It is nothing to do with birds or frogs but there is a lot of it about this year.
Cuckoo spit is the white froth found on plants in early summer. It contains and protects the grub or nymph of the Common Froghopper . They are called Froghoppers because from above they appear frog-like, and they are able to hop significant distances when disturbed.
Problems Caused by Cuckoo Spit
The immature light green grub can be seen in this froth on the back of this leaf which is now distorted and curling round the wound.
Treatments For Cuckoo Spit
Under the dark blemishes are two Cuckoo spit grubs that have sucked the sap and deformed the leaf.
How is the Froth Made ‘The froth is created by the insect excreting a fluid, the result of excess undigested plant sap, exuding through the anus. This sap, as it is excreted, mixes with a secretion from the abdominal glands. Air bubbles are introduced through a special valve on the abdomen which acts like a bellows, and contact with the air causes the liquid to ferment, forming the froth (or spittle).’ from h2g2 It also tastes like it was made in this way so wash your hands!
See also Common Garden Pests
Read more on gardenerstips.co.uk