Seasonal Changes in Birds: Molting, Migration, and Reproduction

Birds have a busy year ahead of them. Between migrating, breeding, and molting, it’s just one challenge after the next, and timing is critical if they are to get everything done on their annual to-do list.

Each season presents its own opportunities and challenges, with some better suited to specific activities than others. As such, most birds follow a regular and predictable cycle. The endocrine system and its associated hormones act in advance of important events in the annual cycle, coordinated by environmental cues like photoperiod.

In this guide, we’ll explore the role of a bird’s endocrine system in regulating vital events and processes on the avian calendar.

Hormonal Triggers for Seasonal Molting

Molting Process

All birds are forced to replace their feathers as the elements and general wear and tear take their toll on delicate plumage over time. Molting and regrowing feathers is usually a highly coordinated and carefully timed process guided by hormonal changes in the body.

For example, post-breeding molt coincides with a decrease in prolactin, the hormone that stimulates parental care. In domestic geese, increased triiodothyronine levels have also been recorded during new feather growth in both the prebreeding and postbreeding molt.

Timing and Types of Molts

With the exception of very large birds like Albatrosses, most species can undergo at least one complete molt each year, typically over a period of several weeks.

Most bird species have a complete molt in the post-breeding period by shedding and replacing their feathers in a sequence that does not prevent them from flying or keeping themselves warm. This prebasic molt typically occurs before migration.

Many birds perform a second, partial molt before breeding. This pre-alternate or prenuptial molt allows birds to look their best and impress a partner. Some birds (Willow Warblers and Bobolinks) go one step further and perform two complete molts each year.

Willow Warblers and Bobolinks perform two complete molts each year. Bobolink perching on a metal post

The Hormonal Drive Behind Bird Migration

Preparation for Migration

Annual migration is a remarkable adaptation that allows birds to make the most of the seasons by moving between different latitudes and altitudes at different times of the year.

Long-distance migrants often fly thousands of miles one-way when traveling between summer breeding territories and overwintering grounds, and some extreme athletes (e.g., Bar-tailed Godwits) even complete these flights without stopping to rest or feed! Such feats require preparation, with important input from the endocrine system.

Fat storage by hyperphagia is an important form of preparation that can double a bird’s weight. As the time approaches, birds begin to experience Zugunruhe, an instinctual restlessness that compels them to migrate. Both are stimulated by the thyroid hormones T3 (triiodothyronine) and T4 (Thyroxine).

Navigation and Timing

Migration is a carefully timed event, usually occurring in the spring, before nesting, and again in the fall, after nesting and the prebasic molt are complete. The sequence varies between species, but birds typically follow a regular schedule where breeding, molting, and migrating do not overlap.

The timing of migration in obligate migrants is linked to external factors like day length and endocrine hormones like corticosterone and ghrelin. Interestingly, finding their way home or migrating may rely in part on a bird’s biological clock to adjust for the shift of the sun in the sky relative to the time of day, highlighting the potential role of the endocrine system in navigation.

Reproductive Cycles and Hormonal Regulation

Breeding Season Onset

Most birds time their breeding with the onset of the warmer months, when plants come out of dormancy, sparking the emergence of invertebrates and a general wake-up call for temperate and high-latitude ecosystems.

Gonadotropin-releasing hormone (GnRH) produced in the hypothalamus stimulates several other hormonal changes that trigger the development of reproductive organs and various breeding behaviors like courtship, territory formation, and nest-building.

Egg-Laying and Brooding

Hormonal cues stimulate the maturation and then ovulation of ova in the female ovary. The eggs are then fertilized in the oviduct, where they are fully formed and shelled before being laid. Once the clutch is complete, elevated prolactin levels cause broodiness and motivate incubation and parental care.

Post-Breeding Hormonal Changes

As the breeding season draws to a close, shortening days trigger the release of gonadotropin-inhibitory hormone, which shuts off the supply of GnRH and effectively ends breeding activity. At this time, the reproductive gonads shrink, and sex hormone production is diminished in advance of coming life stages like molting and migration.

Gonadotropin-releasing hormone (GnRH) produced in the hypothalamus stimulates several other hormonal changes that trigger the development of reproductive organs and various breeding behaviors like courtship, territory formation, and nest-building. Southern Black Masked Weaver

Environmental Cues and Hormonal Responses

Photoperiod and Temperature

Until now, we’ve discussed how birds time their various life stages relative to the seasons, but you may be wondering how birds keep time without a watch or a calendar.

Birds perceive the shortening and lengthening days with their built-in biological clock. Day length provides an important cue since days are shortest in winter and longest in summer.

Climate Change Impacts

Accelerated climate change presents a significant challenge for birds that rely on a light-based biological clock to maintain a circannual rhythm.

The problem is that the timing of food availability is changing while day length remains the same, so birds might be arriving too late to the party. Still, many birds are already adjusting by arriving significantly earlier than they did in recorded history.

Adaptation and Survival Strategies

Species-Specific Adaptations

Many birds follow a similar pattern in the timing of their molt, migration, and breeding seasons. However, there are exceptions to almost every rule when it comes to birds, and many species have evolved alternative strategies.

Let’s look at a few interesting examples:

• Great Horned Owls begin breeding in winter and lay their eggs as early as February to give their slow-growing chicks their best shot at independent success by the following winter.
• Bobolinks defy the norm by undergoing a complete prealternate molt in their wintering habitat and a complete prebasic molt on their breeding grounds.
• Migratory Painted Buntings are unusual in that they molt at staging areas en route between their breeding and overwintering grounds.

Survival Benefits

Most species follow a similar pattern, and in most cases, you can see birds migrating in spring, nesting into the start of summer, molting in midsummer, and then migrating again in the fall.

However, each bird species has evolved to survive in a specific ecological niche that provides its own unique opportunities and challenges. The unique species mentioned above highlight how a one-size-fits-all strategy is not always ideal when it comes to the timing of endocrine cues.

Great Horned Owls begin breeding in winter and lay their eggs as early as February to give their slow-growing chicks their best shot at independent success by the following winter. Great-Horned-Owl Family

Summary

Rather than consulting a calendar, birds rely on their fine-tuned endocrine systems to coordinate vital annual life stages like clockwork each year.

Each complex phase requires preparation, so changes must begin well in advance. Managers and conservationists can better protect the world’s birds by understanding these physiological processes and the challenges modern birds face due to factors like habitat loss and climate change.

There’s much more than meets the eye when it comes to bird watching! As the seasons change and you notice the tell-tale signs of birds breeding, molting, and migrating, consider the complex chemical process going on within our feathered friends.