We're finally working out what our bitterns are up to here at Lakenheath Fen. During a very, very hot bittern watch yesterday morning things started to click into place at various locations. It was a very interesting day, not just in terms of bittern sightings but also understanding more about the surveying technique.
To confirm a bittern nest, you need to record four 'in and out' flights from the same location within a four hour period. These flights are feeding flights, with the female bittern leaving the nest to find food before returning to feed the chicks. The male bitterns have no involvement in raising the chicks – unlike our male marsh harriers! However, as I've mentioned before, the bitterns don't make it easy for us to monitor them. The reedbeds need lots of reed-edge feeding habitat for bitterns, which means lots of channels as well as open water. Sometimes the bitterns fly nicely over the reeds for us to see, other times they use these channels as flight paths - you may catch a glimpse of a wing when they come into land if you’re lucky, otherwise you don’t know they've moved. Bitterns also walk through the reeds and this can make it difficult to keep track of where they are, especially when you're dealing with more than one bittern in the same location! Some bitterns croak in flight and this can be helpful - for example I could hear one in flight yesterday and kept madly scanning the reeds before it popped out of one channel briefly and into another. If I hadn't had my ears as well as my eyes open, I would have missed that one.
I've spent most of our bittern surveys in one particular spot - this can be useful as you build up a feel for what is happening over time. It's a good spot too, although sometimes you need eyes in the back of your head as bitterns can fly behind me as well as in front. Yesterday, I had 20 bittern 'episodes' - this is an amazing count for four hours! I can't call them all flights because on five occasions I was fortunate enough to see bitterns on the ground, for quite long periods of time. There was such a hub of activity right in front of me involving at least three bitterns (I was struggling to keep track!) that I came to the conclusion I was seeing an adult female with young, almost at the point of fledging. When two bitterns came to be standing right next to each other quite happily with a third very nearby, this confirmed my suspicions. How lovely! The timing matches with when we first spotted activity in this particular area too.
Yet up to this point, I had not recorded the necessary number of flights in and out to confirm the nest. We were quite sure there was a nest but up until yesterday it had only been a ‘possible’ nest. It just goes to show how difficult it can be monitoring some species. If a nest fails at the egg stage, chances are we wouldn’t know about it at all as there are no feeding flights to monitor. So the nest figures are always an estimate but with important species like this an educated estimate is better than nothing! So at this stage, we think we have four bittern nests. With one successful nest already this matches last year but we’re hoping that this year the other three nests will go the distance too. Last year we think the wet, cold conditions played a large role in the nest failures so with the weather set fair for the foreseeable future we're feeling hopeful!
I’ll finish with a couple of photos from yesterday. This particular bittern was in full view of me for at least half an hour, preening and generally watching the world go by!
Photo credit: Ali Blaney - chillin' by the pool
Photo credit: Ali Blaney - still chillin'...