Trap-lining or trap line refers to a feeding strategy amongst certain families of bird

Bird

Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...

s, notably hummingbird

Hummingbird

Hummingbirds are birds that comprise the family Trochilidae. They are among the smallest of birds, most species measuring in the 7.5–13 cm range. Indeed, the smallest extant bird species is a hummingbird, the 5-cm Bee Hummingbird. They can hover in mid-air by rapidly flapping their wings...

s and woodpecker

Woodpecker

Woodpeckers are near passerine birds of the order Piciformes. They are one subfamily in the family Picidae, which also includes the piculets and wrynecks. They are found worldwide and include about 180 species....

s. These birds visit a circuit of specific plants, trees, or other feeding sites, much as trappers check their lines of traps.

In the case of sapsucker

Sapsucker

The Sapsuckers form the genus Sphyrapicus within the woodpecker family Picidae. All are found in North America.As their name implies, sapsuckers feed primarily on the sap of trees, moving among different tree and shrub species on a seasonal basis...

s, entrapped insect

Insect

Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

s in sap are an essential source of food

Food

Food is any substance consumed to provide nutritional support for the body. It is usually of plant or animal origin, and contains essential nutrients, such as carbohydrates, fats, proteins, vitamins, or minerals...

. For hummingbirds the relationship is to the seasonally flowering species providing nectar.