The trends in nest success and chick productivity over 25 years
Between 1994 and 2018, over 6000 breeding attempts have been monitored with known outcomes, and these data are summarised in the graph below:
It is immediately obvious that, since 1994, there has been very little change during the period. The fluctuation in nest success (the grey line), which has ranged between 48% and 72%, is probably weather related, and the corresponding trend line shows that there has been a very slight long-term increase.
The fledged brood size of successful nests (orange line) shows a very slight downward trend. This offsets the slight increase in nest success resulting in a near static overall productivity of around 0.9 chicks per breeding attempt (blue line), with the fluctuations also explained by weather.
There has been a claim on a recent television programme that kites are now producing two or three young per pair as a result of supplementary meat provided by feeding stations, and that this explains the unexpected rapid increase in the population growth recorded since the early 1990’s, but the data show that brood size has not increased. Feeding stations could, however, have an effect on adult and 1st winter survival, especially during periods of snow cover when food is difficult to obtain in the wider countryside, and it is changes in survival that are most likely to have led to the population changes witnessed.
The early spring of 2018 will be remembered for an unseasonably cold snap at the end of February, “the Beast from the East” and this was followed by more cold weather in late March and into April. Bad weather early in the season can affect the breeding success of some species due to females perhaps being in poorer condition which in turn means they lay fewer eggs, so how did the cold weather affect kites during the later breeding season?
Well in the end, the summer turned out to be one of the warmest and driest on record, with very few heavy rain events or high winds, and altogether 171 breeding attempts were monitored. The results indicate that the cold snap early on actually had little or no lingering impacts;
% Nests Successful
Fledged per Nest
Fledged per Successful Nest
The number of young fledged per breeding attempt (0.94) is slightly higher than average, and the percentage of breeding attempts that were successful (71.9%) is the highest ever recorded, with the average from 1994 to 2017 being 62.5%.
With Welsh kites, lower nest survival is inevitably associated with bad weather, and the higher figure for 2018 is almost certainly due to the exceptionally warm dry weather during the 2018 main breeding season, resulting in ideal foraging conditions and less risk of exposure to wet and windy weather which is known to increase nestling mortality.
The slightly higher nest productivity in 2018 is due to a higher proportion of successful nests, rather than larger broods, as the mean productivity per successful pair (1.31) is lower than the long-term average of 1.41. It is predicted that brood size will begin to show a decrease as the population approaches its carrying capacity and competition for food limits productivity.