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Irregular migration

Irregular migration

Ranger Steve’s Nature Niche

By Ranger Steve Mueller

Male white-winged crossbill. Photo by Garth McElroy/VIREO via Audubon.org

Various birds, mammals, insects, and even plants migrate. Irregular migration is one type of migration that can be observed. Rough-legged Hawks, Snowy Owls, White-winged and Red Crossbill might show up as a result of irregular migration in winter. I refer to their eruptive appearance in our region as a type of irregular migration. 

When food supplies are scarce in northern wintering grounds, they move south in search of food and survival. When lemmings in the arctic that Snowy Owl depend on are low, it requires them to migrate or starve. Many predators starve with young being most vulnerable. During years of low food supply, these predators frequently have small broods or might lay no eggs. Their efforts concentrate on their own precarious survival. 

Prey populations fluctuate for many reasons and they control predator population numbers. It was thought that predators control prey species numbers but evidence indicates prey numbers often control predator numbers. 

Insect numbers control bird populations for swallows, flycatchers, phoebes, nuthatches, and many other species. Insectivorous birds feed heavily on insects with minimal effect on eliminating insect populations. If you have been in the far north, you might have experienced trillions of mosquitoes. Exposed areas of skin like your face and hands make it a nearly unbearable experience because of biting insects. 

At times I wiped my face with one hand and then the other to remove mosquitoes and then wiped each hand. Immediately I needed to wipe my face and hands again because they were already covered with insects. Head nets and effective insect repellent were necessary. A cold spell after bird arrival in spring reduces insects and can cause tens of thousands of birds to starve. Swallows and longspurs sometimes experience massive death. 

During the breeding season, birds and insect eating mammals gorge themselves on food that appears endless. Some species, like caribou, find it essential to migrate to cold areas near the arctic ocean to get reprieve from insects that dangerously reduce their blood. Breeding and calving grounds near the cold ocean are important to aid caribou health and survival. Migration from inland to coastal areas is a normal and important migration. 

Some species like Monarchs have unique migrations while other butterflies may have irregular migrations. Painted Ladies make two-way migrations that are regular but many have one-way eruptive movements. Several butterflies have one-way immigrations when they their leave normal geographic breeding ranges. Populations of Orange Sulphurs become abundant and move north in late summer as do Variegated Fritillaries and Little Yellows. As fall conditions arrive, they do not return to sites of origin. They live and die here.

Duckweed is a small plant that floats on the surface of ponds and becomes so numerous it hides the visible water it covers. When fall cold takes over, duckweed sinks to the bottom of ponds and will resurface in spring when warming sunlight allows it to increase photosynthesis that produces oxygen and buoyancy. It has its own annual vertical migration. A different migration process occurred for plants during the advance and retreat of glaciers. Individual trees, shrubs, and herbaceous plants did not move. During the slow advance of ice, seeds spread by adult plants allowed new individuals to establish in front of the ice front. As glaciers advanced south or retreated north, plant seeds moved species hundreds of miles to where they colonized newly exposed habitat. 

Unique irregular eruptive migration nature niches occur for White-winged and Red Crossbills. These northern conifer inhabiting bird species feed on cone seeds. The sharp tips on the upper and lower bills overlap. They partially open the beak to insert the tips between spruce or pinecone scales and then close the bill to push scales apart. They reach between the scales to retrieve edible seeds with their tongue. Trees regularly have high cone production years followed by years with few. To survive, the crossbills must move to areas where seeds are available. During lean years in the north, we are treated with their irregular eruptive presence here. 

This only breeches the beginning of migration phenomena. Amphibians migrate and more await future sharing.

Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at odybrook@chartermi.net – Ody Brook Nature Sanctuary, 13010 Northland Dr. Cedar Springs, MI 49319 or call 616-696-1753.

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Ray Winnie
Kent County Credit Union


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