Friday, 29 July 2011

On the shore

As I mentioned in the last post from the beach, there's both a huge density and diversity of invertebrates (mostly Annelid worms and molluscs) in the sand and mud. So much so, in fact, that the intertial mud and sand is a huge feeding resource for wading birds. I was once told that if you extract all the worms and molluscs that birds like to feed on from a sample of estuarine mud 1m square and 10cm deep, you'd have a food sample with a total energetic content of around about 1.5 Mars bars. Now, I've had a quick search online for the primary literature that back this statement up, and I wasn't too surprised to discover that althere the cubic metre equivalent is easily found on a google search (you get many answers from 13 mars bars per cubic metre, to 20, with most around about 14 or 15) none of them pointed me to the primary literature at all. And I can't do so either. But I did find papers like this and this that, once you do the sums, suggest it's not a bad approximation. And it's a nice thing to help people understand just how rich an environment this is - everyone knows what will happen to them if they eat to many Mars bars, and there's lots more than 1 square metre in most estuaries! What's more, the rate of productivity is prolific too, so even as it gets eaten, the resource is being continuously replenished. And, of course, the mud isn't the only place with massive densities of invertebrates either - if there's seaweed left to decompose on the beach it harbous massive densities of arthropods, another favoured food resource.
Spot the white-fronted sandplover on it's nest...

White-fronted sandplover nest - two eggs here.

Unsurprising, therefore, that many birds choose to forage in these rich habitats. Some of them, like the White-fronted Sandplover I found nesting at Ushongo (can you spot it on the nest?! This seems to be the first breeding record for the northern coast of Tanzania) are resident. But most of them are migrants fron the north and now, in mid July, most of them are busy finishing off their breeding season. Not all, though - a few first year birds will hang around in Africa all year, waiting to breed for another year, and already those adults who failed in their breeding attempt have come back. In the next few weeks more and more will pour south - and they don't just look for coastal estuaries either, the soft mud around many shallow lakes are similarly rich feeding areas so even if you're only passing the odd soda lake in the traditional safari areas you'll be able to see many of these birds as they start to return in ever bigger numbers. In fact, places like Lake Manyara will soon be home to truly spectacular gatherings of wading birds from the north - well over 1 million little stint are there in most years, and even more at times (up to 3 million have been estimated by Neil Baker of the Tanzanian bird atlas project).
Turnstones, curlew sandpipers and sanderling

Curlew sandpipers and turnstone

So, let's have a little look at some of these migrants that I saw last week - if only to illustrate some of the amazing migrations these birds are capable of. In the pictures above there's a mixed group feeding on invertebrates in rotting seaweed. Most of them - the ones with the orange legs and beaks - are (ruddy) turnstones. These birds, if they migrated this year, will have attemtped to breed right on the tundra beside the Arctic ocean - check here for a map. That's an extraordinary movement - and amazingly, these birds can easily live for 30 or so years, doing these phenomenenal migrations each year. (They also have a remarkably wide diet, with a memorable series of papers reporting ever increasingly bizarre food items from soap and other rubbish, to dead whales, until the editor finally stopped the correspondence when the title reached "Turnstones feeding on human corpse". Nice.). Anyway... You'll alsoalso see (at the right of the top picture, and several in the lower picture) lots of grey waders with rather longer beaks. These are Curlew Sandpipers, and have a similar breeding distribution in the Arctic (but only the Russian Arctic, not North America). These are clearly birds that didn't migrate this year, as the breeding adults at this time are a beautiful rusty red colour and we'll start seeing them soon. Long, decurved beak, long black legs and a white stripe above the eye help identify this species. And if you look carefully on the top picture you can spot one other grey bird running up the sand bank, a bit stockier looking, with a shorter beak - this is a sanderling, and when not breeding in the Arctic is a typical bird of sandy beaches around the world.
Greater sandplover and turnstones
Standing still among the rushing turnstones in this picture is a non-breeding Greater Sandplover, a species with a rather different migration route - this time breeding in the areas around the Caspian Sea in central Asia - not anywhere near as far as the other two, and obviously closely related to the other plovers we see around here, which a much shorter beak and big looking head. We also saw a few lesser sandplovers, which are extremely similar, but tend to have smaller beaks, small heads and shorter, blacker legs (you can see this is a bit greenish on this bird).

There were plenty of others around too, but I didn't get any good pictures I'm afraid. It's always worth checking through the wading bird flocks, though, as anything might turn up - many species show amazingly long migrations and occassionally extreme vagrants turn up among the flocks of commoner species. All very nice.

3 comments:

  1. So not all migratory birds migrate every year?? How often do they migrate? I'm not a guide in training, or otherwise, but I'm quite enjoying your ecology discussions.

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  2. Yes indeed, many shorebirds and seabirds don't migrate north in their first (or, for some species, second too) year. A few of the larger migrant raptors seem to hang around here all year too - there's usually one or two juvenile steppe eagles to be found. These are young birds of long-lived species that wouldn't nromally breed in their first year. It's far, far rarer for small passerine (songbirds) not to migrate - but they can only really expect to live a few years and must make the most of every opportunity. Also, life in the bush during the dry season is hardly easy, whereas sitting on a coast is much less seasonal.

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