Orphaned in winter, dead by summer: swan tracking moves the problem off the tundra

A month of lateness does not read like a cause of death, but on this route it works as one. Bewick's swans breed on the Arctic tundra of European Russia and winter in the Netherlands and neighbouring countries, flying south in family groups ^[4]. The normal arrangement holds until at least halfway through the young bird's first northward migration, when it is close to a year old ^[5]. A juvenile that does not leave northern Germany until April is following the right route a long way behind the birds it would otherwise have travelled with ^[9], and it arrives on the breeding grounds out of step with everything that sets the timing there.

The arithmetic in this study is worth stating plainly, in both directions. Ten of the 24 tracked juveniles were orphaned over the winter ^[1], which is 42 percent of the cohort ^[14]. The survival split, 40 percent against 86 percent ^[7], is a 46 point gap and a mortality rate roughly 4.3 times higher for the orphans ^[15]. In whole birds, though, that is four survivors out of ten set against about twelve out of fourteen ^[16]. The effect is large; the sample is 24 animals from 12 families ^[3], and the honest reading is a strong signal from a small, well-characterised set rather than a population estimate.

The detection method also sets a limit on what can be concluded about where the young birds died. There is no telephone network in the breeding area, so the transmitters cannot report for months, and coverage returns only when the swans move south in October ^[8]. According to Linssen, for most of the orphaned young no such signal arrived ^[2]. Death is therefore inferred from silence, dated to a summer rather than located or explained. One orphan's last fix came over Latvia, after which it left the records entirely ^[9].

The one bird that recovered is instructive about mechanism. After hunters shot her mother on the wintering grounds, 036T crossed to Great Britain in February and travelled with an unrelated family for two weeks, until the bond broke in mid-March ^[10]. She started north alone on 26 March, met her biological father again in the Russian Arctic in September, and was still alive five years later ^[11]. Whatever the young bird is getting from adult company appears to be transferable, and an unrelated family will supply some of it. What produces that chance is a wintering site where the adults are still present and not being shot at. Linssen's own conclusion is that protection here is less about which fields are inside a reserve and more about quiet during the first winter and the first spring migration ^[12].