Action Synopsis: Bird Conservation About Actions

Provide calcium supplements to increase survival or reproductive success

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • Eight studies from across the world, including a literature review from across the world found evidence for positive effects of calcium supplementation on several bird species.
  • Positive effects included lower incidence of bone disease, higher fledging succes, larger broods, higher quality eggs or chicks and better physical condition of female parents. Not all species reacted similarly.
  • Six studies including a literature review did not find any evidence for increased reproductive success in at least one of the species supplied with supplementary calcium.
  • One replicated study from Europe found that birds took calcium supplied, and birds at polluted sites took more than those at cleaner sites. The effects on fitness were not monitored.


About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A before-and-after study on a northern South African pig farm (Richardson et al. 1986) found that the incidence of osteodystrophy (a bone-deforming disease) in Cape vulture Gyps coprotheres chicks declined from an average of 17% in 1974-6 to 2.5% in 1983, following the establishment, in 1977, of a feeding station where carcass skeletons were crushed to provide small bone fragments. A total of 1378 chicks were examined over the study. The authors note that vulture colonies on game reserves not ranches had far lower levels of osteodystrophy (0-1%), probably due to the presence of bone-crushing mammals such as spotted hyenas Crocuta crocuta.

    Study and other actions tested
  2. A randomised, replicated and controlled trial in woodland patches in Wyoming, USA (Johnson & Barclay 1996), found that house wrens Troglodytes aedon provided with supplementary calcium did not raise more chicks than control (unsupplemented) birds (5.6 chicks/successful nest for 48 supplemented nests vs. 5.4 for 44 controls). In addition, they did not produce larger eggs or chicks (9.0 cm3/egg and 9.9 g/chick for 53 eggs and 49 chicks from supplemented nests vs. 8.8 cm3 and 9.9 g for 53 and 41 controls) or have larger clutches (6.7 eggs/clutch for 54 supplemented clutches vs. 6.4 eggs/clutch for 56 controls). Calcium was provided in the form of an equal mix of crushed oyster shells and chicken eggshells in small pots attached to the outside of nest boxes. Control nest boxes also had pots attached, but they were not filled.

    Study and other actions tested
  3. A small control trial conducted during the 1996 breeding season on an acid bog in the north-eastern Netherlands (Beintema et al. 1997) found that six of eight black tern Chlidonias niger chicks force-fed with supplementary calcium fledged successfully, whereas all 11 unsupplemented chicks died before fledging. The two unsuccessful supplemented chicks died early (within five days), whereas the unsupplemented chicks all showed skeletal deformities and significantly lower rates of weight gain. Commercially-available calcium pills, were given to chicks three times a week, providing a total of approximately 700 mg of calcium over the study period.

    Study and other actions tested
  4. A cross-over study in a mixed forest on calcium-poor soils in the Netherlands from 1990-2 (Graveland & Drent 1997) found that female great tits Parus major supplied with supplementary calcium were more likely to lay eggs (0-3% of supplemented nests empty vs. 10-15% of controls, 622 nests studied) and less likely to desert them (15-25% of supplemented clutches deserted vs. 40-70% of controls, 339 clutches studied). Eggs from supplemented females were less likely to have defective eggshells (15-25% vs. 40-70%, 360 clutches) and a higher proportion of eggs from successful, supplemented nests hatched (80-95% of eggs vs. 55-80% for controls, 204 clutches). There were no significant differences in laying date or clutch size between treatments. This resulted in 5-9 hatchlings/nest for supplemented nests and 2-4 hatchlings/nest for controls. Calcium was provided in the form of 500 mm2 snail shells or chicken eggshells provided three times a week from early March, in feeding cups on the outside of nestboxes. Birds took supplements mostly during the egg-laying and chick-provisioning stages of reproduction.

    Study and other actions tested
  5. A replicated, controlled study from April-June in 1994 in 12 experimental and 15 control plots in mixed oak woodland in Loch Lomond, Scotland (Ramsay & Houston 1999), found that calcium supplementation did not affect the reproductive success of blue tits (Parus caeruleus). Supplementary calcium did not significantly increase egg weight or size (1.12 and 1.14 g / egg; 1075 and 1082 mm3 / egg for experimental and control nests respectively). Similarly, there were no differences in shell weight or thickness (both treatments: 0.07 g and 0.038 mm / egg). Clutch size was also similar between pairs (9.3 and 9.5 eggs / nest for experimental and control nests respectively). Hatching success, expressed as the proportion of all eggs in a clutch that hatched, was not significantly different between experimental or control groups (both median = 1). A dry cuttlefish bone, 200 g of oyster grit and 200 g of crushed eggshell were provided in experimental plots.

