Observations of sand martins on the ground

Photo: Sand Martin on one of the yacht club
 slipways.

Throughout the first half of June I visited the yacht club on the south side of Pennington Flash on average about 3 times per day, early morning, midday and late evening. On the morning of 10th June I noticed up to 50 sand martins landing on the gravelly foreshore of the yacht club apparently feeding on invertebrates. Over the next few days I occasionally saw this behaviour repeated, always in the morning and always in dull rainy conditions.

Early morning on 18th June during a period of moderately heavy rain there were around 300 sand martins swirling around low over the water in front of the yacht club apparently feeding on flying insects. Every so often a large part of the flock would land en masse on the yacht club foreshore and appeared to be feeding on something on the ground while the remainder continued to feed over the water. I accessed the yacht club for a closer look and through the telescope I could clearly see that they were actually eating grit. On a couple of occasions I did see them eat what looked like invertebrates but that certainly wasn't the main reason why they were landing. I watched them behave like this for about 30 minutes, flying around feeding and then periodically dropping to the ground to eat grit. I had a walk over to where they were landing and couldn't find a single invertebrate on the ground.

Photo: Substrate on which
the sand martins were landing.

Sand martins are known to eat grit in order to obtain enough calcium for egg laying. According to Turner (1982), the maximum amount of time they can spend collecting calcium is about 30 minutes per day without impacting on the energy needed for egg laying.

The interesting thing is, it clearly wasn't just the females which were landing because on a few occasions I saw males attempt to copulate with the females while they were on the ground. There was no gentle courtship  ritual or display, the males just jumped on the females from behind, but every time the females shook them off. Opportunistic mating is well known amongst sand martins. Unlike swallows, both sand martin parents share the incubation of the eggs and the feeding of the young, but even so the males if given the opportunity will attempt to mate with other females who are in their fertile period. What better indication of a female in her fertile period than a bird which is landed on the ground and collecting calcium for the production of eggs? Given the time of year I guess that this must be the start of second broods.

What I can't explain is why I only saw the birds behave in this way on cloudy, rainy mornings.

Very occasionally they were accompanied by a handful of house martins but usually it was exclusively sand martins.

The main sources of information for this post were:

"A Handbook to the swallows and martins of the world", Turner & Rose, Poyser (1989)

"Timing of laying by swallows and sand martins", Turner, Journal of Animal Ecology (1982)

Photo: The foreshore of the yacht club
where the sand martins were landing.