This week I completed a task I’ve been putting off for a while – totalling up the results for the 2014 swallow season at Cardiff Riding School. This season will go down in personal history for two reasons. Firstly, although it was my 9th season studying this population it was my first as an MPhil student (all be it part time!). Secondly, it was the worst year for predation
Magpies were the bane of this year. A pair usually nests in a large sycamore next to one of the yards but they generally keep themselves to themselves. Early signs were that it was not going to be the case this year and nor would the mice. The sight of nest lining spread over the floor of a stable is the image that comes foremost when I think of the summer just gone. What affect did this have on the fecundity of this population? The headline statistics for 2014:
- 14 pairs
- 133 eggs laid
- 85 chicks hatched
- 56 young fledged
This is not a great result from a population that has fledged in excess of 120 fledglings in a season. But is predation the worst thing that call befall this population? The best year with which to compare 2014 with is last year. 2013 was similar to 2014 in that fewer pairs bred at the stables than average (15 in 2013, five year average 18 pairs) and both were fairly hot and dry. The key difference; predation was at background levels in 2013 (<2 breeding attempts per annum); the major cause of mortality was down to environmental factors. Here are the same headline statistics for 2013:
- 15 pairs
- 105 eggs laid
- 80 chicks hatched
- 59 young fledged
2013 was the second worst season on record for this population. The third most was 2006; again a year with background predation and like 2013, it was a very dry summer.
- 22 pairs
- 130eggs laid
- 99 chicks hatched
- 69 young fledged
Although this is a very rough and ready look it does illustrate that while it is easy to take a vitriolic view of the magpies for this year’s level of predation, the weather can enact a far greater toll on these birds. Despite a heavy predation rate (c.40% of breeding attempts), 14 pairs of swallows still managed to fledge just three young shy than that of 15 pairs in 2013 and 13 fewer than 22 pairs in 2006; both years which were hot and dry in which nestlings died of dehydration and food was scarce.
Without the magpies’ attention it’s probable this year’s 14 pairs would have produced more young but I’m not sure how many more. The higher number of eggs in 2014 compared to 2013 is the result of replacement clutches after predation; some pairs laid replacements of replacements. Insects too, seemed thin on the ground this year (at least anecdotally) and several of the broods were small for their age when they were ringed. Unfavourable weather conditions can last the entire season, and there is no escape from those. Predation, however, came in waves. These coincided with the magpies having young of their own. Swallows breeding before, between or after these wave were usually successful in rearing at least one chick but there were some sickly looking broods.
It was also interesting to see how different pairs coped with the attentions of the magpies. Some pairs stayed, doggedly relaying in the same nest time and time again. Some moved home, building a new or using an exsiting nest a stable or two away from where they started. Some pairs succeeded. Other pairs didn’t. Some just gave up.
But how do birds cope with changing or unfavourable weather conditions? Unravelling the impacts of weather on seasonal fecundity (the number of fledglings produced per pair in a season) is the focus of my MPhil. I’ll be using the last nine years’ worth of data, combined with next year’s to look at how daily changes in weather affect the different stages in the nesting cycle, from clutch initiation through to the number of birds fledge, as well as some of the mechanism parent and offspring use to compensate for life in an uncertain environment. This year may have not contributed as much data as I would have hoped but it did contribute some novel data, which I’ll blog about at a later date.
The riding school swallows will probably be in South Africa by now. Some will succumb to predators or the weather. But others will make it back next year. This season might have been a bit of a let-down, but there is always next year!