Table. Mortality, fledgling success, carcass removal, and estimated fraction of Sandwich tern breeding population that died or disappeared in 10 breeding colonies, the Netherlands, 2022.
| Breeding colony no. | Date mortality first observed | Fledgling success (no. fledglings produced/no. initial breeding pairs) | Dead adults found | Dead chicks found | Abandoned eggs | Carcass removal | Estimated percentage of breeding population that died or disappeared from colony* |
|---|---|---|---|---|---|---|---|
| Colony 1 | 2022 May 30 | 1% (20/2,100) | 342 | 425 | Yes, all 700 late nests | Yes, with a 12-d delay, on 10 of next 18 d, adults and chicks | 99% |
| Colony 2 | 2022 Jun 21 | 0% (0/150) | 107 | 0 | Yes, all 150 late nests | Once, with a 3-d delay, adults only | 100% |
| Colony 3† | 2022 Jun 4 | 0% (0/1,176) | 170 | Hundreds | Yes, many | Yes, with a 6-d delay, on 6 of next 18 d, adults only | 100% |
| Colony 4† | 2022 May 26 | 0.1% (5/3,374) | 3,316 | Thousands | Unknown | Twice, once 19 d after start, then 16 d later, adults only | 99.8% |
| Colony 5 | 2022 May 29 | 0% (0/220) | 400 | 0 | Unknown | Once, 17 d after deaths started, adults only | 100% |
| Colony 6 | 2022 May 31 | Low (Few/3,016)‡ | 941 | Thousands | Yes, all 245 late nests | Yes, without delay, on 15 of next 36 d, adults only | >92% |
| Colony 7 | 2022 May 31 | 11.1% (45/404) | 115 | Unknown | Yes, many of 40 late nests | Yes, without delay, on 14 of next 36 d, adults only | 78% |
| Colony 8 | 2022 Jun 14 | 47.4% (65/137) | 2 | 0 | No (also no late nests) | No | 5% or not applicable§ |
| Colony 9 | 2022 Jun 6 | 9.5% (665/6,974) | 2,368 | 3,122 | No, there is activity above 400 late nests | Yes, with an 8-d delay, on 14 of next 27 d, adults and chicks | 81% |
| Colony 10¶ |
2022 Jun 14 |
0% (0/600) |
240 |
12 |
Unknown, 150 + 450 late nests |
Yes, with a 7-d delay, on 3 of next 15 d, adults and chicks |
100% |
| Overall | Low (Few/18,151 pairs) | 8,001 (22% of breeding birds) | Thousands |
*Calculated by subtracting the ratio of fledgling success in 2022 over average fledgling success in previous years (0.5 fledgling per pair) from 100%. †After colony 3 and colony 4 were decimated, a very late colony of 600 breeding pairs was established in July on Texel midway between these 2 colonies (53.022°N, 4.819°E), and this very late colony yielded 300 fledglings in September 2022. ‡Exact number not recorded. §Because no carcass was examined, it is unclear if the 2 dead birds observed in this colony died from other causes or the cause of death was HPAI H5N1 but further infections in the colony were aborted. ¶In this colony, some very late birds did end up producing some fledglings (around 155) in August (Appendix 1 Table 1).