Birds need calcium to lay their eggs- where do they get it from?

April 25, 2011

in Birds, Gardening For Wildlife, Slugs & snails

Snail shells in nest material prompts me to research!

Well it was my turn to make the evening meal. I also needed to clean out the tit bird boxes. So being thrifty and not wishing to waste anything, I decided to combine both activities and make birds nest soup! When looking through both the great and blue tits’ nest material I found small fragments of snail shell and wondered why it was there. I had never seen them eating snails or snail shells. Why were they there?

Difficult area of research

Although slowly increasing, there is still a paucity of knowledge regarding the nutritional requirements for wild birds. But this is not surprising. The essential nutrients for a balanced diet are not as easy to study for wild birds as they are for captive birds. Areas such different species, ages, sizes, weight, seasonal requirements, population sizes, behaviour and weather for example, all complex issues in their own right! Then there is amounts of fat, fibre, carbohydrates, proteins, minerals, vitamins and combined with seasonal energy requirements, again all complex issues. Something most of us simply do n’t think about when we throw that old ham sandwich out onto the lawn ‘for the birds’!  Whereas many bird food retailers concentrate on food/energy requirements, I feel an area that should be further investigated is that of calcium intake and I will explain why now.

calcium in the form of a bird’s egg shell

Calcium for eggs and chicks.

Our garden songbird birds families do not ‘store’ the extra calcium needed for egg laying in advance in their skeletons. Perhaps that would make then too heavy, less manoeuvrable and therefore more easily caught by a sparrowhawk or other predators. Storing extra calcium in the skeleton could also affect the functioning of the skeleton. So where do the get it? They have to find a dietary source of calcium and eat it.

Calcium is vital to breeding birds

Without sufficient calcium, eggshells can be defective causing clutch desertion as the hatchlings never hatch. Calcium is needed in large quantities for egg shell production and for the healthy growth of the new born chicks. Without it young birds can be unhealthy, deformed or even die. During the years when DDT, the organochlorine pesticide was used, there was a marked decline in the thickness of certain bird’s egg shells, such as song thrushes and predatory fish-eating birds of prey. Although not used now in the UK, acidification of the soils may be having a similar effect on birds’ egg shells through lack of calcium and even a lack of snails, that need calcium for their shells.

Calcium in their diet

In order to produce calcium quickly for egg laying and then to provide calcium for a couple of weeks or so whilst their young birds grow a strong skeleton, would require huge numbers of insects or seeds from which that extra calcium would be required, that simply are not be available in sufficient numbers to provide the much needed boost in calcium the birds would need. Indeed research has found that insects and seeds alone may only provide 10% of the calcium needed for egg laying. Which insects would be available in such huge numbers and imagine the time spent looking for those insects and how many would they have to eat? During the early part of the breeding season here in the UK, I doubt very much whether that is a likely scenario.

birds need to eat calcium to form their egg shells containing the young bird

Specific calcium foraging RIGHT before they lay

Before eggshell formation begins, the egg content has to be almost completed. That makes sense! Female great tits were found to deposit 60% of the calcium in the shell in less than 8 hours, prior to laying. Whereby, the fat and protein deposited in the egg is extended over a 4 day period. A general feature of many wild birds is to lay down the calcium needed to lay the night before laying.

So at this very critical time, females are forced to reserve sufficient time during the day to forage and collect calcium specifically for the eggs. Valuable time away from feeding themselves. Finding a calcium rich food supply is therefore very important and they are forced into a very narrow time window in which to do so.

Birds have to search harder to find a suitable calcium source in calcium poor soils. Not increasing their search effort will almost certainly result in laying thin-shelled eggs that fail to hatch. Searching harder for calcium means less time searching for food for themselves. Less food intake or more energy expended in searching for calcium may result in a bird laying smaller and fewer eggs and/or reduce the probability of second clutches.

Different ‘Calcium’ strategies for different birds

Birds that spend time searching for a calcium rich source just for their future egg shells are therefore, depending upon the species, spending time away from feeding food for themselves, unless of course they have other strategies. For example, wood pigeons consume vegetable/fruit matter producing a highly nutritious crop milk which is fed to the squabs. However, they only lay 2-3 eggs. Could a lack of calcium in their diet be limiting their egg numbers?

Young blackbirds are unable to get enough calcium for their growth from worms alone, but can from the soil in the worms guts! Robins, wrens and pied flycatchers will forage on the ground to obtain their calcium by eating millipedes and woodlice, both soil dwellers with 3 times higher calcium content than snail shells. Other than the great and coal tit, both of whom forage in autumn on the ground looking for beech mast, the tit members do not generally forage on or near the ground to obtain their insect food, nor do they take millipedes and woodlice. So where do they get their calcium?

A natural calcium source right in your garden? Perhaps!

Springtime is when snails are becoming more active after their long winter sleep. The calcium content of your soil, may determine the number of snails you have in the garden. If you have many millipedes and woodlice in your garden it may well indicate a calcium rich soil although they require less calcium than snails.

Prior to laying, female tits ( as do many other woodland birds) go to roost at night with their gizzards full of calcium by eating snail’s shells. The calcium, if consumed any earlier in the day, would leave the gut by the time it was needed for eggshell formation. Thus they have to contend with a very narrow time window. If they fail to find any and consume it within the required time frame, they will be unable to lay their eggs. Calcium availability can limit the reproduction of wild birds.

