A wonderfully calm and uneventful day today on the osprey nest at Loch of the Lowes, with the two eggs being incubated in the spring sunshine by both parents. We are so pleased to have two eggs (and therefore two possible chances of a live chick this year) but of course we can’t help hoping there might be one more- possibly even laid tonight or tomorrow morning so keep your eyes open! Today we’re reprinting some of the most commonly asked questions about osprey eggs- for more like these please see our FAQ pages by clicking on the tab at the top of this page.
Q: How many eggs do they lay?
A: Two or three is the usual number of eggs for an established breeding pair. Ospreys tend to start breeding at 3 to 7 years of age. In the first year of breeding, however, they often fail to breed successfully, and younger birds often start off with one egg, producing two the following year, and hopefully building up to a standard clutch of three. Rarely four eggs are laid, although these may not all hatch, nor all survive to fledging.
Q: How far apart are the eggs laid?
A: Osprey eggs are usually laid 1-3 days apart, that is with an average of two days between each, although a gap of up to 6 days has been observed, the whole clutch laid over up to 10 days .
Q: How big are the eggs?
A: Surprisingly, osprey eggs are only the size of a large hens or duck egg.
Q: Why are the eggs spotted – they don’t seem very camouflaged?
A: Each of the eggs is unique and is a combination of off-white to pinkish or buff background, with mottled dark brown or reddish splotches. Some eggs have a uniform mottled appearance while some can have more of this reddish brown colouration at one end. Most birds of prey have similarly spotted or specked eggs- in fact many British birds do. Though to us they seem to stand out, from a height or distance the outline of these eggs blurs easily and blends them into their background very effectively.
Q: How long do they incubate for?
A: The general rule is 5 to 6 weeks (35-42 days), the average being 37-39 days.
Q; Do ospreys, like pigeons, not sit on the first egg laid until others are laid to join it, to ensure they hatch together?
A: You are correct that this is exactly what many bird species such as pigeons do, but this strategy is not true of ospreys: they always incubate from the first egg laid and this is why the eggs usually hatch a day or two apart, in the order they are laid.
Q; Could ospreys lay two clutches of eggs in one year?
A: No, ospreys never ‘double clutch’ (lay two lots of eggs and raise two broods in one year) unlike many smaller birds. This is because they know that a late laid brood will not have enough time to mature sufficiently to make an autumn migration safely, and these chicks would be highly unlikely to survive. Long lived birds like ospreys will simply wait till next year to try again. This is why nest disturbance by humans of ospreys is such a problem- it wipes out their whole breeding year. Occasionally if Ospreys lose eggs in the first week or so, they may lay more (such as EJ at Loch Garten who laid egg 3 and 4 after her mate destroyed 1 and 2) but not after a whole incubation has been unsuccessful.
Q; How long can the eggs be left alone?
A: This depends on two things; predators and weather. Obviously any egg unguarded is vulnerable to opportunistic predators, such as crows, pine martins, herons etc. If the weather is mild, the eggs can be fine for up to half an hour or so, but if it is cold and wet they can quickly get cold. Too much cold would cause the embryos inside to die as they need a very steady temperature to incubate successfully.
Q; Is it normal for the male to incubate the eggs?
A: Yes, in general both ospreys will tend to the eggs safety, although the female always does the majority of the incubation. In some pairs males never incubate the eggs, and in other pairs males will incubate for an hour or more whilst the female has a break to fly, toilet and eat. The male is the sole food supplier once the eggs are laid.
Q: Can the eggs overheat?
A: It is unlikely the eggs got too hot (incubation temperature is usually over 30 degrees C) but humidity is also important, and sometimes hot dry weather can make the shells a little tougher than normal.
Q Do chicks hear outside noises as they develop inside the shell? The female ‘chirrups’ to it so it must hear that?
A: As the chick develops inside the shell, all its senses grow. From my limited experience with captive bred birds, I believe this sense ( hearing) only becomes functional in the last week or so of incubation- it is probably the same for osprey chicks.