Summarising MigrantWatch data 2007-2011

This entry was posted Thursday, 13 October, 2011 at 5:23 pm

By Suhel Quader

Up to 11 Oct 2011, 9900 sightings had been reported to the MigrantWatch database. This information is meant to be used, so at MigrantWatch we are gathering our thoughts to put together a comprehensive summary of the data that the project has gathered since it started in July 2007. When it is ready, we’ll send the report to all MigrantWatch participants, with grateful thanks.

In preparing the report, we have been thinking about how best to show a picture of arrival dates of different species (for example, see a crude attempt here). In this post, I’d like to show you some first attempts, and ask for your feedback on improvements and additions.

The picture below shows sighting information from all years of the project (until 11 October 2011) and across all India, for the 23 winter migrants for which there are at least 100 records in the database. Each short vertical line depicts a sighting. The dates are arranged from July to July, corresponding with a typical migration season (rather than a calendar year). To help see the patterns of first sightings, it’s better not to focus on the very earliest sighting dates and instead look at when the main concentration of first sightings is. The yellow boxes in the picture depict when the earliest 1% to 5% of sightings happened. (This is just like the percentile rankings that students get on some exams.) The width of the yellow boxes indicates how spread out the arrival is for each species: wide boxes mean that the earliest sightings are spread out considerably in time. In the picture below, species are arranged in order of the earliest 1% of arrivals (ie, the left edge of the yellow boxes).

What does this summary tell us? We see that shorebirds form a large proportion of early arriving species, which confirms what I’m sure many of us have casually noticed. Ducks tend to turn up a bit later, as also wagtails and warblers. Although only a few MigrantWatch participants keep an eye out for departing migrants (and so information from the end of the season is thin), it seems as though most migrants have left the country by the beginning of May. The departure appears particularly abrupt for species like Rosy Starling, Common Sandpiper, Greenish Warbler and Blyth’s Reed-warbler. Individuals of other species hang on for a while, and several shorebirds have been seen well into June, possibly birds who have decided to skip their return flight altogether and instead over-summer in India.

Of course, this all-India picture obscures variation in migration dates in different parts of the country: migrants don’t suddenly appear everywhere at the same time! To try to look for geographical differences, we have grouped sightings into five regions (excluding the Andaman and Nicobar Islands, and Lakshadweep): North-East, North-West, North, Central, and South. Region-wise sightings for two migrants are illustrated below.

Rosy Starlings first appear in the North-West, as might be expected from their breeding grounds (Eastern Europe to Kazakhstan). Their early initial arrival, in beginning July, is followed by a gradual and unhurried trickle-down to other parts of the country (except to the North-East, where the species does not occur). It takes nearly two months for these birds to reach the South. In contrast, departure dates are more uniform, all around end-April, presumably as the birds hurry back en-masse to their breeding grounds.

In contrast, White Wagtails put in a much later first appearance, starting in the North-East and North (apart from a couple of oddly early sightings in the South). They then appear relatively rapidly in North-West, Central and South — in roughly the opposite East-West sequence as Rosy Starlings. Departure of White Wagtails is less clear, because of the low number of observations.

Do these visual depictions of MigrantWatch information make sense? Are they useful? How can we improve them? Are they too simple? Too complicated? Please do let us know in the comments below; we would like to summarise the data in a manner that is of most use and interest to you! (Of course, as always, you can play around with the data yourself — just login and click on view data/maps to download the entire database.)

And please don’t forget to report your migrant sightings. MigrantWatch is no more and no less than the sum of contributions of all participants; to paint a clear picture of bird migration, the more reports (of first, and last, and general sightings) the better!

1 Comment to Summarising MigrantWatch data 2007-2011

  1. Patrick David says:

    October 16th, 2011 at 5:00 pm

    The second set of graphs on the arrival and departure of the Rosy Starling and White Wagtail is very informative. I hope this is part of larger global effort to track bird movement and the information is used to conserve key habitats.

Leave a comment