13/04/14 - Climbing high

Last week we paid the first visit of the year to the local heronry to ascertain how the breeding season was going so far. A good number of adults were present, and many empty egg shells were located beneath the trees. Calls of young birds could be heard and so the spikes were donned ready to scale the dizzy heights of the larch and pine trees, however upon reaching the top most of the young were newly hatched with just one bird being of a ring-able size. This isn’t what we were expecting to find compared with previous years, with the milder spring and higher temperatures a number of species started breeding earlier than last year and so we were expecting the chicks to be much further on. Herons are one of the species known to be early nesters with some records as early as February for birds on eggs, however March is the usual time for herons to start laying with the incubation period c27 days. We shall now return in c10-12 days to hopefully ring and darvic a number of the young.

Herons have bred in the heronry here for at least 40 years, along with Little Egrets in 2010, which went on to produce two young. These two species are known to nest alongside each other, however the latter is a much later breeder. Herons are rather sociable birds, breeding in close quarters, a single tree has been known to hold as many as 10 nests! Herons are known for nesting in long established heronries with data going back as far as 1928 when the first survey by the BTO took place, since then heronries have grown in number with the biggest heronry being on an RSPB reserve in Kent with over 150 nests, down from 200 in previous years.

Last year we managed to ring and darvic 22 birds over the course of four days throughout April and May, with the first visit being on the 16^th April and the last being on the 17^th May. Out of the 22 birds ringed last year we’ve had two sightings, which is much in line with the national reporting rate of 11%. The first bird was seen briefly locally however the second was seen at Nosterfield Nature Reserve, nr Ripon on the 16^th August, three months after being ringed in our heronry.

From the ringing recoveries that we’ve collected over the years in the valley we’ve found out that our young herons undergo a rapid post breeding dispersal to the north, with recoveries 50-60km north of the LDV by July/August. Given that heron numbers also peak in the valley at this time would suggest that these birds include at least some that have dispersed from other sites presumably further south, and are not just the local breeding population as previously thought. 

Foreign recoveries from birds ringed in Britain only make up 3% of the total with the most being in France, and prior to 1940 the proportion of recoveries in foreign countries has declined from 12% to 2% in the 1990’s but the reasons remain unknown as to why. Despite herons being relatively common and widespread there are gaps in our knowledge of heron movements, including the movement during the year of adult birds and the apparent population shift towards coastal areas in the winter as suggested by counts. The national data does however suggest that general dispersal movements occur over the first five months of life, later returning to the natal site to breed, which fits with what we suspected was happening in the valley.