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Friday, February 21, 2014

NEW ENGLAND BIRDS UNDERGO LATE WINTER MORTALITY


This first winter White-throated Sparrow was moribund before being hand captured and fed a diet of chick feed.  With feeding, it recovered within 24 hours.
With intense cold since mid-November and with a month of continuous, deep snow pack in southern New England, it appears to be the kind of winter that is associated with high mortality among especially ground-feeding birds.  Still, documenting the effects of such a winter on bird survivorship is notoriously difficult. Birds that succumb overnight may simply disappear into surrounding woodlands or be quickly scavenged by nocturnal mammalian predators.  It is also the kind of winter in which we might expect that diseases could become established among populations already weakened by physiological stress.

Hence, it caught my attention when within the past week I found four dead birds on my property- all primarily ground-feeders- Northern Cardinal, Song Sparrow and White-throated Sparrow. Inasmuch as I also raise free-range chickens, this was the sort of observation that raised disease transmission concerns. There has been a Goshawk patrolling my bird feeders in recent days as well, but the untouched dead birds both beneath the feeders and elsewhere on the property appeared to have their demise unrelated to this predator- who left with his prey.  In hard winters past, I had found dead Carolina Wrens about my property, but these wrens- at their northern range limit here in Connecticut- have a history of winter die-offs that has been documented since the nineteenth century.

This week, I contacted the state animal diseases lab to learn if any outbreaks of bird diseases had been reported. They informed me that none had, but I told them I had also just picked up a moribund individual that I would bring to them for autopsy if it succumbed overnight.  However, in the absence of any evidence of respiratory distress or other obvious evidence of disease, I decided to try first feeding the bird to see if it would recover.  Indeed, within 24 hours the bird had regained its strength and continued to show no other signs of disease (photo above).

This observation led me to suspect that this local mortality was primarily starvation-related.  I found it notable that all the birds involved were juveniles- the least experienced and likely most socially subordinate individuals- i.e. the individuals most susceptible to winter mortality.  In my studies on Bald Eagles in years past, juvenile birds indeed appeared less able to meet their winter energetic requirements. In my more recent studies of forest bird communities of eastern Connecticut, I've also demonstrated that forest birds, including the species I've been finding dead locally, vacate northern latitudes and accumulate in energetically less expensive coastal locations.

However, I am still not entirely convinced that no other factors are at work.  Examination of the dead birds showed that they still had reasonable levels of subcutaneous fat and well-developed breast muscles- suggestive that they were not starving.  One thing I have observed with my domestic birds is that socially subordinate hens can show atrophy of the breast muscles.  It is something I have to pay attention to so that I can separately feed such birds before they become weak.

In the meantime, I plan on keeping a close eye on events around my bird feeder and elsewhere on my property.  I recommend that others of you in this region be mindful of the possibility of a local disease outbreak.


2 comments:

  1. Interesting.

    Here, we tend to have American coot die-offs in the winter because we go so many packed in together in certain areas that they kill each other through fighting. This year, it hasn't been so bad. We're having a drought here in Calfiornia and I wonder how it affects some of our seed-eating birds like towhees and sparrows. I have noticed fewer yellow-rumps and white-crowns this year.

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  2. We are now up to 7 dead birds in a week. We'll be bringing them to the state pathology lab this Mon.

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