The events of the year 1830 were originally singled out to examine why Jenyns turned down the offer to be the naturalist on the Beagle the following year, no answer beyond those he stated could be found. What this does show is every detail I could find about his year (and a few months of the next). Most interesting are letters from his cousin the Rev George Vachell in Macao and Canton who was the Chaplain to the British Factory and shows that Darwin wasn't the first to send back specimens to Jenyns and Henslow. His tour that year was to Derbyshire and a most interesting walking tour he had.
The morning was cold and wet, it had been raining now for two weeks.
He checked his barometer, it had risen to an unusual high of 30.61degrees.
Later he heard gunfire outside the vicarage, one of the estate workers
had shot some female tufted ducks on the stream by the side of his
house. These he took into his study to measure and describe by the
light of the tall windows "One a younger bird than the other having
the forehead and other parts marked with grayish white feathers,
probably the sign of immature plumage. This species is the Anas
[Aythya] fuligula. Temm.".
James Francis Stephens wrote to tell Jenyns that he was selling his
duplicate collection of British Insects, most of them had been bought
from other collectors. There were more than 3155 insects and included
more than 2000 species of beetles, butterflies and others, for these
he wanted £20. Stephens had offered as part of the deal to put them in
order with printed labels, but he really wanted to get on with his
"Illustrations of British Entomology"*, and rather resented the time
he had used up. [*started in parts: 1827 to 1837, Suppl.1846].
Stephens wrote "The task I had undertaken turned out to be rather more
trouble than I anticipated, nevertheless I have so far accomplished
it, ready for Tuesday's carrier .... You will observe that I have in
several places introduced new generic names (in the M.S.S. Catalogue
of the insects sent) which are those I shall adopt in the new edition
of my Nomenclature"*. Jenyns seems to have found a buyer for the
insect collection at Cambridge, possibly the Philosophical Society.
Stephens was going to use the money to continue publishing his
"Illustrations" and also offered for sale some of the plates, they had
cost a lot of money to produce. [* The Nomenclature of British
Insects, London 1829 & 2nd Ed. 1833].
Jenyns used Stephens identifications for his insect collections and later gave them to Cambridge Philosophical Society, among them (according to Jenyns' Catalogue in Cambridge Zoology Museum) at about this period are specimens collected by C.Darwin, C.C.Babington, T.Vernon Wollaston and A.Power. Jenyns was also building up a collection of spiders and diptera.
By now a good number of people were collecting birds for Jenyns, both country people who got paid for any interesting specimens and also friends at Cambridge University who wished to help in his scientific endeavors. Some large flocks of Twites [Mountain Linnet] were seen near Cambridge. Some of these were shot and brought for Jenyns to to examine.
It seems that some days before this Jenyns had been to see Charles Darwin at Christ's College at Cambridge, for Darwin commented in a letter to W.D.Fox "Mr Jenyns has been looking at my insects & he was very grateful for a good many insects I gave him". The invitation for Jenyns to visit dates back to the previous November "I am going to Bottisham to see Mr Jenyns cabinet & I believe he is coming to see mine". It seems that Jenyns had made the first move.
The temperature now rose and "from the 3rd to the 8th, with the exception of one night, there was a slight interruption of the frost, attended at intervals by much fog and mizzling rain: still the thermometer, during that period, was seldom many degrees above the freezing point, and in only two instances as high as 40 degrees".
William Yarrell posted a letter to Jenyns, Yarrell was most excited about a new species of swan that he knew was different from the Hooper, he had pointed out some of these to Jenyns in London during his last visit there. Jenyns seems to have spotted some of these new swans at Cambridge and offered to meet Yarrell there, but Yarrell was not so easily tempted "during the late severe weather I have been so fortunate as to procure three specimens of the new swan". The general shortage of food in England during the winter months, meant that almost any edible bird entering this country would have been shot or trapped and offered for sale. Yarrell haunted the London markets to purchase any novelties. Yarrell now had found enough of the new birds to enable him to work at the internal and external descriptions of the new swan, to write a paper for the Linnean Society. In his last letter Jenyns had described to Yarrell a very large swan, Yarrell replies "I am almost afraid you have missed another new swan ... the North Americans have a species which they call the Great Grey headed Swan, both the Hooper and the new one are common at Hudsons Bay, and why may not their Great Grey headed Swan occasionally visit us, as well as the others". Yarrell asked Jenyns to prevent the owner of a hermaphrodite lobster from "fastening it down till I have seen it". He intended to see it in Cambridge in February and he would bring with him for the Philosophical Society's Collection "one or more of every British Bird's egg that I possess beyond a pair, which I keep for my own drawers"
With little natural history available Jenyns turned to his other great interest, Meteorology. "In only two instances [was the temperature] as high as 40 degrees. These occurred on the 5th and 7th of January, both of which were extremely fine days, and the only ones at all pleasurable to the feelings which had been experienced for upwards of three weeks. Indeed, on the second of these occasions, we were almost induced, from the promising appearance of the sky, to anticipate some decided change of the weather; but our hopes were soon at an end. The very next night after the 7th, the thermometer fell again several degrees below the freezing point, attended by more snow; and from that time to the 7th February there was scarcely any further variation in the weather at all, it being marked by one continued succession of frost and snow, with keen winds, principally from the east and North-east".(Mag NH)
How was it that some gnats seemed to have the power to resist cold?
"notwithstanding the severity of the weather a brood of Trichocera
hiemalis, Meig., made its appearance today and many specimens were
seen sticking to the walls of our outhouses and in the windows, as if
they had just emerged from the pupa state; and though they did not
offer to take wing for several weeks, yet they readily moved their
quarters when disturbed; a proof that, notwithstanding the continued
frost, they were not actually torpid".
Charles Darwin at Christ's College, Cambridge, on the 13th he wrote to
W.D.Fox "I rode over the other day to pay Mr.Jenyns a visit. he has a
nice snug little house, & lives very comfortably, & was altogether
very civil, but not particularly liberal. I gave him an awesome lot of
insects and he gave me  good ones and two or three more common
ones, refused me a specimen of the Necroph sepultor, (the Fen one of
which he can get plenty) although he has 7 or 8 specimens by him at
that time. - There is a perfect specimen of liberality for you: a true
disciple of the Curtis school: My Dytici and Colym: astonished his
weak mind:" At this time Darwin saw Jenyns as the man to beat in
collecting beetles and had formed an poor opinion of Jenyns'
personality, probably based on his rather severe features, one he soon
changed when he got to know him better. Jenyns didn't consider
himself a"collector" of beetles, instead he saw them as a subject for
serious scientific study and so resisted giving rare specimens to
Darwin, after all Darwin was only a collector and little else.
However, Jenyns would have thought it perfectly right for others to
give him specimens. It is not clear when Darwin made his visit, but
as the round trip to Swaffham Bulbeck from Cambridge on horseback was
about fourteen miles, Darwin probably picked a dry fine day for it,
Jenyns tells us that these occurred on the 5th and the 7th of January.