. The Canadian field-naturalist. 1979 URQUHART AND URQUHART: MONARCH BUTTERFLY 43 Overnight Roosting Clusters During the autumnal migration the Monarch Butterflies cluster on trees of various species along the migratory routes to remain during the night period (Urquhart 1960). Of the species of deciduous trees chosen Red Maple {Acer rub- rum). Sugar Maple {Acer saccharwn), Mani- toba Maple (Acer negundo), and willow {Salix sp.) were most common. Pine (Pinus sp.) and spruce {Picea sp.) were most frequently chosen since it was possible for the butterflies to obtain a secure hold with the sickle-
. The Canadian field-naturalist. 1979 URQUHART AND URQUHART: MONARCH BUTTERFLY 43 Overnight Roosting Clusters During the autumnal migration the Monarch Butterflies cluster on trees of various species along the migratory routes to remain during the night period (Urquhart 1960). Of the species of deciduous trees chosen Red Maple {Acer rub- rum). Sugar Maple {Acer saccharwn), Mani- toba Maple (Acer negundo), and willow {Salix sp.) were most common. Pine (Pinus sp.) and spruce {Picea sp.) were most frequently chosen since it was possible for the butterflies to obtain a secure hold with the sickle-shaped tarsal claws (Urquhart 1960). During periods of calm weather individuals of a cluster are widely spaced (Figure 1). During stormy conditions with strong wmds the migrants cluster closer together and there are fewer clusters on the trees. The massing together under conditions of strong winds has the distinct advantage of causing the weighted branch of the tree to sway in the wind rather than being whipped about, which would dislodge the migrants. The leeward side is always chosen as a protection against the wind. Since migrants tend to cluster on certain trees year after year, it has been thought that perhaps an odor of some kind may have been left the previous year thus acting as a guide for future generations of migrants. Numerous observa- tions in many localities, however, have not produced evidence to substantiate such a sug- gestion. For example, when a particular group of roosting trees was removed, the migrants selected another group that had not been used previously. As has been demonstrated, the direction of migration, the presence of suitable. Figure 1. Overnight roosting Monarch Butterfhes during the autumnal migration showing the widely scattered nature of the Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly
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