How to tell whether eggshells in Curlew nests have hatched or been predated

Question from Mike Smart (14/05/2017):

Juliet Bailey and I found an apparently predated nest in the Severn Vale in early May (much too early for the eggs to have hatched even in an early year like 2017); the nest had been ripped up, and had two broken eggs, in very large pieces. I am assuming that these large pieces (maybe half the egg) are a sign of predation. But does anybody know for sure how to distinguish hatched eggshells from predated eggshells?

Response from Geoff Hilton (15/05/2017):

In my experience, it is fairly straightforward to distinguish hatched from predated eggs: the hatched eggs have very obvious egg membrane that is usually still attached to the shell in places, but has largely come away from it. The membrane is almost leathery in appearance, and really quite large folds of it are seen. In a predated nest, the membranes are much less obvious and are basically still attached to the shell, barring a few small detached flaps. It’s not very easy to describe – and I don’t think I’ve got photos – but it is very obvious.

Determining whether avian or mammalian etc predator seems slightly more tricky to me. Different people seem to have different theories. It should be possible to really nail it using cameras to ground-truth the observations. Sometimes you get really obvious canine marks on shells, and the distance apart can be used to determine whether fox/otter/mink/polecat/stoat etc. Sometimes there is something that looks more like a beak has just gone through it like a hammer, but I rarely convince myself that that’s definitely what it is. Foxes tend to carry eggs away intact. Badgers tend to just crush them into a million pieces…. But this is all anecdote really.

I think we should really try to get hold of a job lot of temperature loggers next year. They cost a tenner, and are the size of one of those little disc batteries that go in a TV remote. No wires. They measure nest temperature every 20 mins or so (depending how you programme them). For Common Scoter, we have found they give brilliant insights into what’s going on. When the bird is incubating, temperature is high and constant. When off, the temperature drops (obviously!). When off for a long period, even if the weather’s hot, you get a diurnal temperature fluctuation rather than constant warm temperatures.

For Scoters, we measure and weigh the eggs. This allows us to estimate their date of laying, because eggs lose mass at a predictable rate once laid. Hence, we can predict due hatch date. If the temperature logger indicates that the nest ceased to be incubated well before the due hatch date, we know it failed. If it ceases to be incubated on about hatch date, then we can usually infer hatching.

Some folk have used the time of nest failure as an indicator of the predator identity: birds tend to be in daylight hours, badgers and foxes tend to be at night. It’s rough and ready, but it can help.

We’ve also used it to understand incubation schedules – the female taking a break is easy to spot by a sudden temperature drop. This can be useful. We learn when her ‘normal’ daily break time is, and if there is a break in incubation outwith this time slot, it’s usually because of a disturbance event. So we can start to understand how often and when they get disturbed, which can lead to more detective work about what the cause is. Curlews are different to ducks in this regard, because they share incubation, so there will not be regular hour-long periods with no bird sitting. But that might make it even more obvious if there are disturbance events, because basically the temperature only drops if there is a disturbance: there are no ‘normal’ incubation recesses.

The bit I’m less certain of is how robust Curlews are to people faffing around at nest sites. General view of wader workers seems to be that waders are pretty robust, and in general visiting nests does not increase the risk of predation (much). But I’m no expert on waders and I think we should really try to get a consensus view among the experts of what the story is with that.

Again, as a non-Curlew specialist, such accounts seem to be incredibly useful in terms of teaching us a bit about how we interpret Curlew behaviour to get an understanding of the breeding season. From non-Curlew experience, it seems to me highly likely that these birds seen in pairs and/or “not settled”, even though it’s mid-May, are birds that have already failed.

Dr Geoff M Hilton
Head of Conservation Science, Wildfowl & Wetlands Trust (WWT)
Slimbridge, Glos GL2 7BT, UK


Philip Sheldrake wrote (18/05/2017):

This is a photograph taken on 24 May 2016 of a nest I know successfully hatched on the Plain last year.

Hatched nest © Philip Sheldrake (RSPB)

….and another of some eggshell pieces I found some 110m from a nest I concluded to have been predated.

Predated nest © Philip Sheldrake (RSPB)

Phil Sheldrake, Conservation Officer
RSPB, Brewery House, 36 Milford Street, Salisbury, SP1 2AP


Kirsty Brannan commented (24/09/2017):

Phil’s photos and nest outcome look good to me. The 24 May egg remains are within the nest bowl, and consist of large pieces. Two of them show the ‘folding in’ that seems quite characteristic of a hatch; the folded parts are visibly held together by the internal membrane. And you can see the inside of the egg has some blood vessel traces. By contrast the predated remains are small fragments, with cleanly broken edges. And a long way outside the nest!

Kirsty Brannan, Senior Conservation Officer
RSPB Midlands Regional Office 46 The Green, South Bar, Banbury, OX16 9AB

Kirsty Brannan wrote (15/09/2017):

Referring to your notes, I concur with Geoff Hilton’s advice about identifying nest outcomes from the egg remains alone. The presence of the thick internal membrane, alongside with the egg remains being retained in the nest cup, and with no signs of vegetation trampling, are all helpful signs of a successful hatch. However, it can be tricky to make that call in the field, and we’ve certainly found the temperature loggers invaluable in determining nest outcomes with confidence (although I’ve lost a few in the course of predation events as well!).

Hatched nest © Kirsty Brannan (RSPB)

By contrast, a nest that is suddenly empty before the predicted hatch date is probably a sign of fox predation, but may be corvid, as both will carry whole eggs off. Foxes can also be surprisingly active during the day – something to be wary of when interpreting temperature logger data. Even foxes usually leave pretty clear tracks in long grass, but probably not much in a grazed pasture. Badger predation is messy – eggs are typically crunched on the scene, and surrounding vegetation shows a lot of trampling.

In the examples shown from the photos, even some of the fox-predated nests had eggshell fragments found outside the nest – often 20-30 metres or so. These three photos all show fox predation:

Curlew eggs predated by fox © Kirsty Brannan (RSPB)

Curlew eggs predated by fox © Kirsty Brannan (RSPB)

Empty nest with datalogger © Kirsty Brannan (RSPB)

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