BRANTA — S. James Reynolds
Uptake and disposition of calcium and water by egg-laying Zebra Finches (Taeniopygia guttata)
Institution: University of Oxford, U.K.
Supervisors: CM Perrins, RH McCleery
Details: DPhil 1998 (Completed)
Address: School of Biosciences, The University of Birmingham, Edgbaston, Birmingham, UK B15 2TT (Oct 2005) Email
Subject Keywords: Calcium, water, reproductive biology, reproductive physiology
Species Keywords: Zebra Finch Taeniopygia guttata
Calcium and water are two major constituents of eggs. Calcite, a crystalline form of calcium carbonate, makes up 98% of the shell which provides the developing chick with protection and a supply of calcium. By mass, water is the largest fraction of the avian egg. ‘Plumping!, the movement of water into the developing egg, causes it to swell and stimulates shell calcification. A number of studies have examined the role of lipid and protein reserves in egg production but very few have specifically addressed the uptake and disposition of calcium and water during laying.
Egg-laying Zebra Finches (Taeniopygia guttata) retained significantly more administered calcium (45Ca) than did non-laying hens during the whole laying cycle. Localisation of 45Ca was high in the long bones in the first 16 hours of the laying cycle but low in the eight hours before oviposition. In these eight hours, birds were at roost and relying upon skeletal reserves for the calcium for shell formation. 45Ca was high in ovarian tissues at the onset of shell deposition and a partially calcified egg contained some radioactive calcium. Medullary bone is a specialised, short-lived form of bone found only in laying birds. A study of medullary bone in long bones of laying birds demonstrated massive remodelling with very high osteoblastic (calcifying) and osteoclastic (decalcifying) activity. Non-laying hens had heavily calcified medullary tissues throughout the laying cycle. Across a single laying cycle, dramatic changes in medullary bone composition occurred. Medullary reserves may be fundamental to the Zebra Finch as a short-lived, highly labile calcium store when dietary calcium is unavailable for overnight shell formation. Bone scintigraphy revealed that it is possible to use medical imaging to visualise calcium exchange in situ in live birds. Refinements of the techn ique are discussed. Calcium-deprived hens, maintained on a low calcium diet, produced eggs of similar size and shell structure to those produced by hens on an ad libitum calcium diet. They also had similar skeletal masses suggesting that they did not resort to breakdown of structural (cortical) bone, in addition to relying on medullary bone, in response to shortfalls in dietary calcium. Compared with when they were on ad libitum calcium, hens on limited calcium reduced clutch size, but an extended period on a totally calcium-free diet may be necessary if the effects of calcium limitation are to be fully apparent. Even on very low calcium diets, Zebra Finches may obtain sufficient calcium for egg production by increasing their digestive efficiencies.
Total body water was measured twice in the same individuals. It increased dramatically from 7.82 ml during prelaying to 11.92 ml at the onset of laying. This oedematous response provides an enlarged reservoir for exchange of metabolites during egg production. Body mass increased by only 0.84 g suggesting that such a degree of hydration can only be accommodated by atrophy of body tissues. Water intake was higher in hens in the 10 day period before the onset of laying, compared with that in non-laying hens over an equivalent period. Whether increased water intake is the mechanism of the hydration and, if it is, when it occurs, remain to be studied.
Reynolds, S.J. & Perrins, C.M. in press. Dietary calcium availability and reproduction in birds. Current Ornithology.
Reynolds, S.J., Mïnd, R. & Tilgar, V. 2004. Calcium supplementation of breeding birds: directions for future research. Ibis 146: 601-614.
Reynolds, S.J. 2001. The effects of low dietary calcium during egg-laying on eggshell formation and skeletal calcium reserves in the Zebra Finch Taeniopygia guttata. Ibis 143: 205-215.
Reynolds, S.J. 2000. Low dietary calcium during egg laying: effects on egg, clutch and laying parameters, and on skeletal calcium reserves of the Zebra Finch (Taeniopygia guttata). Avian and Poultry Biology Reviews 11: 304-305.
Reynolds, S.J. & Waldron, S. 1999. Body water dynamics at the onset of egg-laying in the Zebra FinchTaeniopygia guttata. Journal of Avian Biology 30: 1-6.
Reynolds, S.J. 1998. Morphological changes in medullary bone during egg-laying in the Zebra Finch Taeniopygia guttata. In: Adams, N.J. & Slotow, R.H. (eds) Proceedings of the 22nd International Ornithological Congress, Durban. Ostrich 69: 383.
Reynolds, S.J. 1997. Uptake of ingested calcium during egg production in the Zebra Finch (Taeniopygia guttata).The Auk 114: 562-569.