Posted | filed under Weekly Banding Reports.

With the arrival of June, spring migration has started petering out. We are no longer seeing birds migrating overhead and haven’t encountered any new species this week. Surprisingly, despite lowered rates of obvious migration, we are still catching decent numbers of birds. We have been averaging about 30 birds a day which brings our spring total up to 739 (almost 100 birds higher than our spring total last year). Most of the birds we captured this week were White-throated sparrow, American redstart, Canada warbler, mourning warbler and common yellowthroat. We still have nine days left in spring migration and we are hoping that the decent banding continues. We feel that given the late arrival dates of many species this year, we could yet see a final push of late migrants, especially considering that we have seen so few Tennessee warbler – a species we usually observe in high numbers.

American redstart

Male American redstart

Besides the lack of obvious movement – the other clear indication that migration is ending is that we are starting to recapture several of the same birds over and over. Our nets are all strategically located in habitat that birds like so it is no surprise that many birds establish territories and build nests near our netlanes. Although some may think it would get rather boring catching the same birds day after day, it is actually very interesting and can teach us a fair bit about local bird ecology. During the breeding season both male and female birds undergo distinct physical changes; for species that aren’t sexual dimorphic (meaning species that the sexes don’t look different) this is the only time of year when we can determine if they are male or female. Male birds develop what is called a cloacal protuberance. This means exactly what it sounds like it means – the cloaca (which is a bird’s only ‘exit’ port) swells significantly to help with copulation. Female birds, on the other hand, develop a brood patch (BP); they lose all the feathers on their breasts and bellies and fluid builds up under the skin. The BP develops so that the mother bird can have skin to egg/chick contact while incubating. Birds need BPs to incubate successfully because feathers are such good insulators that very little of the mother’s body heat would reach the eggs if she retained her belly feathers.

By looking at the breeding conditions of the recapture birds we encounter we can get a very accurate picture of the breeding timing of local birds. We can tell that once the male birds of a certain species start getting cloacal protuberances (CP), that means that the female birds are done building nests and are ready to start producing eggs. The females will lay an egg a day and the males will retain theirs CPs until the female is done laying. Once done laying, the females then develop BPs and we know that they are sitting on eggs, which means that in 7 – 10 days (depending on the species) chicks will start hatching. Information about local breeding timing is important not only to researchers but also to industry as many regulations regarding forest management are based on that information.

mourning warbler

Male Mourning warbler