Expedition to Tierra del Fuego – February 10, 2001
The entire team began the day searching for birds that were finding refuge from the morning high tide. Sherry, Olivia, Mandy and Bruno took position at the mid-bay access point, while Brad, Bruce and Steve did the same at the stone beach or eastern site. Ricardo and I moved around in the van, but ended up at the Bluffs, an access point in between the stone beach and the mid-bay points. We ruled out the western portion of our study site because we knew it would flood completely. The tides were going to get higher than the previous high tide. We would stay at our posts until the tide receded, then we would start scans.
We guessed that the failure of the night before came down to two possibilities. Either the birds left the area, or we had somehow missed the birds. The first possibility was unlikely — the birds would have to fly overland 50 km to Bahia San Sebastian, or 40 km across the Straits of Magellan. But the same tide that flooded Bahia Lomas would also flood Bahia San Sebastian, so the move would be wasteful, and I knew from our survey flight that very little habitat existed on the opposite side of Straits of Magellan. Besides, the howling wind would pose and significant obstacle in either case.
It continued to blow on this day with sustained winds of about 40 mph, gusting much higher. The tide swelled with alarming speed, and in combination with the wind made the sea fearsome. At the stone beach access, the sea foamed and flooded every low habitat. At the mid bay access the team had to retreat to the dunes to avoid the ever expanding waterline. There was actually little danger, but the force of the wind and tide made everyone cautious.
From the start we saw birds. At the mid-bay access Sherry, Olivia, Mandy and Bruno watched flocks of 2,000-4,000 birds moving east, just as we had two nights earlier. The flocks rose in the fierce wind and drifted to the same eastern area that we assumed was the stone beach, but we were obviously misjudging distance. Fortunately, Brad, Bruce and Steven could see the high-tide roost we worked so hard to locate. The birds landed in the low island just off the bluffs and in between the mid-bay and stone beach access points.
From the bluffs, Ricardo and I could see more detail. All of the Knots and Godwits settled on the eastern side of a small sandy bar on the eastern edge of the island. The island rose far enough above the tide that shrubs grew in a swath only a few hundred meters wide but running the entire length of the island. A salicornia marsh ringed the eastern edge of the island. Around that, a sand-muck flat extended into the larger flat that continued throughout this portion of the bay. The birds gathered along the waterline with quanacos feeding all around them, seemingly oblivious to the watery chaos surrounding the islands .
The birds remained on the bar or island for several hours, eventually leaving in small groups and re-distributing throughout the eastern portion of Bahia Lomas. Three hours after high tide Brad, Bruce and Stephen once again observed birds on the western end 12 miles away.
We had run out of time and could not trap the birds on the island even though it presented good possibilities. The roosting site was well suited for both cannon netting and mist netting, which we could try to work out for next year. Much of our work would not stand an empirical test. Capturing, banding and instrumenting birds would provide useful information on movements we could only surmise in this years work.
However, we understood enough to guide future work. Moon tides at first appeared to us to be unusual alterations of the natural rhythm of the bay. But our experience taught us a lesson that mariners since the days of Magellan had known from harsh experience. Unusual tides and currents are the usual on the Straits of Magellan, defining the bay and in our limited experience the ecology of the birds.
Extraordinarily high tides engulf the bay on both new and full moons, or nearly twice a month, every month of the year. Although the new moon tides are a bit weaker, the moon constantly influences the tide.
For example, on January 27 the high tide, influenced by the new moon, rose to 9.2 meters then fell to a normal high tide level of 7.9 by February 3 just as we arrived. By February 11 it will peak at 10.5 meters, falling back to a new normal of 7.5 meters on Febuary19, only to start rising once again. In other words, the increase or decrease in the high tide occurs nearly all the time. This constantly changing wall of water seemed to influence the bird’s movement as much as any other ecological influence.
The shorebirds in the eastern bay required areas to feed, and to roost or rest in secure areas when they aren’t feeding . Their need for food increased daily, as they were moulting into breeding plumage and preparing for the journey back to the Arctic. We judged from some very preliminary invertebrate and feeding bird surveys that they must feed on the flats that flood every day. This is so because flats that flood only on full or new moon tide lack sufficient tidal flow to support clam and marine worm populations in numbers suitable for shorebirds. So, the increased flooding decreased the amount of time the bird’s foraging areas were exposed. This changed rhythmically twice each month. Moreover, when the tide reached extraordinary high levels, the birds were forced to find unflooded roosting sites, free from marauding ground predators. Thus, forcing a 15 miles exodus to the roost on the eastern half of the bay.
The fact that the birds didn’t leave the area, but clustered in a definable high-tide roosts will help us a great deal in the further study of Red Knots.
Lawrence J. Niles, PhD
Chief, NJ Endangered Species Program