaking 32 percent of the time. Desiccation was a result of too thin of an eggshell. In a separate part of this project, researchers gave birds snails fragments and chicken bone fragments. These bits and pieces ended up in nest material and in pellets. Pellets are the fecal matter produced by the baby birds. The experiment was done to ensure the researchers can identify sources of calcium in nests.

Great tits use calcium-rich substances to build their nest and due to the environment, they can’t find enough. Only 56% of their nests contain calcium; the rest of the nests can’t support the babies. The Great Tits droppings only contained about 34% of calcium. Which is mostly contained from eggshells and snails. Only fragmented pieces of snail shells and chicken shells at the Buuunderkamp Forest site meaning that the forest is running out of calcium-rich items. Due to the environment, Snails are dying out in Buuunderkamp forest and the birds are taking the chickens’ eggshells. Calcium was found in the stomach of all females whose eggs didn’t have defects. Int the stomaches of females there were few sources of calcium besides the snail shells. In calcium deficient forests the snail shells ingested where smaller and associated with egg defects. Fragments of large snails found to be enough to have proper forming eggs. These results show that a lack of snails causes poor eggs quality.

 

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A great tit nest. 

 

Great tits need snails to help with recent calcium deficiency they have seen in their environment but they can only adapt so much. Human-generated sources of calcium help reduce their low calcium levels. There were also fewer defects there where more human-generated sources of calcium. Human sources of calcium have to be obtained outside the forests which put the birds at higher risk. When birds were given extra calcium in their nests they ate less of the snails and human sources of calcium. Snails can’t form shells without calcium in the soil therefore great tits and soil quality is linked.The data showed that the habitat played an influential role in the calcium intake of great tits and demonstrated a correlation with eggshell defects in the area. Calcium is particularly low in Great Tits because they are insectivorous and insects contain little calcium. Other insectivorous also have issues obtaining enough calcium. Some birds have even been seen eating window putty, mortar, bird pellets, and ash. Dr. Graveland’s data gives reason to believe that eggshell defects occur in nutrient scarce areas across the globe and not just the Netherlands.

 

 

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