Last summer the entrance to Lake Wollamboola was closed to the sea. The backed-up waters of the lake enclosed hundreds of hectares of sunlit shallows interrupted by rafts of algae, Black Swans and Australian Pelicans. The lake was dominated by wading birds, migratory species such as the Bar Tailed Godwit and endemics like the diminutive Red Capped Plover.
This summer I launched the kayak into a very different lake. It was open to the sea, a strong current flowing through a network of deep channels around the entrance. A huge flock of Little Black Cormorants sat drying their wings at the edge of the main channel. There were a few waders wandering the shallows , including Bar Tailed Godwit, Red Necked Stint, Sanderling and Eastern Curlew, but nothing like in the numbers or diversity of the previous year. Nor could I see any of the thousands of Black Swans that carpeted the lake last summer.
None of which should have been a surprise. Coastal lakes and estuaries change to a remarkable extent depending on whether they are open or closed to the sea. Major rainfall and storm events are the natural forces that determine whether the entrance is open, alternately creating and breaching the ridge of sand that builds up during prolonged periods of small seas and low rainfall.
The water in closed lakes is much more variable than the sea, with salinity changing in response to rainfall and evaporation. Opening the lake re-introduces tidal influences and seawater, allowing larger marine predators to move in and out of the system. All of which explains how lake habitats can change so much from month to month.
The relative paucity of waders in Lake Wollamboola was more than made up for by large flocks of terns and gulls at the lake. The gulls were all Silver Gulls, but there were three different species of tern, the Caspian Tern, Crested Tern and Little Tern. The Caspian Tern is the world’s largest, while, as the name implies, the Little Tern is one of the smallest.
Feeding flocks of terns often wheel above schooling baitfish, acting as sentinels that attract other predators, such as cormorants and pelicans. You can spend a lot of energy racing after the feeding birds, hamstrung by the fact that they can fly faster than you can paddle. That’s just what happened to me. After a frustrating hour I headed over to a long sandbank where a bunch of birds were roosting. I stopped about twenty metres away in the shallows, a distance that seemed to allow the birds to continue acting naturally. Several of the birds on the spit were juvenile Little Terns. Every few minutes an adult would arrive with food in its beak, land for a second or two and quickly hand over its prize to a youngster.
The channel next to the bank was acting as a flyway for other birds, providing the opportunity for me to grab a photo as they flew by.
Some of the Little Terns were carrying small fish, mostly Sandy Sprat and Blue Sprat. Others were carrying beakfuls of School Prawns. The prawns looked big in the beak of the Little Terns, and judging from the number they were capturing were highly valued. It quickly became clear that the other species of terns and gulls were also concentrating on the School Prawns. Silver Gulls, not normally the most effective of predators, were catching large numbers of the prawns in the shallows. A big ‘run’ of prawns was in progress and the birds were making the most of the abundant food source.
The open lake was clearly no less productive than when closed last year, just different, offering different feeding opportunities for a different range of wildlife. The smallest and largest species of tern in the world were both feeding on the same abundant prey.
Although there was some feeding of juveniles at the sandspit most of its occupants were flying in to rest and roost. It was inevitable in the limited real estate of the exposed spit that there would be competition for the prime spots. Watching the resultant squabbling, it was clear that Little Terns take a very ‘canine’ view of their larger relatives, seeing no reason to act in a submissive way towards them.
One slight light sour point in the morning’s kayaking was the appearance of a young fox on the edge of the lake. It was paying keen attention to a nearby group of Black Winged Stilts and seemed little concerned by my scrutiny.
A couple of days later I returned to one of my favourite wildlife destinations, Lake Illawarra. There are always plenty of interesting behaviours on show, especially the balletic feeding of the Little Egrets. For me the biggest appeal of the site is the amazing light on the lake, especially when contrasted to the pure white plumage of the egrets. Good stuff!