Gadwall breeding in Cornwall

Swift Report - Winter 2022

Cornwall’s first successful breeding of Gadwall, at Walmsley Sanctuary in 2012.

This appears to be the first breeding record of Gadwall in Cornwall and represents a considerable achievement for the Society and especially warden Adrian Langdon and his small but enthusiastic group that manage the reserve. Walmsley Sanctuary is now regarded by many as the ‘Jewel in the Crown’ of the Society, not only attracting many rare birds but also, just as importantly, providing a safe environment for breeding birds. Now that the new water level management work has been completed, the site retains a greater area of water, vastly improving its attractiveness to wildfowl and hopefully should produce more exciting breeding records in the future. For that reason, I feel that the details of this breeding should be documented. Although I may not be the best person to do it, I was at least well-placed to witness most of the major events as they unfolded.

In March/April, at least eight individual Gadwall, a fairly equal mix of males and females, appeared to be using the Amble Marshes site, with no shortage of pursuit-flights, calling, whistling and display behaviour by the males. The latter consisted of simply raising and lowering their heads, sometimes facing and sometimes alongside one of the females. These observations are not unusual; this behaviour has been noted many times during spring at this site in recent years at least.

On 5th June a female and seven small ducklings were noted. They were observed for at least an hour, and my guess at the time was that they were around two days old. At the time there was no indication of the exact location of the nesting site, though the family group was first seen on the newly-formed scrape just southeast of the Tower Hide, later moving on to open water inside the bunded area. Inexplicably, on 6th June the female appeared to have eight ducklings in tow and this prompted me to take an interest in the appearance of the individual ducklings as they developed, half expecting to discover an adopted Mallard in their midst.

From the outset, all the ducklings looked alike; upperparts black-brown without olivaceous tinge, light patches on upperparts were cream-buff, not light yellow as Mallard, in all other respects looking right for pure Gadwall ducklings. I use the term ‘pure’, as with a probable first breeding record at a site where there is a history of hybrid pairings, there will always be a strong well founded suspicion of Brewer’s Duck (named by John James Audubon after his ornithologist friend Thomas Brewer, as at the time Audubon thought that the bird shot in Louisiana was a new species, not a hybrid Mallard x Gadwall). Frankly, I would be astonished if the apparent long-term, though recently rapid, increase in the species’ numbers, effectively spreading westwards and spilling into Cornwall (assisted in part by escapes from waterfowl collections and several high-profile reintroductions, fuelled by the existence of newly formed reservoirs, fishing lakes, decoy pools and nature reserves) had helped to prevent gene transfer from closely related species, namely but not exclusively Mallard. Many wild Gadwall already show some diluted Mallard-esque plumage characters in some plumages and many female Gadwall show varying amounts of white in the tail, as does the Walmsley female. You can be certain that if there are observable consequences of gene transfer there will be many more that are not so apparent.

There is certainly no shortage of loose male Mallard groups patrolling the area in spring, seemingly hell-bent on cornering and mating with anything, as long as it’s vaguely brown-looking! I have never identified an obvious mixed-plumage hybrid Mallard x Gadwall on the Amble Marshes, but this isn’t as easy as it sounds. Some presumed males especially, undergo extremely odd and puzzling plumage stages and I find it difficult/impossible to rule out with any certainty possible backcrosses and intersexes. Variant plumage is not the only possible manifestation of mixed genes; at least two apparently normal plumaged male Gadwall have been observed showing an unmistakable and lengthy attachment to female Mallard (one at Amble Marshes and a different male at Treraven Meadow), even at times with female Gadwall in close proximity. However, it’s impossible to speculate if the males were both the product of a mixed pairing Mallard x Gadwall or parasitic egg laying followed by imprinting, there is also the added complication of the possible whereabouts of previous hybrid young, these lacking description (two in 2008) at the same site.

During the observed fledging period of 45+ days, the adult female did a brilliant job of guiding and protecting her brood. As far as I could see she managed to keep them all together and intact, despite the attentions of an accomplished duckling-snatching Peregrine Falcon and numerous Grey Herons and Buzzards, right up to the dog attack on 9th July. The young at this stage were still unable to fly, for the first two hours the female Gadwall did an outstanding job of distracting the dogs from her scattered brood, skilfully managing to feign injury and stay a just few yards ahead of the excited dogs until she eventually tired. At this point, amazingly she dived underwater, travelling out of sight right across the total expanse of open water to join up with two of her frightened and unmistakably traumatized brood (I presume under water not to risk leading the dogs back to her brood). Even after this lengthy, sustained attack by the dogs I saw no evidence that any of her brood had perished; the most likely outcome was that they had dispersed to all corners of the reserve, though I never did see all eight juveniles together again. Six young plus the adult female were noted on 19th July, all having no problem at all flying round and just beyond the bunded area, with four juveniles together and no sign of the female on 25th July. On 26th July, six juveniles were repeatedly flushed by dogs, sometimes flying very high over the reserve and well out over the estuary though always keeping a tight grouping and returning to the quietest part of the reserve as the dogs moved around. Throughout this disturbance there was no sign of the adult female, who by now was probably flightless and understandably focusing on her own survival. An update for the 29th July showed all eight juveniles alive and well.

To go back to the question of proving the parentage of this brood beyond any reasonable doubt to enable the record to be accepted as a first breeding record for the county, the fact that no male Gadwall was seen around this brood is to be expected. It is considered normal for the male to have nothing at all to do with the ducklings’ rearing or the female’s well being. I see nothing in any of the photos of the young (other than the accepted individual Gadwall plumage variation) to suggest Brewer’s Duck. The adult female shows a good amount of chestnut to the wing coverts (not all females do) and each one of her ducklings shows varying amounts of chestnut on the coverts. My only question in regards to the parentage of the ducklings would be in the structure of the head and bill, which may well reflect a lack of experience with the range of structural variation in juvenile Gadwall on my part, but to me all the full grown ducklings show a more Mallard-shaped head profile, lacking the steeper forehead and noticeably thinner bill of typical Gadwall. It could well be a perfectly normal juvenile Gadwall trait or possibly all of the ducklings were males. To me the adult male Gadwall head profile always appears more Mallard-like and heavier than the delicate female head profile, bearing in mind that the perception of a duck’s head profile can be extremely observer subjective and dependant to a large degree on the individual cuck’s attitude, nevertheless I still consider the observation worthy of mention. All of surviving juveniles show a generous amount of white in the speculum, certainly more than is shown in most textbook illustrations depicting the juvenile wing. The only other differences noted in the individual appearance of the ducklings, apart from a slight size variation, being the amount of chestnut on the wing coverts and some dark spotting to the lower mandible, shown by some, presumably females, though I’m not sure if this is considered to be a reliable indicator of gender at this age. When the brood eventually disperses and winter visitors start to arrive, it will not be as easy to comment on any future plumage stages, or developing characteristics without that certainty of knowingly observing members of this particular home grown family group.

Colin Selway