Beside acid rain causing a decline in soil calcium on poor soils, resulting in a decline in snail populations in some European forests, many people use slug pellets to kill snails and slugs to protect the ‘valuable’ flowers and plants’ they have just bought from the garden centre to beautify their gardens. Killing snails with poisons reducing their numbers, can only deplete snail shell availability in our gardens, at this critical time for our birds. Snail shells are considered to be the main source of dietary calcium for many of our garden birds. Something to ponder upon!

Birds do n’t eat snails

Snails try to avoid being eaten by hiding during the day and making their shells stronger or more ornate. This needs calcium, gleaned from the vegetation growing in soils with a calcium content and time for the snail to grow. Young snails have very thin shells and therefore would make a better target for many predators including birds, although again I have never seen them being eaten by any birds other than song thrushes, which is the only bird that has adapted and evolved to crack open snail shells, although I have watched blackbirds wait for a song thrush to crack open the snail shell then pounce and steal the now ‘naked’ snail. A very good example of kleptoparasitism!  Young snails with thin shells would hide from potential predators and more time would have to be spent searching for them.

The other week I was removing an old shed. Underneath it were two wood mice which scampered away. There were numerous empty snail shells and snail shell fragments, underneath the shed. Not one live snail, only empty snail shells, all either with holes in them or cracked open. I also had to thin out some dense ivy on wooden fencing panels. Again I found numerous empty snail shells 4-5 foot up and deep in the ivy itself. The snails must climb up the dense ivy and hide. As wood mice eat snails, I think the wood mice have climbed the ivy and eaten the snails in situ and also found snails in the garden and carried them to eat in relative safety underneath the shed.

Other calcium sources

In experiments, great tits deprived of snail shells, doubled their search effort and started to burrow in the soil and to eat sand, small stones and even their own eggs. Birds may find other sources of calcium, such as bones from dead mammals, bones in owl pellets, calcium rich grit or calcium from lime rich mortar. In my old house, I had very few snails in my garden until I had a brick extension built and their numbers exploded. The weathering of the new mortar provided a rich source of calcium made available to snails as it was washed down into the nearby soil and was taken up by the vegetation which the snail ate. The house was over 40 years old and most of the lime available may have been washed away over that period.

Egg shells from other birds have been found in the nests of blue tits. Wasting nothing, the birds eat the shells from their own nestlings, boosting their own calcium intake.

A calcium take away- eggs shells, food and snail shells


This year, I have started my organic pest control earlier than usual and started to collect young and old snails already and will offer them and crushed hen egg shells on the bird table with food to see if they have a preference or not. Hopefully some birds will participate! I have noticed that the blue and great tits are putting the finishing touches to their nests, i.e. dog hair and feathers I clump in the garden exactly for that purpose. The robin and blackbirds have also helped themselves.  If they do they may well be the luckier one as in gardens that has been acidified by whatever means, then birds there may well suffer from a lack of calcium in the soil, with it’s knock on effect on snail availability and they may not fair as well regarding their calcium intake and hence nestling success. They will be ready to lay soon so now is the time for me to put out the shells. Did I make the birds nest soup? Yes, but nobody liked it! They wanted to know why it was crunchy!


See video of Blue tits in nest box after female had eaten snail shells left out for that very purpose.

Refs and websites…..  Besides my own observations……

Bures, S & Weidinger K (2003) “Sources and timing of calcium intake during reproduction in flycatchers” Oecologia

Cannon, A (1997), “ Secret Ingredient” BTO, The Bird Table, No.10. Mead, C, (1999), “ Focus on the woodpigeon” BTO The Bird Table, No.19.

Green, R. E, (1998), “Long-term decline in the thickness of eggshells of thrushes, Turdus spp., in Britain”  Proc. R. Soc. Lond. B 22 April 1998 vol. 265 no. 1397 679-684

Graveland, J & Wal.R, (1996), “ Decline in snail abundance due to soil acidification causes eggshell defects in forest passerines”,  OECOLOGIA Volume 105, Number 3, 351-360, DOI: 10.1007/BF00328738

Graveland J, Berends AE. (1997), “ Timing of the calcium intake and effect of calcium deficiency on behaviour and egg laying in captive great tits, Parus major.” Physiol Zool. 1997 Jan-Feb;70(1):74-84.

Graveland, J & Drent, RH (1997) “Calcium availability limits breeding success of passerines on poor soils” Journal of Animal Ecology  66, 279-288, British Ecological Society

Mand, R, Tilgar V & Leivits A,  “Calcium, snails and birds: a case study” Web Ecology 1 pp 63-69 (2000)  OECOLOGIA 1996 Volume 105, Number 3, 351-360, DOI: 10.1007/BF00328738

Perrins. C (1979), “British Tits”, The New Naturalist, Collins, London

Reynolds. S J . Perrins C M. “Dietary calcium availability and reproduction in birds” Current Ornithology, 2010, Volume 71, p 31-74.

Web based source which you can find yourself!!!!





{ 2 comments… read them below or add one }

Pat March 12, 2014 at 5:10 pm

Hello – thought you would be interested in what I saw this morning in Norfolk. I have a female blackbird who has been collecting nesting material for more than a week. This morning I saw her pecking at a small patch of yellow material near a shed. After she left, I had a look at it, and it turned out to be yellowish building sand mixed with soft cement, from the hard standing near a shed. It was obviously very handy if she is egg laying.


nurturingnature March 12, 2014 at 5:38 pm

Hello Pat, thanks for sharing that. Amazing where they find calcium sources. Me? Stick with ice cream I think!! Cheers, George


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