Mid-September marks the beginning of a very exciting time of year at Montrose Basin, we're filled with anticipation as we await the return of numerous winter migrants. We have seen wader numbers starting to increase this month, as the birds return from their summer breeding grounds. But why do these birds flock to Montrose Basin in their thousands?
While out on the reserve carrying out ranger duties recently, I was reminded of the rich food source that the mud holds. I was surrounded by mud casts from lugworm and once my eyes focused, I could see thousands of tiny holes made by Hydrobia (mud snails), and further ahead were banks of common mussel beds; surely a bird haven. Since the tidal estuary doesn't freeze over in winter months, it provides a constant supply of food thanks to the invertebrates that call the mud home.
But this isn't the whole story, it would be easy to think that since there is an abundance of food for wintering migrants that birds will always flock to the reserve in great numbers. However, it is important to remember that this food source is sensitive to changes in their environment and so as the invertebrate numbers fluctuate, so will the visiting birds that feed on them.
The presence of invertebrates in the mud is associated with the salinity, exposure, inundation, sediment size and fertility of the substrate. Though the mudflats appear flat and featureless from the shore, there is actually a lot of variation with level and undulating areas. The sediment varies too and can be stony in areas and muddy in others. Over time, this can change and substrates shift across the basin. This means the invertebrates that suit a particular substrate will move with it.
It is not only the location of this food source that alters, but also the quantity. Many of the invertebrates living in the estuary have a large natural variability range. Meaning some years can see a dramatic increase in numbers while others could see the same decrease.
The breeding cycle of the different invertebrates present varies for each species. Lugworms may breed several times in their life cycle, but a ragworm can only breed once. As a ragworm reaches maturity, often after just one year's growth, hormonal changes cause their digestive systems to break down, enabling eggs and sperm to be produced. Spawning is then stimulated by temperature and the lunar cycle, so all mature ragworms in a population will spawn together.
All invertebrates present at Montrose Basin play a vital role to the make-up of the reserve. For instance, species of Corophium (small shrimp) have a very significant role, increasing oxygen levels in the top centimetre of sediment. This creates a thin yellow/brown layer on top of a much darker anoxic layer. Human disturbance to these layers can expose this anoxic layer and release pollutants held in the mud.
So these, often overlooked, species residing in the mudflats are fundamental to the populations of many bird species that visit the reserve.
Those living in close proximity to the basin may have been hearing the call of wild geese for some weeks now. The first geese to arrive at the basin were Canada geese on August 19. However, they don't stay for very long at the basin as they move further south to their wintering grounds.