THE BIRDS OF NORTH KENT. 55 



appear as colours, but simply as light and shadow. A party of a 

 dozen or more of these birds which I was watching one winter's 

 day swimming in the rough water at the edge of the tide was 

 most difficult to distinguish, owing to the fact that the light and 

 dark patches of plumage corresponded almost exactly with the 

 bright lights and deep shadows of the broken water. The further 

 fact that I was once deceived into supposing that some white 

 pieces of stone lying on the bank were Sheld-Ducks illustrates 

 another way in which this striking plumage is an actual aid to 

 concealment ; for, if the bird happens to be standing with a dark 

 coloured background behind it, the dark patches will blend, at 

 certain distances and under certain conditions of light, with the 

 background, and all that will be visible will be the light or white 

 patches of plumage, which bear absolutely no resemblance at all 

 to the shape of the bird. The contrary effect takes place when 

 the conditions are reversed. The light patches will blend with 

 the light background, and the dark portions of the plumage then 

 show up in the same way, as something that does not look at all 

 like a bird. This gives a working theory as to protective colora- 

 tion which may be applied to many more birds than the Sheld- 

 Duck, and it is one worth while keeping in mind by the student 

 of bionomics. It is especially interesting because it points to the 

 possibility of markings which at first sight might be considered as 

 militating against a species, being really a protection to it under 

 several distinct sets of conditions. But, notwithstanding all this, 

 a Sheld-Duck standing in the centre of a meadow green with the 

 spring growth of grass is a sufficiently striking object, and one 

 which may be often seen on these marshes from the end of March 

 onwards through the spring and summer while their nesting 

 operations are in progress. In the raised banks formed by 

 accumulations of bottomings from the ditches there are in many 

 places rabbit-warrens of considerable size, and the burrows 

 are what might be called the natural site in the district for 

 these birds to choose for their nursery. During my first day 

 spent on these marshes a shepherd informed me that Bar-geese 

 nested in the rabbit-burrows, and, although I have not myself 

 found any nests in them, several of my shepherd friends often do 

 so ; in fact, they generally take a clutch or two of eggs every 

 year for some of the neighbouring farmers, who hatch them out 



