Action: Remove eggs from wild nests to increase reproductive output
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- A replicated study from Mauritius found that harvesting entire clutches appeared to increase Mauritius kestrels Falco punctatus productivity more effectively than removing individual eggs as they were laid.
- A replicated study over 30 years in Canada (Kuyt 1996) found that wild whooping cranes Grus americana reproductive success was higher for nests with one or two eggs removed than for control nests.
- A single study from the USA found that removing bald eagle Haliaeetus leucocephalus eggs from wild nests for hand-rearing did not appear to greatly affect the wild population.
If a bird population is at a very low level then removing eggs from nests can be used to encourage females to continue laying, with some species replacing eggs as they are removed. The removed eggs can then be hand-reared (see ‘Artificially incubate and hand-rear birds in captivity’) or fostered (‘Foster eggs or chicks with wild conspecifics’ and ‘Foster eggs or chicks with non-conspecifics’), increasing the productivity of the species.
Alternatively, removing eggs from wild nests may actually increase the success of these nests, possibly due to competition between siblings. Studies discussing this aspect of the intervention are described below.
Supporting evidence from individual studies
A replicated, controlled study in two marshland sites in Florida, USA, between 1985 and 1990 (Wood & Collopy 1993) found that 78% of 58 bald eagle Haliaeetus leucocephalus pairs that had their first clutch removed for hand-rearing (‘donor nests’) between 1985 and 1988 laid replacement clutches within two months. Replacement clutches were slightly smaller than first clutches (58 first clutches averaged 2.1 eggs/clutch vs. 1.8 eggs/clutch for 45 second clutches). In one study area, donor nests produced fewer fledglings than control pairs (1.0 fledgling/nest for 16 donor nests vs. 1.5 fledglings/nest for 39 controls), but this was not true in a second area (1.2 fledgling/nest for 26 donor nests vs. 1.1 fledglings/nest for 41 controls). Donor nests were more productive in the year before eggs were removed than the year after donation (approximately 1.3 fledglings/clutch for 32 pairs the year before donation vs. 0.85 fledglings/clutch for 34 pairs the year after). A demographic model suggested that a donor population would be very slightly smaller than a control population after 25 years. Timing of clutch removal did not affect the speed or probability of replacement clutches being laid.
A replicated 1995 study on the Mauritius kestrels Falco punctatus conservation programme (Jones et al. 1995) found that harvesting whole clutches rather than single eggs was more successful in increasing wild pair productivity: 95% of females re-laid within 14 days of clutch removal, but fertility fell rapidly in clutches where eggs were removed as they were laid. Females laid up to 4 clutches/season as a result of harvesting, but clutch fertility decreased to zero by the fourth clutch. Clutch size was an average of 3.4 eggs/clutch for 96 first clutches, compared with 3.3 for 63 second clutches.
A replicated controlled study in Northwest Territories and Alberta, Canada, between 1967 and 1996 (Boyce et al. 2005) found that the reproductive success of wild whooping cranes Grus americana was higher for nests that had one of two eggs removed, compared to control nests. Both recruitment of juveniles to the population and survival until August (eggs were removed in May) were higher (50% chance of recruitment from nests with eggs removed vs. 39% for unmanipulated nests). A total of 496 eggs were removed from wild nests in the study period, representing 62% of all crane nests during this time period. The success of artificially incubating and rearing the removed eggs is discussed in Kuyt (1996) in ‘Captive breeding, rearing and releases (ex situ conservation)’.
Kuyt, E. (1996) Reproductive manipulation in the whooping crane Grus americana. Bird Conservation International, 6, 3–10.
- Wood P.B. & Collopy M.W. (1993) Effects of Egg Removal on Bald Eagle Productivity in Northern Florida. The Journal of Wildlife Management, 57, 1-9
- Jones C.G., Heck W., Lewis R.E., Mungroo Y. & Cade T.J. (1995) The restoration of the Mauritius kestrel Falco punctatus population. Ibis, 137, s173-s180
- Boyce M.S., Lele S.R. & Johns B.W. (2005) Whooping crane recruitment enhanced by egg removal. Biological Conservation, 126, 395-401