Osprey Egg News 8th July

Our ever helpful friend at the National Museum of Scotland, Bob McGowan , has taken a professional look at our osprey eggs which were removed from the nest after the birds gave up incubating unsuccessfully for 74 days.

Our initial observations were that one egg ( the rounder one) was much smaller than average and only weighed   39.1gms, whereas the oddly elongated one was a more normal weight of 52.9gms. By contrast last years unhatched eggs weighed 46.9, 49.8 and 50.6gms.

Both eggs were intact and showed no sign of structural damage, ruling out the crow attack as the cause of hatching failure. The eggs were candled- a light process similar to xray- and it was immediately obvious that there were no embryos inside either egg. In other words, there were no partly developed chicks inside , and the couple of incidents when the male osprey left them alone and cold for a period during later incubation can also be ruled out as the cause of hatching failure.

The eggs were then carefully blown- the rather smelly contents removed and examined closely. One egg ( the smaller round one) was completely ‘blank’, and had not been fertilized at all and had begun to dry out. Unfertilized eggs are common in nature and we have had instances of this before in this nest.

The second, longer heavier egg had a few spots of blood among the fluid, which means it was fertilized initially but development probably stopped at a few days, very early in the incubation period. We cannot say why this happened , but again this is not unusual in eggs when an embryo is not viable or conditions are not right.

So the conclusion to all this is that the cause of our breeding failure is a fertility issue, rather that any fault during incubation. By far the most likely cause of this is our female ospreys advanced age , though there is a slight possibility that it might be the male who has the fertility issue ( he could be firing blanks so to speak) or that we have a case in in breeding depression ( a term scientists use to describe the low fertility of a pair of animals who are too closely related and therefore not a good genetic combination and don’t produce many viable offspring) .

We cannot blame either bird for the lack of breeding success this year, but we have to accept that this pairs fertility is now been low for 3 years and may always be so.

Lastly, what was fascinating as ever was to see our eggs in the context of the reference collection in the museum with ospreys eggs from all over the UK- there was a fascinating wide variation in colour and size of course.  What was truly heart-breaking though was to see at least two full clutches of osprey eggs that had been robbed by human egg thieves from nests not 5 miles from Loch of the Lowes in the 1990’s. These had ended up in the collection after the thieves had been caught and prosecuted and their collections confiscated- how sad that these may well have been the potential offspring of some of our birds descendants, cruelly denied the chance ever to hatch. What a sad reminder of why we still need to do osprey nest protection watch and how close the threats have come in the past to our precious birds.

Ranger Emma


About rangeremma

Perthshire Ranger Scottish Wildlife Trust
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