    Study and other actions tested
  6. A replicated, controlled trial in deciduous forests in Estonia in 1995-6 (Tilgar et al.1999) found that pied flycatchers Ficedula hypoleuca and great tits Parus major used supplementary calcium supplied to their nest boxes, and that, in base-poor habitats, supplemented flycatchers laid larger eggs with thicker shells, compared to controls. There was no difference in tits, or in either species for hatching success or nestling condition. This study used a subset of data from studies described in detail below.

    Study and other actions tested
  7. A replicated, controlled study in a 30 km2 area of base-poor pine forests in Estonia in 1995-7 (Tilgar et al. 1999) found that great tits Parus major provided with supplementary calcium began laying earlier and had larger clutches than controls (average first laying date of 27th April-6th May for 42 supplemented nests and average clutch volume of 17.6 ml for 36 clutches vs. 29th April-11th May, 84 clutches and 16.6-18.4 ml, 65 clutches). In addition chicks from supplemented nests were larger than controls in 1997, the worst year for reproduction. There were no differences between groups with respect to clutch size (10.6-11.5 eggs/clutch, 40 supplemented clutches vs. 10.3-11.2 eggs/clutch for 81 controls), the size of individual eggs, hatching success or the number of fledglings produced (92-7% success and 7.8-8.9 fledglings/clutch for 29 supplemented clutches vs. 93-5% and 7.0-8.6 fledglings/clutch for 48 controls). Supplementary calcium consisted of snail shell and chicken eggshell supplied constantly in small feeders on the outside of nestboxes, from approximately 2 weeks before the start of nesting. This study uses a subset of the data from Tilgar et al. 2002.

    Study and other actions tested
  8. A replicated study at three alpine sites (one heavily polluted in the Czech Republic, two less so in the Slovak Republic and Norway) (Bureš & Weidinger 2001) found that both meadow pipits Anthus pratensis and water pipits A. spinoletta took calcium-rich supplements placed near their nests, but that both species took supplements more frequently in the heavily polluted site. Snail shells were taken most often by both species, followed by eggshells, bone fragments and plastic snail shells (low in calcium). Quartz (low in calcium) and limestone pieces were taken less frequently than other supplements. A total of 84 meadow pipit nests and 47 water pipit nests were monitored across all sites. The effects of supplementation on reproduction were not monitored.

    Study and other actions tested
  9. A replicated, controlled study in 1997 in 15 experimental nests (containing 72 nestlings) and 14 control nests (containing 65 nestlings) in artificial nestboxes in Oklahoma, USA (Poulin & Brigham 2001) found that purple martin Progne subis nestlings were unaffected by calcium supplementation. Calcium-supplemented nestlings did not grow to a larger size or at a faster rate than control nestlings for any of the growth parameters measured in any brood size (on average 53.9 compared 54.6 g / nestling; 4.2 compared to 4.4 g / day respectively). Additionally wing, leg and tail sizes did not differ between calcium-supplemented and control nestlings (on average 29.4 and 29 mm; 15.9 and 15.9; 22.8 and 22.9 mm respectively). Brood sizes were experimentally manipulated to contain either 4 or 5 nestlings. Nestlings in half of the nests of each brood size were fed a 1 ml dose of liquid calcium with a pipette over 3 weeks. Control nestlings received a water placebo.

    Study and other actions tested
  10. A replicated, controlled trial in pine and deciduous forests in Estonia in 1995-7 and 1999-2000 (Tilgar er al. 2002) continued the study from Tilgar et al. 1999 and found that great tits Parus major supplied with supplementary calcium had significantly larger first broods, containing significantly larger chicks and hatching more eggs, than control (unsupplemented) birds (supplemented nests: 10.1-11.6 eggs/clutch, n = 172 nests, 9.2-10.4 hatchlings/nest, n = 101 nests and lower leg lengths of 19.1-19.8 mm, n = 73 chicks vs. 10.2-11.4 eggs/clutch, n = 254 nests, 8.2-10.1 hatchlings/nest, n = 110 nests and 18.8-19.7 mm, n = 97 chicks, for control nests). Supplemented nests also fledged more chicks in 1997, 1999 and 2000 (the only years tested), but this difference was not significant, unless unsuccessful nests were excluded (7.2-9.0 fledglings/nest for 81 supplemented nests vs. 4.2-8.7 fledglings/nest for 115 controls). In 1999 (the only year investigated), second broods were significantly larger and fledged significantly more chicks in supplemented than control nests (8.4-9.1 eggs/clutch, and 5.5-6.6 fledglings/nest for 13 supplemented nests vs. 9.8-9.6 eggs/clutch and 7.9-8.1 fledglings/nest for 15 controls). The authors note that when clutch size was included in models, fledging rates did not differ between treatments, suggesting that calcium provisioning acts mainly to increase clutch size, rather than  nest survival. There were no differences between pine and deciduous forests. Calcium was provided in the form of small snail shell and eggshell fragments in a feeder within the nest boxes. Control nest boxes were given empty feeders in.

