By Suhel Quader and Uttara Mendiratta from the MigrantWatch team

In 2010 we continued the Pied Cuckoo Campaign, an effort started in 2009 to gather data that would help understand the association between the southwest monsoon and arrival of the migratory population of the Pied Cuckoo to the Indian subcontinent from Africa.

Many people believe that the arrival of the Pied Cuckoo precedes the onset of the monsoon by 1–2 weeks. Is this indeed the case? The Pied Cuckoo campaign was started precisely to answer this question.

Mousumi Dutta, a MigrantWatch participant who works for the Indian Meteorological Department presented an analysis of the 2010 Pied Cuckoo data earlier on this blog; and a year and a half ago, we summarised the findings from the first season of the Campaign.

In this post, we compare the 2009 and 2010 sightings of this species in relation to the onset of the monsoon in these years. (For details of how we filtered and analysed the data, see Details, below.)

The monsoon in 2009 and 2010
The ‘normal’ onset of monsoon on the Kerala coast is 1 June. In 2009, the monsoon arrived 9 days earlier than this; and in 2010, it arrived 1 day earlier than normal. After the onset, cyclones disrupted the normal progress of the monsoon in both years. In 2009, a cyclone in the Bay of Bengal slowed the monsoon’s progress despite an early start across southernmost India, West Bengal, and the northeastern States. In the beginning of June 2010, cyclone ‘Phet’ in the Arabian Sea brought rain to parts of western India, but delayed the northward progress of the southwest monsoon.

Pied Cuckoo sightings in 2009 and 2010
Between 1 May and 15 July, the number of sightings of the Pied Cuckoo reported to the MigrantWatch database was 28 in 2009 and 25 in 2010.

Despite the earlier arrival of the monsoon in 2009 than in 2010, the first sightings of the Pied Cuckoo were on similar dates in both years (17 May and 18 May from Kolkata and Alibag respectively in 2009; and 16 and 17 May from Bhubaneswar and Jalpaiguri respectively in 2010).

First sightings are shown in relation to monsoon arrival in the maps below. These maps show ‘isolines’ (connecting locations with similar monsoon arrival dates) that depict the approximate advance of the monsoon in the two years (simplified and redrawn from maps issued by the Indian Meteorological Department). The coloured points show the locations and first sighting dates of Pied Cuckoo reports submitted to the MigrantWatch database. Twelve of the points have numbers beside them – these are locations for which first sighting dates are available for both 2009 and 2010. These points are: 1-Ahmedabad, 2-Bhubaneswar, 3-Chandigarh, 4-Jaipur, 5-Jalpaiguri, 6-Mount Abu, 7-Mumbai, 8-Nagpur, 9-New Delhi, 10-Panchkula, 11-Pilerne (Goa), 12-Pune.

To examine more closely the relationship between monsoon arrival date and Pied Cuckoo sightings, we show a scatterplot of the relationship below. Each point represents a location for which we have both the first sighting date and the approximate monsoon arrival date (taken from the IMD maps). If a point falls on the solid diagonal line it would mean that Pied Cuckoos were sighted on the date of monsoon arrival at that location; if on the dashed line, 5 days before; and if on the dotted line, 30 days before the monsoon arrived. So, if Pied Cuckoos are good at anticipating the monsoon, we would expect the points to march from the lower left to the top right of the plot. If you click on this graph you will see a larger version, on which twelve points are numbered just as in the map above.

The maps and scatterplots suggest that the degree to which the arrival of Pied Cuckoos precedes the monsoon is highly variable. Sometimes the earliest sighting is no more than a few days before the monsoon arrives, and sometimes the earliest sighting precedes the monsoon by more than 30 days. So Pied Cuckoos often do arrive at a location before the monsoon does, but the degree to which they do so varies with location and year.

Let’s look at the 12 sites for which we have first sighting dates in both years. The monsoon arrival dates and the first sighting dates from these sites are shown below, with points from 2009 in black and those from 2010 in red. The points for a single location are joined by a line to show how the arrival of the monsoon changed in the two years; and consequently how Pied Cuckoo sighting dates changed. Again, the numbers correspond to the numbered locations on the maps, above.

To help interpret this graph, the two asterisks joined by a dashed line show what one would expect for a hypothetical situation in which the Pied Cuckoos adjusted their arrival perfectly with changes in monsoon arrival — we would see a line pointing upwards from left to right. (For a more comprehensive pictorial and verbal description for how to examine this graph, click here.)

What do we see in the actual data? Out of the 12 locations for which we have information, 6 lines point upward and 6 downward, which means that cuckoos appear to show no consistent differences in arrival date with differences in monsoon onset!

Does this mean we have to revise our dearly-held beliefs about the impeccable timing of Pied Cuckoo migration? Let us know your thoughts in the comments.

Acknowledgements: The most important people in creating this report, as in all of MigrantWatch, are the participants who contributed their time and effort in reporting sightings to the database. This link lists all those whose sightings of Pied Cuckoos between May 2009 and April 2011 were uploaded to MigrantWatch. MO Anand helped in various ways with the preparation of this report. We are grateful for comments from Umesh Srinivasan and R. Jayapal.