Friday, 15 July 2011

Laying down the foundations

Almost 70% of monitored nests are now occupied by a breeding pair. In 17 nests, the egg is being incubated in a nest-cup holding a temperature logger, so hopefully some valuable data on frequency of occurrence of egg neglect (breaks in incubation) are being recorded. One of these eggs has, however, been abandoned already and kicked out of the nest (see below). Another 10 nests have been occupied on a handful of occasions over the season, so there may be a few more pairs yet to lay an egg. It may seem late, but, last year, the last egg was recorded on 2nd August.

An abandoned egg, kicked out of the nest-cup. You can see a temperature logger (an 'i-button') in the centre of the photo for recording nest temperature to monitor egg neglect.

A handful of nests that fledged chicks last year have not been occupied at all during the day so far, leading me to assume that those birds are probably not going to breed this year. However, birds do not always occupy the nest during the daytime prior to laying, so there may be a few unexpected surprises yet. Daytime occupation of nests seems highly variable – typically, one or two birds might occupy the nest on two or three days over the course of maybe 3 weeks prior to laying. At one extreme, one or two birds of a pair were present on 8 occasions over the course of a month before laying an egg. Yet, other nests are checked every day and, day after day, recorded as ‘empty’, then all of a sudden, one day, I’ll find a bird sat there on an egg!

A pair of Storm Petrels occupying a nest-box during the day. This nest-box has not been used for breeding before, so these birds are probably young birds that may be about to breed for the first this space...

While Storm Petrels are still laying their eggs, most other seabirds are much further ahead in their breeding season. Of the few Shags (Phalocrocorax aristotelis), Guillemots (Uria aalge) and Razorbills (Alca torca) that have successfully bred this year, most of the young have fledged and left the cliffs. Gull (Larus spp.) chicks are almost fully-feathered (see below) and must be close to fledging. Related to the Storm Petrels, Fulmars (Fulmarus glacialis) also have a lengthy pre-natal development, but breed earlier than Storm Petrels. I’ve noticed the first few Fulmar chicks over the last week, but all still being brooded by a parent, so probably less only a week or two old; a tuft of pale grey downy fluff poking out from underneath an adult usually gives their presence away.

Great Black-backed Gull chick playing the ‘If I can’t see you, you can’t see me’ game

It was such a pleasant day on Mousa yesterday that I took my time ambling around the island, checking nests at a leisurely pace, and keeping my eyes open for anything of interest...I was praying for some Orcas to start leaping out of the flat-calm, shimmering, blue ocean. Passing through areas used by nesting Bonxies (or Great Skuas, Stercorarius skua), I was scanning the ground ahead for any fleeting glimpses of a chick scuffling away - the most I usually see of a Bonxie chick. However, today, I had close-up encounters with two chicks (belonging to different pairs), both of which were almost misfortunate enough to have my foot land on top of them! Frozen to the ground, not moving a muscle, and hunkered down in the long grass, I didn’t see either of them until I almost stepped on them. Each time, I stopped with a start and looked down to see a pair of large black eyes surrounded by golden fluffy down staring back up at me...
A Bonxie chick hunkered down in the long grass. It must be a few weeks’ old, as you can see a lot of the feathers growing in on the back and wings (though apologies for a bad photo taken on my phone!).

Tuesday, 5 July 2011

It's gonna be a cold lonely summer

A key characteristic of the eggs of Procellariiformes (or ‘tubenoses’) – the group of birds that includes storm petrels, shearwaters, albatrosses, petrels and fulmars – is the ability to withstand chilling (i.e. periods without being incubated). Such neglect is unlikely to be tolerated by rapidly-developing embryos, but the slow rate development of Storm Petrel embryos probably makes them less vulnerable to lapses in incubation. This characteristic is probably an adaptation to a lifestyle that involves foraging at such great distances from the breeding colony on unpredictable food resources. Severe weather or food shortages may prevent the off-duty bird from returning before the incubating bird has to leave to replenish its own body reserves. Incubation places a huge energy demand on parents and Storm Petrels undertake long incubation shifts of 2 or 3 days, during which they do not feed. Additionally, since they are restricted to entering and leaving the nest during darkness (due to the risk of predation), if the off-duty bird hasn’t returned by the time the sun is almost rising, the incubating bird may be left with the choice of leaving now or waiting another 24 hours. 


A ‘cold’ (unincubated) egg

In 2010, egg neglect was recorded in 38% of nests, from regular visual inspections of nests to see if an adult bird was present or not. Eggs that endured chilling of up to 12% of the total incubation period still managed to hatch successfully and 70% of those chicks successfully fledged. Neglect resulted in the lengthening of the incubation period by up to about 5 days. It would be desirable to develop a means of monitoring egg neglect that didn’t involve the high labour of checking nests regularly throughout a long incubation period (at least 39 days) and does not involve the intrusion of shining a torch in nests every day. It is hoped that this can be achieved remotely with the use of temperature loggers. I-buttons are the size of a large watch battery and record temperature (as frequently as you like) and store the readings in an internal memory. The i-buttons have to then be retrieved to download the readings.

An i-button which records temperature at intervals of your choice up to a maximum of 1048 readings

It may be relatively straight-forward deploying i-buttons in the nests of ground-nesting birds, which are readily accessible and have an obvious nest-cup (where the egg(s) are laid), but it is rather more of a challenge when it comes to Storm Petrels. The cavities and crevices that they nest in are, more often than not, out of sight, let alone within reach. Those that are accessible generally involve squeezing and manipulating your hand and arm through the maze of cavities within the dry-stone walls (or ‘dykes’ as they’re known in Shetland) and relying on your sense of touch, rather than sight. The many bruises and scrapes we have endured are evidence of this. In addition, it is often not obvious where a pair is likely to lay their egg. Storm Petrels don’t build much of a nest; at the very most, there may be a slight depression, which may be lined with a bit of dead grass. A logger can only be deployed if the substrate is soft mud, within which it can be sunk into, such that it is level with the surface; if it were to protrude from the surface, a fragile egg could be knocked against it and broken.

Struggling to access a nest-cavity and deploy a temperature logger within the nest-cup

If a bird is incubating an egg over a temperature logger, it is anticipated that the temperature logger will record a higher temperature than ambient temperature. However, Storm Petrels incubate their eggs at a fairly low temperature – a lot lower than most other birds – which is probably an adaptation to enable them to endure such long incubation shifts without feeding and explains the slow development of embryos. Consequently, to confirm the presence or absence of a bird, a second temperature logger is deployed within the nest, but outside of the nest-depression. When temperature loggers are retrieved, once a chick is hatched (or an egg has been abandoned), we will be looking for a difference in temperature between the two loggers.

It is hoped that this remote means of monitoring egg neglect will provide an absolute measure of neglect for the entire period of incubation, without the need for somebody to check nests every day and without causing any disturbance to breeding birds. Fingers crossed it works!