    Study and other actions tested
  11. A replicated, controlled trial in a pine forest site in Estonia in 1995-7 and 1999 (Mänd & Tilgar 2003) found that pied flycatchers Ficedula hypoleuca provided with supplementary calcium laid larger eggs and their chicks were larger, compared to control (unsupplemented) birds (average size of 1.63 cm3 for 172 supplemented clutches vs. 159 cm3 for 178 controls; average leg length of 17.3 mm for 81 supplemented chicks vs. 17.2 mm for 89 controls). In addition, supplemented female flycatchers laying seven or more eggs were significantly heavier than controls. There were no differences in laying date, clutch size, number of fledglings or chick weights between treatments. Calcium was provided in the form of small snail shell and eggshell fragments in a feeder within the nest boxes. Control nest boxes had empty feeders in. A subset of the data from this study is discussed in (6).

    Study and other actions tested
  12. A 2004 literature review (Reynolds et al. 2004) looked at 14 studies of calcium supplementation across a total of seven species. Several studies are discussed in this section. Positive effects on at least one egg-related trait were detected in three species: house wrens Troglodytes aedon (clutch size but not egg size), pied flycatchers Ficedula hypoleuca (eggshell thickness and egg volume) and great tits Parus major (fewer females not nesting, fewer abandoning nests, thicker eggshells and higher hatching success; uncertain evidence for increases in clutch size and advanced laying date (4,7,10)). However, such effects were missing in all traits measured in a study of blue tits P. caeruleus in Scotland (egg mass and volume, eggshell thickness, laying date, clutch size, fledging success). Similarly, positive effects on chick traits were detected in four species: cape vultures Gyps coprotheres (reduced incidence of bone deformation (1)), black terns Childonias niger (higher weight gain (3)), pied flycatchers (higher chick growth rates (11)), great tits (chick growth and fledging success (10)). Such impacts were absent in house wrens (fledgling body mass or number of fledglings, (2)) or purple martins Progne subis (growth rate of fledglings, (9)). One study (11) also reported a positive effect on adult female body condition in pied flycatchers but not great tits in Estonia.

    Study and other actions tested
  13. A replicated, controlled trial in mixed forest site in Estonia in 2000-1 (Tilgar et al. 2004) found that hatching success of great tit Parus major chicks did not differ between calcium-supplemented and control nests, but that a higher proportion of supplemented chicks fledged in 2000 (93% of hatched eggs from 14 supplemented nests vs. 78% from 19 controls), but not 2001 (78% of hatched eggs from 25 supplemented nests vs. 70% from 17 controls). At 15 days old, supplemented chicks were no larger or heavier than controls, but showed significantly lower levels of bone calcification activity. This suggests that supplemented nestlings already had fully developed bones by the time measurements were taken. The consequences for fitness and future reproduction may include decreased predation or parasitism in chicks and a lower reproductive costs for parents (because of shorter nestling periods), but these are not investigated in this study. This difference was only apparent in 2000, possibly because 2001 was a very poor year for reproduction, lowering growth rates and fledging success for all broods. Supplementation consisted of chicken eggshell provided in excess in small feeders on the outside of nest boxes.

    Study and other actions tested
Please cite as:

Williams, D.R., Child, M.F., Dicks, L.V., Ockendon, N., Pople, R.G., Showler, D.A., Walsh, J.C., zu Ermgassen, E.K.H.J. & Sutherland, W.J. (2020) Bird Conservation. Pages 137-281 in: W.J. Sutherland, L.V. Dicks, S.O. Petrovan & R.K. Smith (eds) What Works in Conservation 2020. Open Book Publishers, Cambridge, UK.


Where has this evidence come from?

List of journals searched by synopsis

All the journals searched for all synopses

Bird Conservation

This Action forms part of the Action Synopsis:

Bird Conservation
What Works 2021 cover

What Works in Conservation

What Works in Conservation provides expert assessments of the effectiveness of actions, based on summarised evidence, in synopses. Subjects covered so far include amphibians, birds, mammals, forests, peatland and control of freshwater invasive species. More are in progress.

More about What Works in Conservation

Download free PDF or purchase
The Conservation Evidence Journal

The Conservation Evidence Journal

An online, free to publish in, open-access journal publishing results from research and projects that test the effectiveness of conservation actions.

Read the latest volume: Volume 21

Go to the CE Journal

Discover more on our blog

Our blog contains the latest news and updates from the Conservation Evidence team, the Conservation Evidence Journal, and our global partners in evidence-based conservation.

Who uses Conservation Evidence?

Meet some of the evidence champions

Endangered Landscape ProgrammeRed List Champion - Arc Kent Wildlife Trust The Rufford Foundation Save the Frogs - Ghana Mauritian Wildlife Supporting Conservation Leaders
Sustainability Dashboard National Biodiversity Network Frog Life The international journey of Conservation - Oryx Cool Farm Alliance UNEP AWFA Bat Conservation InternationalPeople trust for endangered species Vincet Wildlife Trust