Stonechat pairs in winter territories

Species like the Robin keep territories during the winter, with males and female individuals defending their own separate territories, while in the summer, territories are defended as a pair. There are some exceptions in which pairs defend winter territories, often in different areas that summer ones. One of these species is the Stonechat. In their wintering grounds on the coast and around the Humber estuary, it is frequent to encounter Stonechat pairs in the same area, feeding near each other and following each other, with no aggression (top shot, a pair at Alkborough, Lincolnshire, 11/12/2017). Of course, winter territory defence has nothing to do with reproduction, and often happens in areas that don't even hold breeding pairs in the summer. This unusual behaviour must offer some benefit to persist in the population, maybe allowing individuals to increase their chances of survival to predation or competition and to secure sufficient food resources.
A study of wintering Stonechats in the Negev desert (Israel), shed some light onto the behaviour. A total of 89 individuals were colour banded and followed through two wintering seasons. Their territorial behaviour was noted. The most surprising result of the study is that wintering Stonechat pairs are very fluid. Pairs often form after arrival to winter areas, and individuals rarely stay together all winter, instead pairs change frequently, with an average of just 4 weeks together per pair. In fact, none of the studied pairs stayed together all winter long and individuals arrived in the area and left at different times. This rules out that breeding pairs migrate together or settle on the same winter territory. Up to a third of individual are unpaired at any given time, but all associations are always between a male and a female. There were some aggressive interactions between members of a pair, which were commonly initiated by the male. Both males and females engaged in territory defence, with males more aggressive against intruders than females.

Male Stonechat at Kilnsea, 10/12/18.

Why pair in the winter?
If pairs are not stable, don't migrate together, or don't form in preparation for the winter season, what is the point in defending territories as a pair? Males and female stonechats feed on the same prey, therefore there may be a cost on sharing a territory during the winter, when food resources may be very scarce. A possible benefit outweighing the cost may be joint territory defence: two individuals are better spotting and chasing away conspecific intruders or detecting predators and defending the territory against individuals of other species with similar food resources. In this study, individuals spent a lot of time defending the territory against Mourning Wheatears, but there was no mention of differences when pairs or individual Stonechats were involved.

Another pair at Kilnsea (13/1/2020)

Why male/female pairs?
These results don't clarify why associations are not between two males or two females defending a territory. The authors speculate that as Stonechats are sexually dimorphic, even in the winter, there may be constraints that prevent two males from living peacefully in the same territory, and the increased male aggressiveness may mean two females may be unable to defend a territory against a male/female pair.

More information
Gwinner, E., Rödl, T. and Schwabl, H. Pair Territoriality of Wintering Stonechats: Behaviour, Function and Hormones. Behav. Ecol. Sociobiol. 34, 321–327 (1994).

Moorhen fight

As I arrived by the pond this morning, two moorhens seemed to be having a dispute, squeaking and chasing. Then, another two individuals took it more seriously and fought in the water, feet to each others chest, trying to push each other underwater, and pecking each others bills furiously, while flapping their wings for balance. It was uncomfortable viewing and I filmed half a minute. I wish I had carried on, as the fight ended shortly after, both moorhens exhausted but unharmed, and, after a quick chase, the resident moorhen came back to its territory by the lake island. Coots get the press for their aggressiveness, but Moorhens are not much behind.

Territorial Great Crested Grebes

The breeding season of the Great Crested Grebes is approaching. I have never seen their spectacular courtship so I thought I would pay them a visit in a local park where they breed regularly. I had almost given up on finding them when, from the bridge, I located a winter plumage individual at the far end of the lake. When I got there, I saw that there were two winter plumage individuals, and then an individual with full breeding plumage emerged from the water quite close to me (above). Breeding plumage individuals have a long dark crest and orange and black 'tippets' on the sides on the head, while these are lacking in the winter plumage. I sat down and watched the interactions between the breeding plumage and one of the winter plumage ones. The breeding plumage individual watched the other one, and approached slowly, nonchalantly, occasionally preening its back, or shaking its head, but the winter plumage one appeared to take offence, and swam in the aggressive posture of this species, with head low and stretched forward over the water, and then started to fly, while paddling on the water, towards the breeding plumage one and kept it away. Occasionally it would try to approach diving (see video below). This behaviour went on for quite a while. Both winter plumage individuals appreared tolerant to each other and at some point they stayed close together, so I wonder if I they were displaying early territorial behaviour, and the breeding plumage individual was trying to find a new mate.
 Great Crested Grebes show some variability in their head moult and in February a range of plumages from full winter to full breeding can be seen, so the three individuals were likely adults in different stages of their moult.

Changes in the plumage of the head. The plumage types of individual grebes in relation to date are

given (males = filled and females = open symbols). Dots refer to scores based on examining photographs of grebe-heads, whereas the triangles indicate the results of examining museum-skins. (from Piersma, 1988)

Winter plumage individual on agressive posture,  a few crest feathers and the dark tips of the tippets have already emerged.

 Breeding plumage individual preening its back feathers

It appeared to try and grab the attention from the winter plumage individual, and... 

...got very close. 

But it got the wrong sort of attention, as the winter plumage individual threatened it

the summer plumage individual flies away. 

The two winter plumaged individuals close to each other, watched by the breeding plumage one

A couple of short videos on the interactions between the two individuals.

More information

Theunis Piersma (1988). The annual molt cycle of Great Crested Grebes. Ardea, 76, 82-95

The Common Pochard

Common Pochards, Aythya ferina, are common wintering ducks in the UK. I find intriguing that most of the birds in my local park are males. However, this is the case for the whole UK winter population, with males outnumbering females in a 2:1 ratio. In sites where supplementary food is given this biased sex ratio can be much more extreme (5:1). Females appear to migrate further south than males, and the reason for this could be that males are much more aggressive towards females than they are to other males. Confronted with this intraspecific competition, females move on to areas where there are fewer males. Males might benefit from shorter migration distances by coming back earlier than females to establish breeding territories.

  Pochards, unlike other birds, seem to have the same pattern of activity between day and night, they are cathemeral. Bouts of feeding are followed by bouts of sleeping day and night, and they are often found resting in flocks during the day (they still sleep about 50% of the time).

  In the winter, these diving ducks feed mostly on seeds and fuiting bodies of aquatic plants (Chara, Nitella, and Potamogeton being a large fraction of their diet). Their distribution is limited by their diet, as they require large bodies of water with good population of aquatic plants. They feed by diving and up-ending, and, they probably rely on the sense of touch to feed, which may explain they do not need light to feed.

One of the few photos I have got of a female Pochard, in the company of Tufted ducks.