Don't tell him Pike

I saw my first Bittern in Nightingale Wood in 1972.  The London Bird Report has it as:  Cheshunt GP, one on Mar 4^th (GJW).  Hmm, rather brief for what I seem to remember was a good find at the time.   However, over the years this has become a regular haunt for wintering bitterns.  So why do Bitterns return to this spot year after year?

Nightingale Wood is a wonderfully scruffy few acres of wet self–set woodland on the edge of a badly restored gravel pit.  It’s landform is varied to say the least: hollows, holes, slopes, cliffs, pools and piles of reject gravels.  Initially colonised by Hawthorn and willow scrub but now succeeding to Ash and Alder, with masses of dead wood. The modern mineral planning officer would never allow it.  They would smooth everything out, cover in top soil and insist on regimented rows of planted trees, and in doing so would eliminate any decent habitat.

Nightingale Wood? I hear you say.  Well yes, once upon a time in the scrubby days, 2 or 3 pairs were to be found.  I found my first Nightingale nest in this wood and watched the youngsters fledge.  Many a Willow Tit nest was also noted.  Over the years the wood has provided regular Woodcock, Water Rails, Siskins and Redpolls , but also 3 Golden Orioles, 2 Firecrests, 2 Wood Warblers, Pied Flycatcher, Great Grey Shrike, Bearded Tits and a roost of 9,000 Fieldfares one night.  The edge of the wood grades into the water-filled pit through tumble-down wet woodland and marshy spits to a reedbed fringe of about an acre.

This is where the Bitterns come in. So why do Bitterns return to this spot? The varied topography of the wood continues out under the water with deeper pools interspersed with reedy shallows.  Research papers will tell you that Rudd and Eels are the preferred prey of breeding birds in the UK, but Perch and Pike aplenty tuck into these watery hollows in the reedbed during the winter and form the main prey for Lee Valley Bitterns in the lean months. Good feeding at this time of year is crucial for getting the birds through the winter and into good condition for breeding.

I returned to Nightingale Wood yesterday for an hour or so at dusk and a couple of Bitterns were on show.  There are currently 7 or 8 Bitterns wintering in the Lee Valley, with 5 in the Cheshunt pits complex.  The reedbed now has excellent viewing rides cut into it by the Lee Valley Park Authority.  One Bittern duly appeared on the edge of a reedy ride no more than a dozen paces from my original sighting in ‘72.   It caught a nice stripey Jack Pike and soon dispatched it.  At dusk both birds moved with just a brief few wing-beats from the feeding spots to the safe roosting areas on the edge of the reedbed.   That’s why they come back here.

Cranes

One frosty morning last week, I was out in the Fens again looking at Cranes.  The nesting pair were back on territory, after some weeks of touring the Fens.  Last year’s youngster was still in tow but will surely be given the boot as spring approaches. Cranes have had their most successful year in the UK since returning as a breeding bird, with up to 24 pairs rearing at least 13 youngsters.  The Fens offshoot of the Broads population now seems well established

The winter food of these birds is a mix of agricultural remains and more natural foods from wet grasslands.  The Cranes had been feeding extensively on beet tops earlier in the winter.  One family party were feeding in a maize strip, picking cobs off the standing crop.  Spuds seem to be a good food also.  Spuds left in the ground after harvest may be favoured as they are fresher, but good farming practice usually leaves few available for the birds.  Piles of dumped spuds have been used to attract birds but it appears Cranes prefer them fresh, so replenishing stocks may be the best approach.  The birds I was watching were feeding on waste grain put out for them.

Winter roost sites are important, with birds favouring shallow flooded areas. Ideally they will be close to feeding areas but birds will frequently fly some distance to a suitable roost.

So if we want to attract Cranes, or keep them in a particular area for viewing, why not grow small areas of sacrificial crops - spuds or maize, adjacent to wet grassland etc.   Also, ensure that suitable winter roost sites are present.   Shallow flooding of some cropped land would be beneficial.    These techniques are well known to attract cranes and geese in the USA and such management would also benefit other farmland birds.  

Shufflers in freezing feeding frenzy

Feeding frenzy - Shufflers, Coot-muggers and a Great Crested Degree.

With a few inches of snow and an easterly breeze, I decided it was time to give Siri a go on my up-graded iPhone rather than get frozen fingers scribbling in the notebook. As I wandered around the valley, a summary of each site was carefully dictated before I headed home, hoping to find my day list magically appearing on my computer via iCloud. And there it was. Holyfield Marsh was particularly good. Apparently I saw a lot of Shufflers, 157 apparently. And also 13 Descenders, 350 Tufted Darks, 4 Snooze (3 drunks and a redhead), 15 Great Crested Degrees and a Yellow-laked Goal. I’m not sure whether I need to train Siri better or change my accent.

