HUNKERING and BUNKERING

The unseen design elements of bird nests.

((c) Dyfi Osprey project 2013)

Many species of birds build nests which – to our eyes – seem unnecessarily vulnerable to the elements. Herons, most corvids, ospreys and many others choose to make their nests in exposed trees or other high places. Given the vagaries of typical British weather, we worry about them and their broods when the rain falls (as it usually does) and strong winds start to blow. However these nests – or at least the central “egg cup” area of them – could be better protected than you might think. But to understand why this might be, we have to look first at a seemingly unrelated subject: Golf.

All keen golfers will tell you that they love to play on the classic style of seaside course: the “links” - which take their name from the term for an area of sandy, infertile ground between farmland and the sea. Golf courses were first laid out in such places because the land is nutrient-poor and of little use for anything else. Links course are challenging to play on, and test the full skill set of golfers. We rejoice in the wild scenery, the undulating fairways and the tricky sloping greens, and we all agree that seaside courses would be perfect, except for one thing...

Rory McIlroy has a practice bunker

in his back garden at home

Pot bunkers.

We all hate them.

Unlike the wide shallow sand traps found on parkland courses, pot bunkers are small and deep, often with high revetted faces and a cup-shaped bowl of tight-packed sand at the bottom. It is all-too easy to get the ball in one, and difficult – sometimes impossible – to get it out again. Almost every amateur golfer believes that pot bunkers are put there to make the course harder, but that's not the real reason for their existence... 

It's often windy at the seaside, and strong winds would remove the material from a conventional sand hazard in a matter of days, meaning that the grounds staff would be forever having to replace it. The design of pot bunkers means that this doesn't happen: the deeply-shaped recess actually forms a high-pressure area above the sand surface, sheltering it from wind gusts.

Large bird nests have a similar layout to pot bunkers, and the same general aerodynamics. A wide raised rim surrounds a lower bowl-shaped interior, in which the eggs are laid. With this “design”, the weather may be as bad as it likes but the eggs – and later, nestlings – are protected from the worst of the wind, as is the bird which broods them. Rain gets in, of course, but rain itself is not the primary threat to eggs and chicks – as long as there is not too much. It is the WIND, with its effect of carrying away heat, which is the real problem.

So... the next time you watch a poor bird hunkering down in her nest while a howling gale blows around it, don't worry too much. Things may not be quite as bad as they look.

Seals and Eiders

There's no blog this weekend because my mother is ill in hospital and we have been rallying round, fetching stuff, doing good works and generally looking busy.

To fill the gap, here's a picture taken at Rathlin Island, Northern Ireland, during my last trip out there:  Seals and female eider with brood.  (There were actually 5 eiderlings, but they steadfastly refused to gather for a group photo.)

(Click for full-size)

"TRUE OR FALSE" OSPREY QUESTIONS (2)

(Originally at FB Osprey Webcam Group)

(c) There are no ospreys on the Indian sub-continent 

FALSE. Ospreys are still reported from India and Bangladesh, where they appear to be coastal, confined to specific areas and not common anywhere. One hot spot seems to be the Sundarbans wetland in Bengal, where ospreys are regularly spotted by visitors. The most recent sighting I could find was 2/2/2013 and this photo by Nazul Islam was taken in 2010.

Osprey at Sundarbans lagoon. (c) Nazul Islam 2010

The population appears to be migratory and there have been a few ring recoveries – one from a bird originally ringed in Norway. In their survey in the 1980's, Sakeur & Sakeur listed ospreys as one of 14 raptor species known to be in decline, citing habitat loss as the major factor in all cases.

Things must have been different in the recent past... A young Army officer named Elliot, on posting in the Punjab, complained to his diary of 1891 that there were no interesting or exotic birds on and around the lake outside his kitchen window, “...only the usual ospreys, everywhere to be seen.”

 

(d) GPS bird tracking devices are less accurate when it's raining.

Pretty accurate: Niacom active antenna with Garmin Q3600 using SBAS differential corrections for a combined error of less than 2 metres

FALSE. Occasionally, I am asked to explain to someone exactly how the Global Positioning Satellite System works. I try to avoid this, because usually the explanation is only a quarter way through before the “someone” has fallen into a deep and restful sleep. The GPSS is one of those things like Corporation Tax, or the Immaculate Assumption, or open heart surgery – if you needed to know how it works, you would already know exactly how it works.

The space-to-ground segment of GPS is a radio signal. In fact, two primary radio signals, known as the “L1 and L2 carriers.” When the system was being designed, the wavelength for this signal (20cm L-band) was carefully chosen because these frequencies are least likely to be affected by water vapour in the atmosphere. Clouds and rain have no measurable affect on GPS accuracy, although many internet “experts” will still try to tell you that this happens. The myth probably arose because people that are carrying GPS -equipped smart phones might take shelter from rain under structures or dense foliage. These local obstructions could have some effect on reception, but the rain itself does not.

 

(e) Ospreys don't like flounders because they taste funny and are the wrong shape.

FALSE. Poor flounders... rejected by chefs, despised by ospreys, and as ugly and stupid as Ian Duncan-Smith, they get a universally bad press. The fact that ospreys don't like them is well supported by data, though it might be truer to say that while ospreys are prepared to eat flounders – they'd just prefer to eat other things instead. But why? Well camouflaged bottom-feeding flounders are abundant around the British coast in summer, and you'd think they would be perfect sustenance for a nest full of baby ospreys.

The best theory we have is to do with net energy inputs. Compared with other seasonal estuarine species ( e.g. mullet, bass, sea trout) the flounder has about 8-10% less fat content. And for birds, as we all know, fat = flight fuel. Emyr Evans (who is not keen on eating flounders himself) has pointed out to me that they have tougher skin than these alternative species, and so the upshot of all this is a three-way disadvantage in terms of diet:-

• More difficult to catch,
• More energy expended dealing with the food,
• Less energy obtained after ingestion.

Put together, this would explain why the birds – especially breeding females – have evolved a preference for more energy-rich prey items. Shape and taste are unlikely to have much to do with it.