Have red knots declined to a new low? Tierra Del Fuego 2011
In one of the pivotal papers chronicling the long, sad decline of the red knot, Allan Baker of the Royal Ontario Museum argued that a population of shorebirds can reach a tipping point that spells extinction. This tipping point can occur when the population appears robust thus fooling everyone into thinking there is no serious problem. This happened to many long-lost species, like the passenger pigeon. The population numbered in the millions then in the thousands. Then one day they were gone — too few adult birds, too few young and a sudden natural loss dealt the knockout blow. Is this now happening to the red knot?
On one of our final days on Bahia Lomas we spread out and counted all the knots on the entire bay. We do this every year and compare our ground count with Guy Morrison’s count done from an airplane. Our ground count is a reality check of the aerial count to make sure there were no serious errors such as wrong identification or missed groups. In the past our two counts have been nearly identical.
This year both ground and aerial counts pointed to a significant decline in the number of red knots. We didn’t know this in the first part of our trip because we mainly trap on the western side of Bahia Lomas. Our counts of red knots on the west side were actually higher than previous years, so we were hopeful the baywide number of birds would be the same or even higher. However, the number of knots on the eastern side of the bay was low.
Bahia Lomas can be divided into three zones: the western intertidal flat where we trap, a much muddier central intertidal flat, and an expansive sandy flat on the eastern side. Knots don’t use the central flat because it is mud; knots always feed on the eastern and western sandy flats where bivalves (their main prey) are present. Counting the eastern flat, however, is both difficult and dangerous.
One landowner owns all but a small portion of the eastern intertidal flat and will not allow anyone access. So we must travel nearly 2 hours on very rough roads to Punta Catalina to get to the eastern extreme of Bahia Lomas. To gain access to the sand flat, we must cross a tidal river that rises and falls 30 feet twice every day. We must wait until low tide, cross the sandy-bottom river on our ATV, and stay out on the intertidal flat until the next low tide allows us to cross the river again. This could take 12 hours depending on the height of the tide.
stunned us. We searched the entire eastern flat and knew we missed nothing. Where in previous years we counted 12,000 knots, this year we counted 6,000. With the count of the western flat done simultaneously at high tide, the total Bahia Lomas population came to less than 11,000 knots — 6,000 less than last year. We found out later that day that the aerial count of knots by Guy Morrison and Ricardo Matus was approximately the same. The consistency of the two counts meant the main wintering population of red knots in the western hemisphere had fallen to a new low.
What does this drop in numbers mean? At this point it is still too early to know. Did the knots move to another wintering area? This is unlikely since the aerial count agreed with our count and it is more expansive. Moreover, the only other significant wintering population in Patagonia, at Rio Grande in Argentina, also fell this year. It once numbered over 5,000 knots, this year Allan Baker, Patricia Gonzalez and Luis Benegas counted only 400 birds. Luis performs an ongoing surveillance of red knot numbers in Rio Grande throughout the Austral summer – red knot numbers remained the same in Rio Grande during our mid-February count so we knew we were not missing birds.
Is there something wrong with Bahia Lomas that led to this drop? It’s possible because the bay’s Hudsonian godwit population fell this year compared to last year’s count, although it is within the range of counts made in previous years. It also does not help explain the decline of red knots at Rio Grande.
The drop in red knot numbers could be from natural causes. The year before last we tracked one bird with a geolocator through several late-summer tropical storms that threw it off course by 1,000 miles. This past year the Atlantic Ocean and Caribbean were wracked by frequent tropical storms and hurricanes. Did large numbers of knots succumb to these dangerous storms while en-route to Tierra del Fuego? Or is this decline another consequence of the overharvest of crabs on the Delaware Bay — too many birds couldn’t breed or died because of the collapse of the Delaware Bay stopover and the population fell too low to withstand natural conditions that have always caused mortality. A loss of 6,000 birds is bad for a wintering population of 56,000, but for a population of 16,000 it is catastrophic. Have red knots reached the tipping point?