Expedition to Tierra del Fuego – February 6, 2003
Adding to our concern over the low numbers of red knots on the bay is the low proportion of immatures. In healthy shorebird populations one could expect 1 out of every 3-5 birds to be young of the year. For example in 2002 we found 1 out of every 3 godwits (36%) to be young of the year. For red knots the figure was much lower, 1 out of 17 (6%).
This year only 5% of the knots were immatures. Why the lower rate? One possibility is juveniles don’t come to Tierra del Fuego to winter but stop short in Brazil. Without any evidence this is only a hypothesis. The more reasonable explanation is the decline in red knot productivity caused by gradual reduction in the number of birds reaching the Arctic, a reduction caused by the decline in the Delaware Bay’s horseshoe crab population
Identifying immature red knots commands great skill. In November, the recently arrived young of the year sport yellowish legs which can be seen from a distance. But in Febuary, the leg color is not reliable so we must have the bird in our hands to look for small, subterminal bands of brown hidden on small wing feathers (coverts) that lie atop the main flight feathers (primaries and secondaries). An immature pattern of moult also helps. Immature knots moults their feather earlier than adults, so if a bird has 10 new primary feathers, it suggests the bird may be an immature and other characteristics must be checked to verify age.
Our big catch, and two smaller catches on subsequent night, allowed us to individually mark 50 birds. Our birds will join other knots individually marked in Rio Grande by Allan Baker team in November as well as birds banded in the Delaware Bay last year. Although the number of bands is intrusive, it allows us to create an alternative method of estimating population numbers. The current method relies on the gradual loss of banded birds of each year thus providing a yearly survival rate which can be used to estimate the total population. Individually marked birds allow the estimation of survival, based on the resighting of individuals, which is more accurate.
Guy and Ken finished most of their survey of the Argentine coast and the Bahia Lomas portion of the Straits of Magellan. Once again they encountered difficult flying conditions preventing them from finishing a small part of the strait. The preliminary numbers were a bit lower than last year (about 23,000 birds). In other words, the decline of red knots is real; ignoring it any longer will not only further endanger this bird but makes the costs of it’s restoration higher and more difficult
Guy and Ken also solved the riddle of the missing knots. Several days ago we lost the daytime population of red knots, 12000 birds gone without a trace. Several days later, Guy and Ken found the birds on the eastern shore of the bay in one 20000 bird flock. Jorge Jorden, our pilot ( and host in Punta Areans) said ” it thought it was a big lake in the middle of the mudflat but it was all birds”. We think this extrodinary combination of the two distinct populations on Bahia Lomas a consequnce of the new moon tides. In our previous work we have determined that there are two distinct populations on Bahia Lomas, each defined by full moon tides, with the birds on each side using a salicornia marsh as a roost site once the 10 m tides covered all mudflat. But on the new moon tide, which is about 1 meter lower, only the western portion is fully inundated. The eastern side still present unflooded mudflat that the birds can use instead of the salicornia marsh.
Our last day on the bay was packed with activity. After a full night on the nets, falling into our sleeping bags at 4:00 a.m., we decided to attempt a cannon net catch the next morning. The previously stormy night gave way to a wonderfully warm morning under a gauzy sky. We hoped to catch godwits forced to roost on the higher salicornia marsh by the ever-higher high tides. We set the net quickly and soon about 5000 godwits filled the site. With some judicious twinkling from Mark, Brad, Luis and Ricardo we finally started a small group of knots towards the net. It looked promising until with a skua flew low over the flock creating a sudden lift off and disappearance of nearly all the birds. Discouraged and fatiqued by many nights with little sleep the team still agreed to mist net on our final night on the bay. We were rewarded with a small catch and our day ended once again at 4:00 a.m.