When to go seabird watching in Cornwall

Cornwall is, without a doubt, one of the top, if not the top, seabird watching sites in the UK. If, like me, you are prone to an occasional bit of seabird watching but Cornwall is a long, in my case  4 hour, drive getting the timing right of when to go is important. A poor day after a 2 am start or overnight stay to be on site by 6 am is, to put it mildly, very dispiriting! The perceived wisdom is that mid-July through to the end of September is the best time of year and that ideal conditions are moderate south-westerly winds. Hence the south-west facing headlands of Porthgwarra and Pendeen are widely recognised as the best spots in Cornwall.

To try and be a bit more scientific about this I decided to collect the daily reports from these two sites and analyse them according to wind strength and direction. The source of my data was my default site for bird news, Rare Bird Alert, and the excellent Cornwall Bird website.  I have tried to remove duplicated data.

 

The rarer seabird reports considered were Cory’s and Great Shearwater and Storm, Sooty, Fea's type and Wilsons Petrel. The wind speed and direction was taken from the met office site.

I have used the wind direction at 07:00 as most reports are from early morning. I have not allowed for wind direction changes during the day as this would require exact timings of reports against met office records.

 

The data was collected over 30 days starting 25^th July 2022.

The results are very stark – if there is not a westerly component to the wind forget it! - 95% of reports were on days when there was a westerly component to the wind. Totalled across the two sites south westerly was the favoured wind direction with 1781 reports.

sum of records from both sites

A westerly component to the wind is hence essential but it should be stated that it is unfortunately not a total guarantee of success as on 15% of days with a westerly component to the wind vector there were no reports from at least one of the two sites.

 

In total over this period Porthgwarra came out on top with 2081 records as opposed to 656 records at Pendeen but it is possible that Porthgwarra is better watched.

Taking the analysis a further level down and looking at the individual sites over the 30 day period still shows the importance of a westerly component to the wind. This clearly shows that the favoured wind direction is north westerly at Pendeen and south westerly at Porthgwarra.  Perhaps this is not so surprising as a  more northerly wind direction will push birds towards Pendeen while a more southerly will do the same for Porthgwarra. At this level of analysis the data sets are, of course, smaller so the results are less statistically significant.

Pendeen

Porthgwarra

Another important variable is how many days the wind was blowing in a certain direction. For example, there were no days with a pure southerly wind over the data collection period so there were obviously no reports associated with this wind direction. In order to check that this does not distort the conclusions I looked at the average number of reports per day for each wind direction. This again confirms the total domination of records on days with a westerly wind component.

Now a strong caveat! There may be observer bias in this data in that local seabird watchers my tend to only go to Pendeen or Porthgwarra when the wind is westerly.

The totals by bird for both sites over this period were dominated by Storm Petrel's  as follows

• 1334 Storm Petrel  
• 642 Great Shearwater
• 558 Cory's Shearwater
• 174 Sooty Shearwater
• 23 Wilsons Petrel
• 6 Fea's type Petrel

Given the rarity status of Wilson's Petrel, the common consensus being that you need to be out at sea on a boat to see them, I found the 23 records during this period to be somewhat surprising but, again, with such small numbers much more data is needed over a longer period to validate this. Similarly the 6 records of Fea's type Petrels  were unexpected.

As with any analysis of this type, more data makes its predictions more reliable. I will hence  continue to collect this data until the end of September and will update this blog accordingly.

Conclusions

The analysis strongly concludes that, for a successful days seabird watching,  a westerly component to the wind vector is essentially at these two sites. While more data is required to improve the statistical significant of this analysis, the correlation between westerly winds and reports is so strong that this is very unlikely to change with more data.

  Footnote – my blogs are posted with sometimes rather imaginative spelling and grammar due to my extreme dyslexia!