Bohai Bay 2024 Update 2

Posted on: 2024-05-19 11:20:46

As all our regular readers will know we are here to focus on Red Knots. No bird, carrying a mark, passes our telescopes without being recorded but undoubtedly Red Knots remain our focus.

Three subspecies occur in the EAAF but one of them, roselaari only occurs as a breeding bird on the Wrangel Islands. Roselaari’s main non-breeding area is the Pacific coast of California and Mexico. Its migratory route does not pass through the Yellow Sea so, we don’t encounter that subspecies here. The two subspecies that we see are rogersi and piersmai. Rogersi breeds in Chukotka in far north east Russia and predominately spends the non-breeding season in New Zealand and south east Australia. Piersmai breeds on the New Siberian Islands, Russia and its main non-breeding area is northern Australia with some birds in South East Asia. Both subspecies can be found in each other’s main non-breeding area.

When in full and fresh breeding plumage the two subspecies can be told apart if good views
are obtained.

A ‘typical’ rogersi, © Adrian Boyle

A ‘typical’ piersmai, © Adrian Boyle

Rogersi’s breeding grounds are snow-free and available to the birds before those of piersmai and this is reflected in their migratory schedules. We conduct daily assessments of the proportions of the two subspecies and this difference in timing is apparent from the simple graph shown below.

The x axis is ‘week beginning’.

Counting Red Knot here on the Luannan Coast is tricky (counting is tricky everywhere actually). We don’t have the resources to do full counts of the area. So, we do an estimate every day when we are working at the core area of Nanpu. And, when we have time, we do more careful and structured counts. The numbers this year for the same date range as 2023 (the year with the lowest number 2010 - 2023) are considerably higher as shown below. This is still way below the peak numbers we recorded of 48,000 in 2018 and 2019. But the highest since 2020.

2023 2024 estimate/count
13-May 1150 700 estimate
14-May 1288 10000 estimate
15-May 1370 10000 estimate
16-May 1400 11830 count
17-May 900 - no estimate or count
18-May 900 11000 estimate
19-May 1300 11810 count

All numbers mentioned are point counts or day counts and don’t have any statistical work done to them. If turnover analysis was done the total numbers using Nanpu during the full spring migratory season would of course be much higher.

The dramatically lower numbers between the years aren’t associated with a corresponding drop in the Red Knot population throughout the EAAF. The Monitoring Yellow Sea Migrants in Australia (MYSMA) count programme that has been running for 20 years shows ‘no significant change’ in the north west Australia Red Knot population. We think the fluctuating numbers are associated with the local food source here at Nanpu. The Red Knots feed predominately on Potamocorbula laevis (pots) and the numbers of this small mollusc fluctuate wildly. This year the density of pots is ‘good’ and considerably higher than in the previous 4 years (Hebo Peng pers. comm.) This matches with Red Knot counts mentioned above.

Additional data that we collect to help inform this story is abdominal profiles (AP). This is a visual assessment of the fat stores a bird has. We are not catching birds so we can’t get their mass from weighing. AP is scored from 1 (very thin) to 5 (very fat). We take the AP score on all individually marked Red Knots if we obtain a suitable view. This year we are seeing more birds at AP 4 and 5 than during the same date range as last year, presumably a direct reflection of the more abundant pots in the mudflats.

The intriguing question to us is, how do the Red Knots know about the numbers, density, and availability of pots here in Nanpu year on year? And how do they ‘decide’ if there are enough pots for the numbers of birds present to allow them, as an individual, to gain enough weight for successful migration? And then logically they would need to decide to either stay at Nanpu or to move on to ensure the necessary weight gain? Birds are not on ‘auto-pilot’ driven by instinct alone. They must make decisions throughout the year and some of these are critical to get correct.

Early morning scanners 2025-05-12

All the figures and graphs in this update will be finalized in the Bohai 2024 final report but are presented here to give you all a feel for the Red Knot situation at present.

Chris Hassell, Katherine Leung, Liu Yang. May 19 2024.