With large numbers of birds restricted to small areas of unfrozen water, the apparent heightened and frenzied feeding activity was interesting. Several groups of diving Coots not only had their associated kleptoparasitic Gadwalls (the ‘Coot-mugger’) but had attracted a range of other ducks and gulls. Nearby, a swirling mass of 60+ Shovelers, sorry Shufflers, continued to stir up and sift planktonic invertebrates from a small patch of water. Such feeding behaviour is typical of the Shoveler, but I can’t recall such a large group or such frenzied activity.

I had followed almost the same route around the lake back in the freezing conditions of January 1985. On that visit, the count of Smew reached a remarkable 40 birds (with at least 19 drunks), along with a Slavonian Grebe and 300 Wigeon. A Bean Goose (‘rossicus’ say my notes) had joined the adjacent Canada Goose flock but it eventually and illogically got dismissed as an escape. I think even Siri would have got that right.

Let’s hear it for Billy Bloodworm

As waders pile through northwards as spring progresses, and we search for those that drop down to muddy pools, we might ponder what they are eating. Now I have always been a fan of the humble chironomid, the ‘non-biting midge’, such that my CB ‘handle’ on the Isles of Scilly (back in the days when a CB was the height of fashion) was ‘Billy Bloodworm’ (bloodworm = chironomid larva). These little beasts play a key role in many a food chain, notably forming an important component of the diet of many birds. Pochard have been found to dive selectively over areas with high chironomid larvae densities. In one study, they formed 80% of the spring diet of Pied and Yellow Wagtails. Furthermore, the pupa, emerging sub-adult and adult insects are vital for breeding duck, forming a major food item for ducklings. And of course those passage waders; we know they stop and feed at wetlands where high densities of larvae are available at appropriate water depths (54-84% of diet in one study of 4 calidrid species). Nothing much new here perhaps, but how many sites deliberately try to maximise this food at the appropriate times?

Many other things eat chironomids as well, for example, bigger invertebrates and fish. But chironomids can rapidly colonise new wetlands and reach high densities by exploiting the nutrient flush resulting from flooded decomposing vegetation, before being pegged back by their predators. So managing early successional wetlands (‘scrapes’) to maximise their biomass is possible; any reserve manager worth their salt should be thinking a lot about the requirements of these little critters. Chironomids have a large range of foods, however, algae and detritus are commonly consumed. Temporary pools frequently have high production levels of chironomids. Flooded vegetation within pools acts as both a substrate for the larvae and food source, first as a substratum for algae, then as a source of bacteria as it decomposes. So, a careful rotational management regime of flooding, drying, vegetating, grazing (dunging), rotovating and re-flooding should all be considered to get the best out of Billy Bloodworm.

So how often should ‘scrapes’ be allowed to dry out? Chironomids pass through four stages: egg, larva, pupa and adult midge. The last two stages are generally very short. 33% of studied species have one generation a year (uni-voltine), 44% have two (bi-voltine) and 18% have several. Uni-voltine species mostly emerge in spring/summer and pass the winter as mature larvae. Bi-voltine species usually emerge in spring and autumn. So, flooding for 12-18 months, before drying might be ideal. With several scrapes being flooded and dried in different rotations, maximum production might be achieved. Drying scrapes in spring by 2-4 cm a week, to reveal mud and shallow water of 2-5 cm should make enough chironomids available to feed a succession of those passage waders. Let’s hear it for Billy Bloodworm.

To feed or not to feed?

The recent severe weather has been tough for Bitterns, with many being seen around the remaining areas of unfrozen water, frequently in unusual situations.  Bitterns traditionally suffer in such cold winters.  At least 17 sites attempted to feed their Bitterns, usually with Sprats or Sardines, during the coldest spell of weather and 8 sites report that Bitterns were seen taking the fish. See Steve Blain's video of a Bittern eating sprats  (or below) provided by local birders at Stewartby Lake.  At Leighton Moss, three Bitterns came to the feeding site regularly (about once every half hour), took some fish and then returned regularly during the day.  At most of the other sites, Bitterns were seen in the vicinity of the food and may well have taken it.  So it would appear that feeding in hard weather may help some Bitterns survive.  We feed Blue Tits, so why not a bird that is a conservation priority in the UK?


On the other hand, is this just another example of unnecessary human tinkering?  Will it make any difference at all to the population in the coming spring, or will it even hinder a natural dispersal of young birds to new sites?  What do you think?

Edit - In a rather low poll, 16 out of 16 respondents thought we should be feeding Bitterns.

Bittern eating sprats from Steve Blain on Vimeo.